The Vienna Convention on the Law of Treaties

Introduction

The Vienna Convention on the Law of Treaties is the central legal instrument governing the formation, interpretation, performance, amendment, invalidity, suspension, and termination of treaties between States. Adopted in 1969 and in force since 1980, it gives treaty relations a common legal grammar at the point where diplomacy becomes binding law. Its importance lies not only in the number of States formally bound by it, but also in the fact that many of its provisions are treated as codifications of customary international law or as authoritative statements of general treaty law (United Nations, 1969; International Law Commission, 1966).

Treaties are among the main techniques through which international law becomes operational. They establish borders, create international organizations, regulate trade, protect human rights, structure alliances, govern diplomatic relations, allocate maritime rights, and build procedures for cooperation and dispute settlement. Yet the legal force of a treaty does not arise merely because States sign a document or announce a political agreement. Treaty law asks more precise questions: who had authority to bind the State, what form of consent was used, when the treaty entered into force, how its terms should be interpreted, and when a State may lawfully withdraw or suspend performance.

The Convention is often described as the “treaty on treaties,” but that phrase should not obscure its deeper function. It does not simply list diplomatic procedures. It translates State consent into legal obligation, disciplines the interpretation of written commitments, and limits opportunistic attempts to escape treaty duties. Rules such as pacta sunt servanda, good faith performance, non-retroactivity, the object and purpose test for reservations, and the interpretive method in Articles 31 and 32 give treaty relations a measure of stability without denying that legal commitments may need amendment, adaptation, or termination.

A serious account of the Convention must also recognise its boundaries. The 1969 text applies directly to treaties between States, not to every form of international commitment. Political declarations, informal arrangements, oral understandings, and treaties involving international organizations raise questions that the Convention does not fully answer. Even within its own field, it operates alongside customary international law, specialised treaty regimes, institutional practice, and judicial reasoning. Its lasting authority comes from this combination of formal law and practical use: it remains the starting point for understanding how treaty obligations are made, read, challenged, and brought to an end in public international law.

1. The Architecture of Treaty Law

Treaty law gives legal form to one of the most common acts in international relations: the written commitment between States. A treaty may settle a boundary, create an international organization, regulate diplomatic privileges, establish a trade regime, protect human rights, structure maritime rights, or set rules for armed conflict. These fields differ in substance, but they all depend on prior questions of treaty law. Was there a treaty at all? Did the State consent to be bound? When did the obligation begin? How should the text be interpreted? Can a reservation alter the legal effect of a provision? When may a treaty be terminated, suspended, or treated as invalid?

The Vienna Convention on the Law of Treaties answers those questions through a general framework. It does not replace the substantive law contained in individual treaties. Rather, it determines how treaty obligations come into existence and how they operate once they exist. That distinction is essential. The Convention is not the law of trade, human rights, diplomatic relations, or maritime delimitation, but those regimes cannot be applied coherently without rules on consent, interpretation, performance, amendment, and termination.

Some of its provisions bind States because they are parties to the Convention. Others are treated as reflections of customary international law, particularly the general rule of treaty interpretation in Article 31 and the supplementary role of Article 32 (International Law Commission, 1966; ICJ, Territorial Dispute, 1994; ICJ, Kasikili/Sedudu Island, 1999). The Convention’s importance lies in that dual character: it is both a treaty text and a widely used statement of general rules about treaties.

1.1 The legal life of a treaty

A treaty does not become legally significant only at the moment of signature, nor does treaty law end once a text enters into force. The Convention regulates the legal life of a treaty across several stages. It begins with capacity and representation, then moves to adoption, authentication, consent to be bound, entry into force, reservations, performance, interpretation, amendment, invalidity, suspension, and termination. Each stage has a different legal function.

This structure matters because treaty disputes often arise long after the diplomatic ceremony has ended. A State may argue that its representative lacked authority, that ratification was required, that another State’s reservation is impermissible, that a later treaty prevails, or that performance has become impossible. These are not merely procedural objections. They determine whether an international obligation exists, what it means, and whether it can still be invoked.

The Convention also protects stability. Treaty obligations would lose much of their value if States could escape them by invoking ordinary political change, domestic inconvenience, or a preferred reinterpretation of the text. At the same time, the Convention does not freeze treaty relations permanently. It recognises amendment, modification, withdrawal, suspension, invalidity, and termination under defined conditions. Its method is controlled flexibility: treaties must be reliable, but they also need legal mechanisms for change.

1.2 The “treaty on treaties”

The description of the Convention as the “treaty on treaties” captures its basic function, but the phrase can be misleading if treated as a slogan. The Convention is not merely a catalogue of diplomatic formalities. It supplies the legal rules that allow treaty regimes to function as law. Without such rules, the binding force of international agreements would depend too heavily on political assertion, diplomatic habit, or unilateral interpretation.

The Convention also performs a constitutional function in a limited international-law sense. It does not create a world constitution, and it does not stand above all other legal instruments in the way a domestic constitution may stand above ordinary legislation. Its authority is more technical and more practical. It provides the legal conditions under which treaty obligations are created, interpreted, preserved, altered, or ended. That makes it central to the structure of public international law without turning it into a supreme code.

Its value is clearest when legal disagreement arises. States rarely dispute treaty obligations by openly denying the relevance of law. More often, they argue over consent, scope, interpretation, breach, exceptions, reservations, later agreements, or termination. The Convention gives those arguments a disciplined legal vocabulary. It does not remove conflict from international relations, but it narrows the terms on which legal claims can credibly be made.

2. Codification Before and After 1969

The Convention belongs to the broader post-1945 codification project carried out through the United Nations and the International Law Commission. After the Second World War, treaty-making expanded rapidly. The United Nations Charter created a new institutional framework, decolonisation increased the number of States participating in multilateral law-making, and technical cooperation increasingly required written agreements across many fields. Treaty law had to serve a wider and more diverse international community than the one reflected in older European diplomatic practice.

Codification did not mean simple transcription of settled custom. The law of treaties contained long-standing principles, such as pacta sunt servanda and consent to be bound, but many questions required clarification. Reservations to multilateral treaties, the effect of coercion, the role of peremptory norms, and the method of interpretation were not merely academic issues. They affected how newly independent States, established powers, and international institutions would use treaties as instruments of legal order.

The Convention should be understood against this background. It was not produced by a single scholar, court, or government. It emerged through a long institutional process that combined legal research, State comments, International Law Commission drafting, and diplomatic negotiation. That origin helps explain why the Convention remains authoritative: it was built through practice, doctrine, and political compromise rather than abstract theory alone.

2.1 The International Law Commission

The International Law Commission was central to the Convention’s formation. Its task was to prepare draft articles that could organise the existing law while also clarifying unsettled questions. The Commission drew on State practice, judicial decisions, diplomatic materials, legal scholarship, and government observations. Its work on treaty law was especially significant because treaties were already a primary instrument of international legal practice, yet the general law governing them was dispersed across custom, doctrine, and case law.

The ILC’s draft articles gave treaty law a systematic form. Instead of treating treaty-making as a collection of isolated diplomatic techniques, the Commission organised the subject around definition, conclusion, reservations, entry into force, observance, interpretation, amendment, invalidity, termination, and depositary functions. This structure shaped the final Convention and still influences how treaty law is taught, argued, and applied.

The Commission’s work also matters because its commentaries remain part of the legal context in which the Convention is read. Courts, tribunals, and scholars often consult the ILC materials not as binding law, but as evidence of the reasoning behind particular provisions and of the relationship between codification and customary international law (International Law Commission, 1966). That is especially relevant where the Convention is invoked against or by States that are not parties to it.

2.2 The Vienna Conference

The United Nations Conference on the Law of Treaties, held in Vienna in 1968 and 1969, transformed the ILC draft into a treaty text adopted by States. That diplomatic stage matters. The Convention was not simply approved as a technical report. States debated its provisions, proposed amendments, defended institutional interests, and negotiated language on issues with major legal and political consequences.

Several subjects were particularly sensitive. Reservations raised the problem of how to balance broad participation in multilateral treaties with the integrity of treaty obligations. Coercion and invalidity raised the question of how treaty law should respond to pressure, fraud, corruption, and violations of fundamental rules. Peremptory norms introduced the idea that some rules of general international law are so fundamental that treaties conflicting with them are void. These questions could not be resolved by drafting technique alone; they required political acceptance by States.

The final text reflects that negotiated character. Some provisions state rules with considerable clarity. Others preserve flexibility through standards such as object and purpose, good faith, manifest violation, material breach, and fundamental change of circumstances. That mixture is not a defect. Treaty law often has to combine legal certainty with room for judgment, because the factual settings in which treaties operate vary widely.

2.3 Entry into force and authority

The Convention was adopted in 1969 and entered into force in 1980 after the required number of ratifications. Its formal effect is straightforward: as a treaty, it binds States that are parties to it according to ordinary rules of treaty law. A State that is not a party is not bound by the Convention merely because the Convention exists.

The more difficult question is broader authority. Many provisions of the Convention are widely treated as codifications of customary international law. This is particularly clear in the rules on treaty interpretation, which the International Court of Justice and other tribunals have repeatedly treated as reflecting general international law (ICJ, Territorial Dispute, 1994; ICJ, Kasikili/Sedudu Island, 1999).

This distinction is essential for legal analysis. The Convention’s authority cannot be reduced to a list of parties. Nor can every provision be assumed to bind non-parties automatically. A careful lawyer must ask whether a specific rule applies as treaty law, as customary international law, or as persuasive evidence of general treaty practice. The Convention’s strength lies in the fact that it often operates in all three ways, but not always with the same legal basis.

3. What Counts as a Treaty

The Convention begins with a definition because treaty law cannot operate until the legal character of the instrument is clear. States produce many texts in diplomatic practice: declarations, communiqués, joint statements, memoranda, exchanges of notes, agreed minutes, protocols, and political pledges. Some are binding treaties. Others record policy alignment, diplomatic intention, or negotiated language without creating legal obligations. The difference is not semantic. It determines whether breach may give rise to international responsibility, whether rules on interpretation apply, and whether a State can invoke treaty-law mechanisms such as reservation, amendment, suspension, or termination.

Article 2(1)(a) defines a treaty as an international agreement concluded between States in written form and governed by international law, whatever its particular designation. This definition has two important consequences. First, treaty status depends on legal criteria, not ceremonial form. Second, the instrument must be governed by international law. A written arrangement between governments may still fall outside treaty law if the parties intended only a political commitment or an arrangement governed by domestic law.

The question is often practical rather than theoretical. A State may later deny that a document was intended to bind it. Another State may insist that the wording, context, signatures, and subsequent conduct show legal commitment. International courts have treated this inquiry carefully. In Qatar v Bahrain, the International Court of Justice found that signed minutes could constitute an international agreement creating rights and obligations, while in Aegean Sea Continental Shelf, it treated a joint communiqué more cautiously because its wording and context did not establish the same legal effect (ICJ, Qatar v Bahrain, 1994; ICJ, Aegean Sea Continental Shelf, 1978).

3.1 Written agreements between States

The 1969 Convention applies to written agreements between States. This formal limit does not mean that international law recognises only highly formal treaty instruments. A treaty may be contained in one document or in two or more related instruments. The written form may be a convention, protocol, exchange of notes, agreed minutes, or another documentary arrangement, provided the parties intended it to be governed by international law.

This point is especially important in diplomatic practice. States often use simplified forms to conclude agreements quickly or to regulate technical matters without a lengthy treaty ceremony. An exchange of notes between foreign ministries, for example, may be legally binding if the notes show agreement to that effect. The law looks beyond external form to the juridical substance of the exchange.

The Convention’s focus on written treaties also has limits. Article 3 makes clear that the Convention does not deny the possible legal force of international agreements outside its formal scope. Oral agreements, agreements involving international organizations, and agreements involving other subjects of international law are not made legally irrelevant merely because they fall outside the 1969 definition. They are simply not governed directly by the Convention as treaty law between States.

3.2 Legal effect over labels

The name given to an instrument is not decisive. States may call a binding instrument an “agreement,” “protocol,” “memorandum,” “charter,” “statute,” “exchange of notes,” or “minutes.” They may also call a non-binding instrument a “declaration” or “memorandum of understanding.” Treaty law is concerned with legal intention, not diplomatic branding.

That approach prevents formal labels from defeating legal responsibility. If two States use language of obligation, identify the parties, set out commitments with sufficient precision, and sign through authorised representatives, the instrument may be a treaty even if it avoids the word “treaty.” Conversely, an elaborate document with solemn language may remain non-binding if the parties clearly intended only political cooperation.

The decisive question is whether the parties intended to create obligations governed by international law. That intention may be found in the text, the circumstances of conclusion, the identity and authority of the signatories, the procedure followed, the presence or absence of entry-into-force clauses, and the conduct of the parties after signature. No single factor is always conclusive. Legal character is usually established through the instrument as a whole.

3.3 Political commitments and soft law

Modern diplomacy frequently relies on instruments that are deliberately not treaties. Political declarations, guidelines, joint statements, action plans, codes of conduct, institutional conclusions, and memoranda of understanding may influence behaviour without creating treaty obligations. These instruments can be useful because they allow States to cooperate where a binding law would be too slow, too rigid, or politically unacceptable.

The legal distinction still matters. A political commitment may carry reputational cost if ignored, but it does not normally create the legal consequences of breach. It cannot be interpreted, amended, suspended, or terminated under the Convention in the same way as a treaty. Nor does it automatically trigger State responsibility as a breach of international law.

Soft law should not be dismissed as legally meaningless. It may shape expectations, guide institutional practice, influence later treaty negotiations, or contribute to the formation of customary international law when linked to State practice and opinio juris. But its force is different from treaty law. The Convention helps draw that boundary by asking whether the parties created a written agreement governed by international law.

4. Capacity, Representation, and Consent

Once an instrument qualifies as a treaty, the next question is whether the State has become legally bound by it. The Convention treats consent as the foundation of treaty obligation, but it does not treat consent as informal political assent. Consent must be expressed through recognised legal methods and by persons with authority to act for the State.

Article 6 states the basic rule: every State possesses the capacity to conclude treaties. That capacity belongs to the State as an international legal person. Yet capacity alone is not enough. The State must act through representatives, and those representatives must have authority under international treaty law. The Convention addresses this through rules on full powers, signature, ratification, acceptance, approval, accession, and exchanges of instruments.

This structure protects both flexibility and certainty. Diplomacy requires practical methods for negotiating and concluding agreements. International law also needs safeguards against uncertainty, unauthorised commitments, and later disputes over whether a State has actually accepted legal obligations.

4.1 State capacity

State capacity to conclude treaties is a basic incident of statehood. A State may enter into agreements on political, economic, territorial, environmental, humanitarian, security, technical, or institutional matters. The Convention does not restrict treaty capacity to particular subjects, although other rules of international law may affect whether a treaty’s content is valid.

The 1969 Convention concerns treaties between States. It does not directly govern treaties between States and international organizations or treaties between international organizations. Those instruments are addressed by the 1986 Vienna Convention, although that Convention has not acquired the same practical status as the 1969 text.

The separation is analytically important. International organizations may possess treaty-making capacity, but that capacity depends on their constitutive instruments, functions, and powers. States possess general treaty-making capacity; international organizations possess treaty-making capacity within the limits of their legal personality and mandate.

4.2 Full powers and representatives

States act through human representatives, which creates a practical problem: how does another State know whether a person has the authority to bind the State? The Convention answers this through the concept of full powers. A representative normally produces a document showing authority to adopt, authenticate, or express consent to be bound by a treaty.

The rule is not absolute. Certain office-holders are treated as representing the State by virtue of their functions. Heads of State, heads of government, and ministers for foreign affairs may perform treaty acts without producing full powers. Heads of diplomatic missions may adopt treaty texts between the sending and receiving States, while representatives accredited to an international conference or organization may adopt texts within that setting, subject to the Convention’s conditions.

These rules reflect diplomatic reality. Requiring full powers for every act by senior State officials would be artificial. Allowing anyone to bind the State would be dangerous. The Convention’s solution is to combine formal authorisation with functional presumptions for officials whose position normally carries treaty-making authority.

4.3 Signature, ratification, and accession

Consent to be bound may be expressed in several ways. Article 11 lists signature, exchange of instruments, ratification, acceptance, approval, accession, and any other agreed means. The method depends on the treaty’s terms, the negotiating practice, and the intention of the parties.

Signature has more than one possible function. In some treaties, signature authenticates the text and indicates political approval, but the State is not legally bound until ratification. In others, the signature itself expresses consent to be bound. The difference must be determined from the treaty, the circumstances of conclusion, or the parties’ agreement.

Ratification usually confirms consent after completion of domestic procedures. It allows a State to sign a treaty while reserving final legal commitment until constitutional or legislative requirements have been met. Acceptance and approval often perform similar functions. Accession allows a State to become party to a treaty without having signed it, commonly after the treaty has already been adopted or entered into force.

4.4 Exchange of instruments

The Convention recognises that treaties may be concluded through an exchange of instruments. Article 13 provides that consent to be bound by a treaty constituted by exchanged instruments is expressed by that exchange when the instruments provide for that effect or when it is otherwise established that the States agreed the exchange should have that effect.

This rule reflects a common diplomatic practice. States often conclude agreements through exchanges of letters or notes, especially for bilateral, technical, administrative, or time-sensitive matters. The legal agreement may not appear in a single document. One note may set out proposed terms, while the reply confirms acceptance. The two instruments together may constitute the treaty.

The decisive point is not the simplicity of the form but the legal effect intended by the parties. An exchange of notes may be as binding as a formal convention if it satisfies the Convention’s requirements. A courteous diplomatic exchange, by contrast, remains non-binding if it lacks the intention to create legal obligations under international law.

5. Legal Effects Before Entry into Force

A treaty normally binds a State only after the conditions for entry into force have been satisfied. The Convention still recognises that the period before entry into force may have legal significance. Signature, ratification, acceptance, approval, or accession can create expectations and interim duties even before the treaty is fully operative for a particular State. This is especially relevant where a State has participated in the treaty-making process but has not yet completed the final legal step required to become bound.

The Convention deals with this period through two different mechanisms. The first is the obligation not to defeat the object and purpose of a treaty. The second is a provisional application. These rules should not be merged. The first is a restraint on conduct before full treaty obligation arises. The second is a positive agreement to apply a treaty, or part of it, before formal entry into force.

The distinction matters because the legal consequences are different. A State subject to Article 18 is not required to perform the treaty in full. It must avoid conduct that would seriously undermine the treaty’s basic purpose. A State accepting a provisional application, by contrast, may be required to comply with specific treaty obligations immediately, depending on the terms of the provisional application clause or agreement.

5.1 Object and purpose before entry

Article 18 of the Convention provides that a State must refrain from acts that would defeat the object and purpose of a treaty in two main situations. The first arises after signature, or exchange of instruments subject to ratification, acceptance, or approval, unless the State has made clear its intention not to become a party. The second arises after a State has expressed consent to be bound, pending the treaty’s entry into force, provided that entry into force is not unduly delayed (United Nations, 1969).

This rule gives legal weight to the act of signature without treating the signature as full consent in every case. A State that signs a treaty subject to ratification has not necessarily accepted all treaty obligations. Yet it cannot behave as if the signature has no legal consequence. The Convention protects the integrity of the treaty-making process by preventing a State from undermining the very bargain it has indicated an intention to consider or complete.

The standard is not easy to apply. The “object and purpose” of a treaty may be clear in some instruments and contested in others. A treaty prohibiting a defined category of weapons, for example, may be easier to assess than a broad framework convention containing institutional, procedural, and programmatic provisions. The test should not be used to impose full performance before entry into force. Its function is narrower: to prevent conduct that would make later participation legally or practically hollow.

The rule also preserves State choice. A State may avoid the Article 18 obligation after signature by making clear that it does not intend to become a party. That possibility is important because a signature may occur before domestic political approval, legislative scrutiny, or constitutional review. The Convention does not force ratification through the back door; it imposes an interim duty of restraint while the State’s position remains legally open.

5.2 Provisional application

Provisional application is different. Article 25 allows a treaty, or part of a treaty, to be applied provisionally before entry into force if the treaty itself so provides or if the negotiating States have otherwise agreed. The mechanism is voluntary, but once accepted, it may produce immediate legal obligations according to the agreed terms.

States use a provisional application where waiting for formal entry into force would defeat the practical value of the arrangement. Economic agreements, energy instruments, technical cooperation treaties, institutional arrangements, and urgent administrative regimes may require immediate operation while domestic procedures continue. The legal advantage is speed. The legal risk is that a State may assume meaningful obligations before ratification or parliamentary approval is complete.

The scope of a provisional application depends on the instrument. Some treaties apply almost entirely on a provisional basis. Others apply only selected provisions, such as institutional clauses, technical rules, funding arrangements, or dispute-management procedures. Careful drafting is essential because a provisional application can otherwise generate disputes about which obligations apply and when they cease.

Article 25 also addresses termination. Unless the treaty provides otherwise or the negotiating States agree otherwise, provisional application ends when a State notifies the other States of its intention not to become a party. This rule confirms that a provisional application is not identical to final treaty consent. It is an agreed interim legal regime, not a substitute for the ordinary process by which a State becomes fully bound.

6. Reservations in Multilateral Treaties

Reservations are one of the most significant features of modern multilateral treaty law. They allow a State to join a treaty while excluding or modifying the legal effect of one or more provisions in their application to that State. The device reflects a practical difficulty in multilateral law-making: a treaty seeking broad participation may fail if every State must accept every clause without qualification.

The Convention’s reservation rules respond to that difficulty. They seek to preserve participation without allowing States to hollow out the treaty. This balance is delicate. A reservation may make it possible for a State to join a treaty that it would otherwise reject. The same reservation may weaken the coherence of the treaty regime, complicate relations between parties, and create uncertainty about the content of obligations.

The problem became more acute as multilateral treaties moved beyond reciprocal exchanges between States. Human rights treaties, environmental agreements, disarmament instruments, and institutional treaties often protect collective interests. In those regimes, a reservation does not merely adjust bilateral relations. It may affect the integrity of a broader legal arrangement.

6.1 The function of reservations

Article 2(1)(d) defines a reservation as a unilateral statement, however phrased or named, made by a State when signing, ratifying, accepting, approving, or acceding to a treaty, by which the State purports to exclude or modify the legal effect of certain treaty provisions in their application to that State (United Nations, 1969). The definition focuses on legal effect, not terminology. A State may call its statement a declaration, understanding, condition, or reservation; its character depends on what it seeks to do.

Reservations serve a practical function. They allow States with different legal systems, political constraints, constitutional rules, or policy positions to participate in a multilateral treaty while limiting specific obligations. Without reservations, some treaties might attract fewer parties. With reservations, participation may increase, but uniformity may decline.

The legal cost is fragmentation within the treaty regime. If many States enter different reservations, the treaty may no longer operate as a uniform set of obligations. Party A may owe one version of the treaty to Party B, another to Party C, and a different set of obligations in relation to Party D. The Convention accepts this risk but seeks to control it through rules on permissibility, acceptance, objection, and legal effect.

6.2 The object and purpose test

Article 19 sets the central rule. A State may formulate a reservation unless the treaty prohibits reservations, permits only specified reservations that do not include the one proposed, or the reservation is incompatible with the object and purpose of the treaty. The third condition is the most influential and the most difficult.

The object and purpose test reflects the International Court of Justice’s approach in the Reservations to the Genocide Convention advisory opinion. The Court rejected the idea that every reservation required unanimous acceptance by all parties, but it also rejected unlimited freedom to reserve. Compatibility with the treaty’s object and purpose became the controlling standard for participation with reservations in many multilateral settings (ICJ, Reservations to the Genocide Convention, 1951).

The difficulty lies in application. Some reservations clearly target central obligations. Others affect procedural clauses, dispute settlement, monitoring mechanisms, territorial application, or interpretive provisions. Whether such reservations undermine the treaty’s object and purpose depends on the treaty’s structure and the importance of the provision reserved against. A shallow analysis will simply invoke “object and purpose” as a slogan. A serious analysis must identify what the treaty is legally designed to achieve and how the reservation affects that design.

This test is also sensitive to the type. In a bilateral or reciprocal commercial setting, reservation logic may be easier to manage through consent between parties. In a human rights or disarmament treaty, the same approach may be inadequate because the treaty protects interests beyond a simple exchange of benefits between States.

6.3 Acceptance and objection

The Convention gives other States a role in responding to reservations. A State may accept a reservation, object to it, or remain silent in circumstances where silence may have legal consequences. The effect of those responses depends on the treaty, the reservation, and the position taken by the other parties.

Acceptance generally establishes treaty relations between the reserving State and the accepting State, with the reservation modifying the relevant provisions between them. Objection produces more complicated results. An objecting State may object without opposing the treaty’s entry into force between itself and the reserving State. In that case, the provisions affected by the reservation may not apply between them to the extent of the reservation. An objecting State may also make clear that it opposes treaty relations with the reserving State.

This system is flexible, but it can be difficult to apply. It requires attention to the wording of the reservation, the terms of the objection, the treaty’s reservation clauses, and the character of the obligation affected. The Convention does not create a single automatic answer for every reservation. It creates a framework for determining the legal relationship between the reserving State and other parties.

The practical lesson is direct: reservations and objections must be drafted with precision. A vague reservation may create uncertainty about the State’s obligations. A vague objection may fail to produce the legal effect intended by the objecting State. Treaty practice rewards careful language.

6.4 Human rights treaty reservations

Human rights treaties expose the limits of ordinary reservation doctrine. Many treaty obligations are designed to protect individuals within a State’s jurisdiction, not to exchange equivalent benefits between States. A reservation to a human rights treaty may weaken protection for individuals without causing an obvious reciprocal injury to another State in the traditional sense.

This makes broad reservations especially problematic. A reservation that excludes central rights, empties monitoring procedures of effect, or subjects treaty obligations to domestic law in open-ended terms may threaten the integrity of the treaty regime. Human rights bodies have repeatedly treated reservations as a serious issue because participation alone is not enough if the reservation removes the substance of the obligation (Human Rights Committee, 1994).

The legal difficulty is institutional as well as doctrinal. Under the Convention, States respond to reservations through acceptance and objection. Human rights treaty bodies may also assess reservations when supervising compliance, but the legal consequences of their views have generated debate. The issue is not whether reservations matter; it is who has the authority to determine incompatibility and what follows when a reservation is invalid.

A careful approach avoids two errors. It should not treat every reservation to a human rights treaty as illegitimate. Some reservations may be narrow, temporary, or compatible with the treaty’s object and purpose. It should also reject the idea that human rights reservations are harmless because other States suffer no reciprocal loss. The central concern is the integrity of the protection regime and the legal position of the individuals the treaty was designed to protect.

7. Pacta Sunt Servanda and Good Faith

The binding force of treaties rests on a simple rule with large consequences: every treaty in force is binding on the parties and must be performed by them in good faith. Article 26 of the Convention gives treaty law its central discipline. Once a State has validly accepted a treaty and the treaty is in force for that State, performance is not optional, provisional, or dependent on continuing political enthusiasm (United Nations, 1969).

Pacta sunt servanda is older than the Convention, but codification gives the principle a clear place within the modern law of treaties. It links consent to legal obligation. States are free to decide whether to enter into many treaty commitments, subject to rules on capacity, validity, and peremptory norms. After consent has been validly expressed, the legal position changes. The State is no longer dealing only with a diplomatic preference; it is bound by an international obligation.

Good faith is part of the same rule. A State cannot comply with a treaty only at the level of appearance while frustrating its legal operation. Treaty performance must respect the obligation undertaken, not merely the most convenient reading of isolated words. The International Court of Justice has treated good faith as a principle governing the performance and interpretation of legal obligations, although it does not create unlimited duties detached from the treaty text (ICJ, Gabčíkovo-Nagymaros Project, 1997).

7.1 Binding force

The binding force of a treaty means that a State must perform the obligations that are in force for it as law. This is the core difference between a treaty and a political commitment. A State may regret a treaty, face domestic opposition, or consider its obligations costly, but those facts do not remove the legal duty to comply.

This rule is essential because treaty relations depend on reliability. International cooperation would be unstable if States could treat treaty duties as policy choices that remain binding only while convenient. Border settlements, investment protections, disarmament commitments, environmental obligations, human rights treaties, and institutional arrangements all require a legal presumption that promises formally accepted will be performed.

The binding force does not mean that treaty obligations are eternal or immune from legal change. The Convention permits amendment, modification, withdrawal, suspension, invalidity, and termination under defined conditions. The point is narrower and stricter: a State must use legal routes to alter or end obligations. It cannot replace the law with unilateral convenience.

7.2 Internal law as no excuse

Article 27 provides that a party may not invoke its internal law as justification for failure to perform a treaty. This rule protects the international legal effect of treaties against domestic instability, constitutional disagreement, administrative difficulty, or legislative delay. A State cannot answer an international obligation simply by saying that its parliament, courts, constitution, federal structure, or internal procedure made compliance difficult.

The rule is especially important because every State has its own constitutional system for approving, implementing, and applying treaties. Some require legislative approval. Others allow executive treaty-making in wider terms. Federal States may face a distribution of powers between central and regional authorities. International law does not make those internal arrangements irrelevant, but it prevents them from being used as ordinary excuses for non-performance.

Article 46 creates a narrow and separate issue. A State may invoke a violation of its internal law concerning competence to conclude treaties only if the violation was manifest and concerned a rule of fundamental importance. This is not a broad constitutional escape clause. The threshold is deliberately high because treaty stability would be damaged if States could easily challenge their own consent by relying on internal procedural disputes after the fact.

7.3 Good faith performance

Good faith prevents treaty performance from becoming a technical exercise in evasion. A State may not rely on artificial readings, procedural manoeuvres, or calculated omissions to preserve the appearance of compliance while undermining the obligation. The principle directs attention to the legal commitment as a whole.

This does not mean that good faith allows courts or other States to rewrite a treaty. Good faith operates within the treaty relationship. It disciplines performance and interpretation, but it does not create new obligations unrelated to the text, context, and object of the agreement. Its value lies in preventing abuse, not in replacing consent.

The principle is especially relevant where treaty obligations leave room for judgment. Clauses requiring cooperation, consultation, due regard, equitable treatment, progressive implementation, or institutional participation cannot always be applied by mechanical command. In such cases, good faith gives legal weight to sincerity, consistency, and respect for the agreed procedure, while still requiring analysis of the treaty’s actual terms.

8. Application in Time and Territory

Treaty obligations must be located in time and space. The Convention does not only ask whether a State consented to a treaty or how the treaty should be interpreted. It also asks when the treaty applies and to what territory it extends. These questions often determine the outcome of disputes.

Temporal application concerns whether a treaty governs events that occurred before it entered into force for a State. Territorial application concerns the geographical reach of the treaty once it binds the State. Both rules serve legal certainty. States and other actors need to know the date from which obligations apply and the territory covered by them.

Articles 28 and 29 provide the basic framework. The default rules are straightforward, but they can be displaced by the treaty or by a different intention of the parties. Drafting matters. A treaty that deals expressly with retroactivity, territorial exclusions, dependent territories, federal clauses, or special administrative regions can avoid later uncertainty.

8.1 Non-retroactivity

Article 28 states the presumption against retroactive application. Unless a different intention appears from the treaty or is otherwise established, treaty provisions do not bind a party in relation to acts, facts, or situations that occurred before the treaty entered into force for that party (United Nations, 1969). The rule protects legal certainty and guards against imposing treaty obligations on conduct that was not governed by the treaty when it occurred.

The principle does not mean that past facts are always irrelevant. A treaty may apply to a continuing situation that began before entry into force but persisted afterwards. The legal distinction is between applying a treaty to completed past conduct and applying it to present or continuing legal situations. This distinction can matter in human rights law, territorial disputes, environmental harm, investment claims, and institutional obligations.

The parties may also agree to a retroactive effect. Some treaties expressly regulate earlier situations, confirm prior arrangements, validate previous acts, or apply to claims arising before entry into force. When States choose that route, the legal basis is consent. Without such wording or a clearly established intention, the presumption remains non-retroactivity.

8.2 Territorial application

Article 29 provides the default rule on territorial application. Unless a different intention appears from the treaty or is otherwise established, a treaty is binding on each party in respect of its entire territory (United Nations, 1969). The rule avoids uncertainty by linking treaty obligation to the State’s territorial sovereignty as a whole.

Territorial clauses matter because State territory is not always administratively simple. Federal constitutions, overseas territories, dependent territories, autonomous regions, occupied territories, and special administrative arrangements may create practical difficulties in applying treaty obligations. A treaty may include clauses extending, limiting, or excluding territorial application. Without such clauses, the Convention’s default rule applies.

The rule also prevents States from avoiding obligations through internal territorial arrangements. A State cannot normally say that a treaty binds only part of its territory because domestic law divides authority between national and regional institutions. If a State wants a different territorial arrangement, it must rely on the treaty’s terms or on a clearly established intention accepted under international law.

9. Treaty Interpretation Under Articles 31 and 32

Treaty interpretation is one of the most litigated and practically significant parts of the Convention. A treaty rarely applies itself. Its terms must be read, contested, argued, and applied to facts that may not have been fully anticipated when the text was negotiated. Articles 31 and 32 provide the standard method for that exercise. They are widely treated as reflecting customary international law and are regularly used by international courts and tribunals, including the International Court of Justice (ICJ, Kasikili/Sedudu Island, 1999; ICJ, Whaling in the Antarctic, 2014).

The Convention rejects two weak methods of interpretation. The first is pure literalism, which isolates words from their legal setting and treats treaty interpretation as a dictionary exercise. The second is uncontrolled purposive interpretation, which treats the perceived aim of a treaty as permission to override the agreed text. Article 31 avoids both errors. It requires the interpreter to read treaty terms in good faith, according to their ordinary meaning, in context, and in light of the treaty’s object and purpose (United Nations, 1969).

This structure matters because interpretation is often where diplomatic disagreement becomes legal argument. States may agree that a treaty is binding but disagree over the scope of a clause, the meaning of an exception, the effect of a protocol, or the relevance of later practice. Articles 31 and 32 do not remove judgment from interpretation. They discipline it.

9.1 Ordinary meaning in context

Article 31 begins with the ordinary meaning of treaty terms, but ordinary meaning is not bare literalism. Words do not have legal effect in isolation. They must be read within the sentence, article, treaty structure, preamble, annexes, related instruments, and the wider legal context accepted by the parties. A term that looks simple in ordinary language may acquire a more precise meaning in a technical treaty regime.

Context includes more than the surrounding words. Article 31 refers to the text, including the preamble and annexes, as well as agreements or instruments made in connection with the treaty’s conclusion. This matters because States often place interpretive signals outside the operative clauses. A preamble may identify the concerns that shaped the treaty. An annex may contain technical definitions. A protocol may clarify the scope of obligations.

The International Court of Justice has repeatedly treated treaty interpretation as a single combined operation rather than a sequence in which text, context, and object are mechanically separated. The interpreter does not first exhaust the literal meaning and only later consults the context. The ordinary meaning is identified through context and legal purpose, not apart from them (ICJ, Territorial Dispute, 1994; ICJ, Kasikili/Sedudu Island, 1999).

9.2 Object and purpose

Object and purpose give treaty interpretation direction. It helps identify the legal design of the treaty and prevents readings that would defeat the agreement as a functioning instrument. In a boundary treaty, the object may include finality and stability. In a human rights treaty, it may include effective protection. In an environmental treaty, it may include prevention, cooperation, or conservation, depending on the text.

Object and purpose must still be anchored in the treaty. It cannot be invented by the interpreter or used to correct an agreement that one party later considers inadequate. A court may use object and purpose to choose between plausible readings, but it should not use it to impose obligations that the parties did not accept. The Convention’s method is interpretive, not legislative.

This distinction is often mishandled. A weak argument invokes the broad purpose of a treaty and then treats any preferred outcome as legally required. A stronger argument identifies the treaty’s object and purpose through the text, structure, preamble, title, negotiated context, and related instruments. Purpose has legal weight because it arises from the treaty, not because it floats above it.

9.3 Subsequent agreement and practice

Article 31 also recognises that the parties’ later conduct may clarify treaty meaning. A subsequent agreement between the parties regarding interpretation or application is strong evidence because it shows a shared understanding after the treaty has entered into force. Such agreements may appear in protocols, decisions of treaty bodies, agreed statements, or other forms accepted by the parties.

Subsequent practice may also be relevant, but the Convention sets a demanding standard. Practice must establish the agreement of the parties regarding interpretation. Isolated conduct by one State, silence without legal significance, or inconsistent behaviour across parties will usually carry less weight. The legal value lies not in practice alone, but in practice showing common interpretive acceptance.

Other later practices may still assist analysis, especially in institutional regimes where treaty bodies, conferences of parties, or monitoring mechanisms develop interpretive patterns. The weight of that practice depends on the treaty, the powers of the institution, the participation of States, and whether the practice can fairly be attributed to the parties. The Convention leaves room for such material, but it does not treat every institutional statement as a binding interpretation.

9.4 Supplementary means

Article 32 permits recourse to supplementary means of interpretation, including preparatory work and the circumstances of conclusion. These materials may confirm the meaning reached under Article 31 or assist where Article 31 leaves the meaning ambiguous, obscure, or leads to a result that is manifestly absurd or unreasonable (United Nations, 1969).

Travaux préparatoires can be valuable, especially where negotiations reveal why a phrase was chosen, why a proposal was rejected, or how delegations understood a technical term. They may help distinguish deliberate ambiguity from a drafting accident. They may also prevent an interpretation that contradicts the negotiating record.

Their role remains supplementary. Preparatory work is often incomplete, politically selective, multilingual, or difficult to reconcile. Not every delegation speaks. Not every silence means consent. Not every rejected proposal proves the opposite rule. Article 32 reflects a sound caution: travaux can assist interpretation, but the treaty text adopted by the parties remains the primary legal instrument.

9.5 Multilingual texts

Many treaties are authenticated in more than one language. Article 33 addresses this by establishing the presumption that each authenticated text is equally authoritative unless the treaty provides otherwise or the parties agree otherwise. This rule reflects the multilingual reality of international law and prevents one language version from automatically dominating the others.

Divergences between authentic texts are not rare. Legal terms may not map perfectly across languages. A phrase that appears narrow in one language may appear broader in another. The task is not to select the most convenient version but to reconcile the texts in light of the treaty’s object and purpose.

If comparison of the authentic texts reveals a difference that cannot be removed through ordinary interpretation, Article 33 directs the interpreter toward the meaning that best reconciles the texts, having regard to the treaty’s object and purpose. The result is a disciplined reconciliation exercise. It respects linguistic equality while preserving the treaty as a single legal instrument.

10. Treaties and Third States

The Convention’s rules on third States express a basic principle of treaty law: a treaty does not create obligations or rights for a State without its consent. Article 34 states the general rule with clarity. Agreements bind their parties. They do not impose legal burdens on outsiders simply because the parties wish to regulate a wider situation (United Nations, 1969).

The principle reflects the consent-based structure of international law. If two States could create duties for a third State through their own agreement, treaty law would become a means of indirect legislation over non-parties. The Convention rejects that result. Consent remains the legal foundation for treaty obligations.

The rule should not be overstated. Treaties may affect the factual or legal environment in which third States act. They may establish institutions, settle boundaries, create regimes that later influence custom, or confer benefits that third States may accept. The point is narrower: legal obligation as a treaty obligation cannot be imposed on a third State without consent.

10.1 No obligations without consent

Article 35 provides that an obligation may arise for a third State only if the parties intend the provision to create the obligation and the third State expressly accepts it in writing. This is a strict rule. The burden cannot arise by implication, political pressure, or convenience to the parties.

The rule is easy to understand in bilateral settings. State A and State B cannot agree that State C must close its ports, patrol its waters, pay compensation, or accept a boundary arrangement simply because A and B find that solution useful. If C is to bear a treaty obligation, C must consent.

The rule also protects legal stability. States must be able to identify which treaties bind them and why. Without the third-State rule, the treaty obligations of a State could be altered by agreements to which it was not a party. That would be incompatible with the basic structure of treaty consent.

10.2 Rights for third States

The Convention treats rights for third States differently, but consent still matters. Article 36 provides that a right may arise for a third State if the parties intend to confer that right and the third State assents. Assent is presumed so long as the contrary is not indicated, unless the treaty provides otherwise.

A genuine treaty right must be distinguished from an incidental advantage. If a trade treaty between two States improves market conditions for exporters in a third State, that third State has benefited as a matter of fact. It has not necessarily acquired a legal right under the treaty. A legal right exists only where the parties intended to confer one and the conditions of Article 36 are met.

Rights conferred on third States may also be subject to conditions. A treaty may grant access, navigation, participation, or another legal advantage only on specified terms. If the third State relies on the right, it must respect the conditions attached to it. The Convention treats treaty rights as structured legal positions, not casual benefits.

10.3 Wider effects of treaty regimes

Some treaty regimes create wider effects without formally imposing obligations on third States. Boundary treaties are a clear example. A boundary agreement binds the parties, but the resulting territorial situation may have legal significance for other States. This does not mean that third States are bound by the treaty as parties. It means that certain legal facts created by a treaty may become relevant in the broader international order.

Institutional treaties may also have wider effects. The United Nations Charter, constituent instruments of international organizations, and multilateral regimes in areas such as the law of the sea, aviation, trade, or environmental protection may shape the legal environment even for States that are not parties to every related instrument. The legal basis for those effects must be identified carefully. It may lie in membership, custom, Security Council authority, recognition of a status, or general international law, not simply in the third-State provisions of the Convention.

Treaty rules may also contribute to customary international law. If a treaty provision is accompanied by general State practice and opinio juris, it may become binding on non-parties as custom. In that situation, the non-party is not bound by the treaty as a treaty. It is bound by the customary rule. This distinction is essential because it prevents treaty law and customary law from being confused while recognising that the two may interact.

11. Successive Treaties and Conflicting Duties

States often become parties to more than one treaty dealing with related subject matter. A State may accept a regional trade agreement while also belonging to the World Trade Organization. It may join a human rights treaty and later accept a specialised convention dealing with the same right in more detail. It may conclude a bilateral security agreement while also being bound by broader obligations under the United Nations Charter. Overlap is normal in a treaty-dense legal order.

The Convention addresses this problem through Article 30, which concerns successive treaties relating to the same subject matter. The provision does not treat every overlap as a conflict. Two treaties may concern the same field and still be capable of concurrent performance. A true conflict arises where compliance with one obligation would breach another, or where two instruments regulate the same issue in incompatible ways.

The Convention’s approach is careful because treaty conflict cannot be solved by a single formula. The legal answer depends on the parties to each treaty, the wording of the instruments, any priority clauses, the dates of conclusion, and whether another rule of international law gives one obligation special status. Treaty lawyers must identify the actual incompatibility before invoking the rules of priority.

11.1 Earlier and later treaties

Article 30 distinguishes situations where all parties to the earlier treaty are also parties to the later treaty from situations where the parties are not identical. If the same States are parties to both treaties, the later treaty normally prevails to the extent that its provisions are incompatible with the earlier one, unless the treaty provides otherwise. This reflects the idea that the same parties may revise their legal relations through a later agreement.

The problem is more difficult when party membership differs. If State A and State B are parties to both treaties, their mutual rights and obligations are governed by the treaty that prevails under Article 30. If State A is party to both treaties but State C is party only to the earlier treaty, the treaty common to A and C governs their relationship. If State D is party only to the later treaty, the later treaty governs relations between A and D. The same State may consequently owe different treaty obligations to different partners.

This arrangement protects consent but creates practical complexity. A State that joins a later treaty cannot use that treaty to remove obligations owed to a State that remains bound only by the earlier instrument. If the State’s later commitment makes performance of the earlier treaty impossible or unlawful, the problem may become one of international responsibility. Article 30 manages priority between treaties; it does not automatically excuse breach.

11.2 Conflict clauses and special regimes

Many treaty conflicts are managed before they reach a court because treaty drafters include express clauses on priority, compatibility, savings, or non-derogation. A treaty may state that it prevails over earlier agreements, that it does not affect obligations under another convention, or that it must be applied consistently with a specialised regime. These clauses are often more important than abstract rules because they show how the parties intended their obligations to interact.

Savings clauses are common where States want a new treaty to coexist with older obligations. A human rights treaty may preserve more protective domestic or international rules. An environmental treaty may operate without prejudice to obligations under the law of the sea. A trade or investment treaty may include clauses preserving regulatory powers or obligations under other international instruments. The legal effect depends on the wording, not merely on the existence of the clause.

Lex specialis reasoning may also assist where two rules address the same subject at different levels of generality. A specialised treaty rule may govern a particular issue more directly than a general treaty provision. This reasoning must be used carefully. A rule is not special merely because it is preferred by one party. It must regulate the relevant issue with greater specificity and be applicable between the parties.

Specialised treaty regimes may develop their own techniques for managing overlap. Trade, human rights, environmental, investment, and humanitarian law regimes often contain institutional procedures, monitoring bodies, dispute clauses, and interpretive practices that affect how conflicts are identified and handled. The Convention supplies the general law, but it does not erase the internal logic of specialised regimes.

11.3 UN Charter priority

Obligations under the United Nations Charter require separate treatment because Article 103 creates a special rule of priority. If the obligations of UN members under the Charter conflict with obligations under another international agreement, Charter obligations prevail (United Nations, 1945). This is not an ordinary lex posterior rule and not simply a clause in one treaty among many. It reflects the special legal position of the Charter in the post-1945 international order.

The most practical examples arise in the field of international peace and security. Security Council decisions adopted under Chapter VII may require States to freeze assets, impose sanctions, restrict travel, prohibit arms transfers, or take other measures that conflict with obligations under other treaties. Article 103 gives priority to the Charter obligation in that conflict.

The priority rule should not be exaggerated. It applies where there is a genuine conflict between Charter obligations and obligations under another international agreement. If both obligations can be interpreted and performed consistently, no priority question arises. The first task remains legal analysis of the obligations themselves.

Article 103 also does not mean that every political statement by a UN organ overrides treaty law. The relevant obligation must arise under the Charter. In practice, binding Security Council decisions are the clearest source of such obligations. The Convention’s rules on successive treaties remain relevant, but they operate subject to the Charter’s priority rule.

12. Amendment, Modification, and Adaptation

Treaties need stability, but they also need mechanisms for change. A treaty that cannot adapt may become obsolete, ineffective, or politically unsustainable. A treaty that can be altered too easily may lose reliability. The Convention deals with this tension through rules on amendment and modification.

An amendment usually concerns a formal change to the treaty as a treaty instrument. Modification usually concerns a change in treaty relations between some parties only. The distinction matters most in multilateral treaties. A bilateral treaty can normally be changed by agreement between its two parties. A multilateral treaty raises harder questions because a change accepted by some parties may affect the rights, expectations, or legal position of others.

The Convention’s rules protect consent and treaty integrity. States may revise their commitments, but they cannot use amendment or inter se modification to impose new burdens on non-consenting parties or to undermine the treaty’s object and purpose. Adaptation is allowed, but it must remain legally structured.

12.1 Formal amendment

Article 39 states the basic rule that a treaty may be amended by agreement between the parties. The treaty itself may contain more detailed amendment procedures. Many modern treaties do so because multilateral regimes often require regular technical, institutional, or normative adjustment.

Article 40 deals specifically with the amendment of multilateral treaties. Unless the treaty provides otherwise, any proposal to amend a multilateral treaty must be notified to all contracting States, and each has the right to take part in the decision on what action should be taken and in the negotiation and conclusion of any amending agreement. This reflects the basic principle that parties should not be excluded from formal revision of the legal instrument to which they belong (United Nations, 1969).

Formal amendment procedures are especially important in institutional and technical regimes. Environmental treaties may need updated annexes or emissions commitments. Trade agreements may require revised schedules or procedural rules. Treaties establishing international organizations may need institutional reform. Technical conventions may need rapid adjustment as science, technology, or regulatory practice changes.

An amendment may also create layered treaty relations. Not every party to the original treaty will necessarily become a party to the amendment. The original treaty may continue to govern relations involving States that have not accepted the amendment, while the amended version governs relations between States that have accepted it. This can complicate treaty administration, but it respects the principle that States are not bound by amendments without consent unless the treaty itself validly provides a different mechanism.

12.2 Modification between some parties

Article 41 addresses agreements by which only some parties to a multilateral treaty modify the treaty as between themselves. This is often called inter se modification. It allows flexibility within multilateral regimes, but only under controlled conditions.

Inter se modification is permitted if the treaty allows it. If the treaty is silent, modification between some parties is allowed only if it is not prohibited by the treaty, does not affect the enjoyment by other parties of their rights or the performance of their obligations, and does not relate to a provision whose derogation is incompatible with the effective execution of the treaty’s object and purpose (United Nations, 1969).

These limits are necessary because modifications between some parties may damage the wider regime. A small group of States should not be able to rewrite central obligations among themselves in a way that weakens collective guarantees, undermines institutional procedures, or changes the legal balance accepted by all parties. The more integrated the treaty regime, the more carefully inter se modification must be assessed.

At the same time, inter se modification can be useful. It may allow a group of parties to deepen cooperation, adopt higher standards, or test more advanced commitments without requiring immediate universal agreement. The legal acceptability of that practice depends on whether the modification remains compatible with the treaty and respects the rights of the other parties.

The Convention’s method is neither rigidity nor complete freedom. It allows treaty regimes to adapt while preserving the consent-based structure of treaty obligations. That balance is essential for long-term multilateral agreements, where legal durability often depends on the capacity to change without breaking the regime itself.

13. Invalidity and Defects of Consent

The Convention treats invalidity as exceptional. Treaty stability would be weakened if States could avoid obligations whenever a treaty became inconvenient, unpopular, expensive, or politically damaging. For that reason, the rules on invalidity are narrow, structured, and tied to serious defects in consent or to conflict with fundamental norms of international law.

Invalidity is not the same as breach. A State may breach a valid treaty by failing to perform it. Invalidity goes deeper: it concerns whether the treaty, or the State’s consent to be bound, has legal force at all. The Convention recognises several grounds, including error, fraud, corruption, coercion, manifest violation of internal law on treaty-making competence, and conflict with jus cogens (United Nations, 1969).

The system is deliberately restrictive. A State invoking invalidity must identify a recognised legal ground and satisfy the conditions attached to it. General unfairness, later regret, domestic opposition, or a change of government is not enough. Treaty law protects consent, but it also protects reliance on consent once validly expressed.

13.1 Error, fraud, and corruption

An error may affect the validity of consent where it relates to a fact or situation assumed by a State to exist at the time the treaty was concluded and forming an essential basis of its consent. The rule is narrow. A mistake about political consequences, future costs, legal strategy, or diplomatic advantage will not usually qualify. The error must concern a factual premise central to the decision to be bound.

The Convention also limits reliance on error where the State contributed to the mistake through its own conduct or where the circumstances put it on notice of possible error. This prevents a State from using its own negligence as a route out of a treaty. The legal concern is defective consent, not poor judgment.

Fraud and corruption are more serious because they attack the integrity of the treaty-making process. If a State is induced to conclude a treaty by fraudulent conduct of another negotiating State, or if its representative is corrupted directly or indirectly by another negotiating State, the affected State may invoke that defect. These rules protect the basic honesty of treaty negotiations without allowing every allegation of diplomatic pressure or hard bargaining to become a claim of invalidity.

13.2 Coercion of representatives or States

The Convention distinguishes coercion of a State representative from coercion of the State itself. Coercion of a representative concerns acts or threats directed at the person authorised to act for the State. If consent is procured through coercion of the representative, the expression of consent has no legal effect.

Coercion of the State is treated with even greater gravity. Article 52 provides that a treaty is void if its conclusion was procured by the threat or use of force in violation of the principles of international law embodied in the United Nations Charter (United Nations, 1945; United Nations, 1969). This rule reflects the post-1945 legal order, in which military coercion cannot be used to manufacture treaty consent.

The rule does not convert every form of inequality or diplomatic pressure into coercion. States often negotiate under political, economic, or strategic pressure. The Convention’s rule is directed at coercion through the unlawful threat or use of force. Its function is to deny legal validity to treaties produced by aggression or comparable Charter-violating force, not to police every imbalance in bargaining power.

13.3 Manifest violation of internal law

Article 46 addresses one of the most sensitive points in treaty law: the relationship between international consent and domestic constitutional rules. A State may not ordinarily invoke internal law to escape treaty obligations. Yet the Convention recognises a narrow exception where the State’s consent was expressed in manifest violation of an internal rule of fundamental importance concerning competence to conclude treaties.

The threshold is high. The internal rule must concern treaty-making competence, not any domestic rule that later becomes politically relevant. It must be of fundamental importance. The violation must also be manifest, meaning objectively evident to any State conducting itself in the matter in accordance with normal practice and good faith.

This is not a broad constitutional escape clause. A State cannot simply point to parliamentary disagreement, administrative irregularity, federal difficulty, or later judicial criticism and claim that the treaty is invalid. Article 46 protects exceptional cases where another State should have recognised a serious defect in treaty-making authority at the time consent was expressed.

13.4 Conflict with jus cogens

Article 53 gives the Convention its clearest rule of normative hierarchy. A treaty is void if, at the time of its conclusion, it conflicts with a peremptory norm of general international law. Such a norm is accepted and recognised by the international community of States as a whole as one from which no derogation is permitted and which can be modified only by a later norm of the same character (United Nations, 1969).

Jus cogens should be used with precision. It does not include every important rule, every widely accepted obligation, or every principle with moral force. The category is reserved for norms with a special hierarchical status in general international law. Common examples discussed in legal materials include prohibitions relating to aggression, genocide, slavery, torture, racial discrimination, and crimes against humanity, although the identification of specific peremptory norms must be made carefully (International Law Commission, 2022).

The legal consequence is severe. A treaty conflicting with jus cogens at the moment of conclusion is void. The rule protects the international legal order against agreements that attempt to legitimise conduct no State may lawfully authorise by treaty. It also shows that consent is fundamental in treaty law, but not unlimited.

14. Termination and Suspension

Treaties may end or be suspended, but the Convention does not allow States to abandon obligations casually. Termination and suspension are governed by treaty terms, party agreement, or specific legal grounds recognised by the Convention. This protects stability while acknowledging that legal relations may change.

Termination removes treaty obligations for the future. Suspension pauses the operation of treaty obligations, either in whole or in part, without necessarily ending the treaty. Both consequences must be distinguished from breach. A State that unlawfully stops performing a treaty has not terminated it; it has violated it.

The Convention’s rules also prevent abuse. A State cannot rely on hardship, political dissatisfaction, domestic pressure, or a more favourable later policy as sufficient grounds to end a treaty. Lawful termination or suspension requires a valid legal basis.

14.1 Termination by agreement or treaty terms

The clearest basis for termination is the treaty itself. Many treaties contain provisions on duration, expiry, denunciation, withdrawal, review, or termination. These clauses are not technical afterthoughts. They determine how long the obligations last and how a party may leave the treaty without committing a breach.

Parties may also terminate a treaty by later agreement. This follows the same consent-based logic that allows States to create obligations. States that can bind themselves by treaty can also agree to release themselves, provided the agreement is valid and does not violate superior rules of international law.

Careful drafting reduces future disputes. A treaty should specify notice periods, withdrawal procedures, effects on existing rights or obligations, depositary notifications, and whether withdrawal affects pending claims or continuing duties. Poorly drafted termination clauses invite conflict precisely when political relations may already be strained.

14.2 Material breach

Article 60 addresses material breach as a ground for termination or suspension. In a bilateral treaty, a material breach by one party may entitle the other to invoke the breach as a ground for terminating the treaty or suspending its operation in whole or in part. In multilateral treaties, the consequences are more complex because several parties and collective interests may be involved.

A material breach is not any breach. It includes repudiation of the treaty not sanctioned by the Convention or violation of a provision essential to the accomplishment of the treaty’s object or purpose. The threshold protects treaties against overreaction to minor or technical violations.

In multilateral treaties, the response may depend on whether all parties act together, whether a specially affected State invokes suspension, or whether the breach radically changes the position of every party regarding further performance. The Convention is careful because the automatic collapse of multilateral treaties would be dangerous. One State’s serious breach may justify a legal response, but it does not always destroy the treaty relationship for all parties.

Certain humanitarian obligations receive special protection. The Convention excludes reliance on material breach as a basis for terminating or suspending provisions relating to the protection of the human person contained in treaties of a humanitarian character. This prevents a State from using another party’s breach as a reason to abandon core humanitarian protections.

14.3 Impossibility and changed circumstances

Article 61 allows termination or withdrawal where performance becomes impossible because of the permanent disappearance or destruction of an object indispensable for the execution of the treaty. The rule is narrow. Difficulty is not impossibility. Financial burden, administrative inconvenience, political resistance, or reduced benefit does not satisfy the standard.

The Convention also restricts reliance on fundamental change of circumstances under Article 62. The doctrine, often associated with rebus sic stantibus, applies only where the existence of those circumstances constituted an essential basis of consent and the change radically transforms the extent of obligations still to be performed. It cannot be invoked lightly.

The International Court of Justice has treated this ground with caution. In the Gabčíkovo-Nagymaros Project, the Court rejected an expansive reliance on changed circumstances and stressed that stability of treaty relations requires a restrictive approach (ICJ, Gabčíkovo-Nagymaros Project, 1997). That caution is necessary. If ordinary political or economic change were enough, long-term treaty commitments would become fragile.

Boundary treaties receive special protection. Article 62 excludes fundamental change of circumstances as a ground for terminating or withdrawing from a treaty establishing a boundary. The reason is clear: boundary stability is a central condition of peaceful international relations.

14.4 New peremptory norms

Article 64 addresses a different situation from Article 53. Article 53 concerns a treaty that conflicts with jus cogens at the time of conclusion. Article 64 concerns a treaty that was valid when concluded, but later conflicts with a newly emerged peremptory norm of general international law. In that situation, the treaty becomes void and terminates (United Nations, 1969).

This rule connects termination to the hierarchy of norms. It recognises that international law may develop in a way that makes an earlier treaty legally unacceptable. Once a new peremptory norm emerges, States cannot preserve treaty obligations that conflict with it.

The rule remains exceptional. It requires the emergence of a genuine peremptory norm, not a new policy preference or ordinary customary rule. It also requires conflict between that norm and the treaty. Because the consequence is the termination of treaty obligations, the analysis must be rigorous. Article 64 protects the integrity of the international legal order, but it is not a general mechanism for revising old treaties.

15. Depositaries, Registration, and Transparency

The administrative machinery of treaty law may appear secondary beside consent, interpretation, invalidity, and termination. In practice, it is essential to the reliability of treaty regimes. Multilateral treaties often involve many parties, accessions, reservations, objections, declarations, amendments, territorial notifications, and communications over many years. Without a trusted administrative centre, the legal position of the parties can become uncertain.

The Convention deals with this machinery through rules on depositaries, notifications, corrections, and registration. These rules do not usually determine the substantive content of obligations. They support the conditions under which treaty obligations can be identified, verified, and publicly known. Treaty law depends not only on valid consent but also on accurate records of that consent.

This practical dimension is especially important in large multilateral regimes. A State may need to know whether another State has ratified a treaty, whether a reservation was entered, whether an objection was made within time, whether an amendment has entered into force, or whether a withdrawal notification has been deposited. Depositary and registration rules make those questions legally manageable.

15.1 The depositary’s role

A depositary is the custodian of a treaty and the administrative channel through which treaty acts are received, recorded, and communicated. The depositary may be one or more States, an international organization, or the chief administrative officer of an organization, depending on the treaty. The Secretary-General of the United Nations frequently acts as a depositary for major multilateral treaties.

The depositary’s functions include keeping custody of the original text, receiving signatures, receiving instruments of ratification, acceptance, approval, or accession, receiving reservations and objections, examining whether instruments appear to be in due form, notifying parties of relevant acts, and performing other functions provided for in the treaty (United Nations, 1969). The role is administrative, but its legal consequences can be significant.

Neutrality is central to depositary practice. A depositary should not use its position to decide contested substantive issues beyond its functions. At the same time, it must maintain procedural clarity. If an instrument is defective, late, ambiguous, or accompanied by a controversial reservation, the way the depositary records and circulates it can affect how other parties respond.

Depositary practice also preserves continuity. Treaties often remain in force for decades. Governments change, ministries reorganise, States succeed or dissolve, and treaty regimes expand. A reliable depositary record allows legal advisers, courts, and other States to reconstruct the treaty position without relying on political memory or informal diplomatic claims.

15.2 Registration and publication

Treaty registration is linked to transparency. Article 102 of the United Nations Charter requires treaties and international agreements entered into by UN members to be registered with the Secretariat and published. A party to an unregistered treaty may not invoke that treaty before any organ of the United Nations (United Nations, 1945).

The rule reflects a historical rejection of secret treaty diplomacy. Before 1945, secret agreements were widely associated with distrust, alliance politics, and diplomatic instability. The Charter did not make every unregistered treaty void, but it created a public-law consequence: non-registration limits invocation before UN organs.

The Convention reinforces this system. Article 80 provides for the transmission of treaties to the UN Secretariat for registration or filing and recording, as the case may be, and for publication. The rule connects treaty law to institutional publicity. A treaty is not merely a private document between governments; it forms part of the public record of international legal relations.

Registration does not by itself prove that an instrument is valid, nor does publication cure defects in consent or content. It does, however, improve legal certainty. It helps other States, courts, researchers, international organizations, and domestic institutions identify the existence and terms of treaty commitments. In a legal order without a central legislature, that transparency has practical value.

16. Customary Law and Non-Parties

The Convention binds States that are parties to it as treaty law. Its influence, however, extends beyond formal membership. Many of its provisions are widely treated as codifications of customary international law, while others have helped shape later practice and legal argument. This is why courts and tribunals often rely on the Convention even where one or more States before them are not parties to it.

The point must be handled carefully. The Convention is not automatically binding on every State in every provision. Its authority outside party relations depends on the legal status of the specific rule being invoked. Some provisions reflect custom. Some may contribute to the development of customs. Others may operate mainly as treaty rules between parties or as persuasive evidence of a well-organised legal framework.

This distinction matters in litigation and legal advice. A lawyer cannot simply cite the Convention and assume universal application. The correct question is whether the relevant rule applies as treaty law, customary international law, general principle, or persuasive codification. The answer may differ across provisions.

16.1 Codified custom

Codification records customary law by giving written form to rules already supported by general practice accepted as law. The Convention contains several provisions commonly treated in this way. Pacta sunt servanda, the basic rule that internal law cannot ordinarily justify non-performance, and the general approach to treaty interpretation are strong examples.

Codified custom is valuable because custom can otherwise be difficult to identify. State practice may be dispersed across diplomatic correspondence, pleadings, national statements, treaty practice, and judicial decisions. A codification treaty can organise that material into a clearer rule. It does not create custom merely by being drafted, but it may provide strong evidence that a customary rule exists.

The International Court of Justice has treated some parts of the Convention as reflecting customary international law. Its use of Articles 31 and 32 on treaty interpretation is especially significant. The Court’s reliance on those provisions has helped make them the standard vocabulary of treaty interpretation in international adjudication (ICJ, Kasikili/Sedudu Island, 1999; ICJ, Whaling in the Antarctic, 2014).

16.2 Later influence on custom

A codification treaty may also influence the later development of customary international law. Once a text is widely accepted, cited by courts, used by governments, applied in negotiations, and relied on by international organizations, it may contribute to the formation or consolidation of custom. The Convention has had that effect in several areas because it provides a stable reference point for treaty practice.

This process is not automatic. A provision does not become customary simply because it appears in the Convention or because many States consider the Convention useful. Custom still requires practice and opinio juris. States must act, or accept the rule, because they regard it as legally required, not merely convenient or well drafted.

The caution is especially relevant for provisions that were innovative, controversial, or dependent on compromise at Vienna. Some parts of the Convention may have reflected progressive development rather than existing custom in 1969. Their later status must be assessed provision by provision, using ordinary methods for identifying customary international law.

16.3 Judicial reliance on the Convention

Courts and tribunals frequently cite the Convention because it gives structure to legal reasoning about treaties. Articles 31 and 32 are the clearest examples. They allow courts to explain interpretation through text, context, object, purpose, subsequent agreement, subsequent practice, and supplementary means. This gives judgments a disciplined method instead of leaving interpretation to impression or preference.

Judicial reliance also appears beyond interpretation. Courts may refer to the Convention when dealing with reservations, treaty succession questions by analogy, third-State effects, invalidity, termination, or the relationship between treaty law and internal law. The weight of such reliance depends on whether the provision reflects custom, binds the parties, or supplies a persuasive analytical framework.

The Convention’s authority in adjudication does not erase the need for careful reasoning. A tribunal should identify why a Convention rule applies in the dispute before it. If both parties are bound by the Convention, the legal basis is treaty law. If not, the tribunal must explain whether the provision reflects custom or assists interpretation in another legally acceptable way.

This is the Convention’s broader achievement. It has become more than a treaty among its parties without becoming a universal code detached from consent and custom. It operates as treaty law, as codified custom in important areas, and as a shared method for legal argument about treaties.

Also read

• The Sources Of International Law

• Customary International Law: How It Forms and Works

• The Principle of Good Faith in International Law

• Jus cogens and the hierarchy of international norms

• Articles on the Effects of Armed Conflicts on Treaties

• State Responsibility in International Law

• State Recognition And Succession In International Law

• Principle of Sovereign Equality of States

17. Practice, Limits, and Legal Order

The Convention is not only a source for courts. It is also a working instrument for legal advisers, diplomats, negotiators, ministries, treaty sections, international organizations, and counsel in disputes. Its rules appear in the background whenever a State decides whether to sign a treaty, enter a reservation, accept provisional application, object to another State’s reservation, invoke a termination clause, or argue about the meaning of a contested provision.

This practical authority should not lead to overstatement. The Convention is foundational, but it is not exhaustive. It governs treaties between States within its formal scope and provides general rules that often reflect customary international law. It does not answer every treaty-related question. Modern treaty practice also depends on specialised regimes, institutional procedures, domestic constitutional rules, customary law, and later developments such as the rules on the effects of armed conflict on treaties.

Its strongest contribution is discipline. The Convention gives treaty practice a common method. It does not make every issue easy, but it requires States and lawyers to ask the right legal questions: was there consent, what was the text, what is the context, what legal effect was intended, what procedure was followed, and what rule permits change, suspension, or termination?

17.1 Treaty drafting and legal advice

The Convention shapes treaty drafting before any dispute arises. A careful treaty lawyer drafts with the Convention in mind: how consent will be expressed, when the treaty will enter into force, whether reservations are allowed, whether provisional application is possible, how amendments will be adopted, how withdrawal will operate, which languages are authentic, who will act as depositary, and how disputes will be settled.

These choices are not cosmetic. A vague entry-into-force clause can delay legal certainty. A poorly drafted reservation clause can weaken the treaty regime. Silence on a provisional application may prevent urgent cooperation. Ambiguous authentic-language provisions can create interpretive disputes. A weak withdrawal clause may leave parties fighting over whether the exit is lawful.

The Convention also matters inside government. Legal advisers must compare international treaty requirements with domestic approval procedures. Article 27 prevents internal law from becoming an ordinary excuse for non-performance, while Article 46 leaves only a narrow route for manifest violations of internal rules of fundamental importance concerning treaty-making competence. This makes advanced legal review essential. A State should not discover after signature that domestic authority, parliamentary approval, territorial application, or implementation was misunderstood.

Good drafting reduces litigation risk. It also protects diplomatic credibility. States that negotiate with precision are less likely to rely later on strained interpretations, defective reservations, or contested withdrawal claims. Treaty law rewards clarity at the point of commitment.

17.2 Litigation and treaty argument

In litigation, the Convention supplies the grammar of treaty argument. Counsel before international courts and tribunals regularly rely on Articles 31 and 32 when arguing over the meaning of treaty terms. Those provisions are not decorative citations. They require argument through text, context, object, and purpose, subsequent agreement, subsequent practice, and supplementary means.

A strong treaty argument does not begin with a desired outcome. It begins with the legal materials that the Convention recognises. The ordinary meaning of the terms must be read in context. The preamble and annexes may matter. Later agreements or consistent practice by the parties may clarify the meaning. Preparatory work may confirm a reading or assist where the Article 31 analysis leaves ambiguity, obscurity, or an unreasonable result.

This method is visible across international adjudication. The International Court of Justice has repeatedly treated the Convention’s interpretive rules as reflecting customary international law, including in cases concerning territorial disputes, river regimes, maritime issues, and treaty-based obligations (ICJ, Kasikili/Sedudu Island, 1999; ICJ, Whaling in the Antarctic, 2014). Arbitral tribunals and specialised courts have used the same structure, even where the dispute arises in a technical regime.

The Convention also disciplines claims about invalidity, termination, suspension, and reservations. A party cannot simply assert that a treaty is unfair, obsolete, or politically unacceptable. It must identify a legal ground recognised by treaty law and satisfy the relevant conditions. That is why the Convention remains central in advisory work as well as litigation.

17.3 Issues outside the 1969 Convention

The 1969 Convention has limits. It applies directly to treaties between States. Treaties between States and international organizations, or between international organizations, fall outside its formal scope and are addressed by the 1986 Vienna Convention. That later instrument reflects many similar principles, but it has not acquired the same level of practical authority as the 1969 Convention.

The Convention also does not directly govern oral agreements. Article 3 preserves the possible legal force of international agreements not covered by the Convention, but such agreements are not regulated by the 1969 text in the same way as written treaties between States. This matters because international law may recognise legal commitments outside formal treaty documents, although proving their content and legal effect is usually harder.

Informal instruments present another limit. States frequently use memoranda of understanding, political declarations, joint statements, guidelines, action plans, and codes of conduct. Some are deliberately non-binding. Others may sit close to the boundary between law and diplomacy. The Convention helps identify when an instrument is a treaty, but it does not supply a complete legal theory of soft law or political commitment.

Specialised treaty regimes may also develop their own procedures and interpretive practices. Human rights bodies, environmental conferences of parties, trade institutions, investment tribunals, and disarmament regimes may apply treaty law through institutional rules that cannot be understood by reading the Convention alone. The Convention remains the general framework, but it operates alongside the internal law of those regimes.

The effects of armed conflict on treaties also require separate treatment. The Convention does not provide a full regime for that issue. Later work by the International Law Commission addressed how armed conflict may affect treaty obligations, but that subject has its own difficulties and should not be collapsed into the ordinary termination doctrine (International Law Commission, 2011). The point is not that the Convention is weak. It is that treaty law extends beyond one instrument.

Conclusion

The Vienna Convention on the Law of Treaties gives treaty relations their basic legal architecture. It explains how States create binding written commitments, how consent is expressed, how treaties enter into force, how reservations operate, how treaty terms are interpreted, how obligations apply in time and territory, and how treaties may be amended, suspended, terminated, or treated as invalid.

Its importance is doctrinal because it codifies and organises the central rules of treaty law. It is also practical because it guides treaty drafting, diplomatic correspondence, litigation strategy, judicial reasoning, and legal advice within governments and international organizations. A treaty lawyer who ignores the Convention is not merely overlooking a source; he is missing the framework through which treaty obligations are usually identified and argued.

The Convention’s authority does not mean that it answers every question. It does not fully govern treaties involving international organizations, oral agreements, informal political instruments, specialised treaty regimes, or the effects of armed conflict on treaties. Nor does every provision automatically bind non-parties as customary international law. Each rule must be located on its proper legal basis.

Its enduring value lies in the way it turns treaty practice into legal order. States may still disagree about meaning, performance, breach, reservations, invalidity, and termination. The Convention does not remove those disputes. It makes them legally intelligible. Treaty law is not a technical annex to public international law; it is one of the main ways international law becomes operational.

Recommended Book

For readers who want a broader foundation after studying the Vienna Convention on the Law of Treaties, International Law Book Review: Is Malcolm Shaw Worth Buying? is a useful next step. Shaw’s textbook helps place treaty law within the wider system of public international law, including sources, custom, state responsibility, international courts, and the legal framework in which treaty obligations operate.

References

Aust, A. (2005) Handbook of International Law. Cambridge: Cambridge University Press.

Charter of the United Nations (1945) 26 June 1945, entered into force 24 October 1945, 1 UNTS XVI. Available at: https://www.un.org/en/about-us/un-charter/full-text (Accessed: 12 June 2026).

Corten, O. and Klein, P. (eds.) (2011) The Vienna Conventions on the Law of Treaties: A Commentary. Oxford: Oxford University Press.

Human Rights Committee (1994) General Comment No. 24: Issues relating to reservations made upon ratification or accession to the Covenant or the Optional Protocols thereto, or in relation to declarations under article 41 of the Covenant, CCPR/C/21/Rev.1/Add.6, 4 November. Available at: https://digitallibrary.un.org/record/200963 (Accessed: 12 June 2026).

International Court of Justice (1951) Reservations to the Convention on the Prevention and Punishment of the Crime of Genocide, advisory opinion, 28 May, ICJ Reports 1951, p. 15. Available at: https://www.icj-cij.org/case/12 (Accessed: 12 June 2026).

International Court of Justice (1978) Aegean Sea Continental Shelf (Greece v Turkey), judgment, 19 December, ICJ Reports 1978, p. 3. Available at: https://www.icj-cij.org/case/62 (Accessed: 12 June 2026).

International Court of Justice (1994a) Maritime Delimitation and Territorial Questions between Qatar and Bahrain (Qatar v Bahrain), jurisdiction and admissibility, judgment, 1 July, ICJ Reports 1994, p. 112. Available at: https://www.icj-cij.org/case/87 (Accessed: 15 June 2026).

International Court of Justice (1994b) Territorial Dispute (Libyan Arab Jamahiriya/Chad), judgment, 3 February, ICJ Reports 1994, p. 6. Available at: https://www.icj-cij.org/case/83 (Accessed: 15 June 2026).

International Court of Justice (1997) Gabčíkovo-Nagymaros Project (Hungary/Slovakia), judgment, 25 September, ICJ Reports 1997, p. 7. Available at: https://www.icj-cij.org/case/92 (Accessed: 20 June 2026).

International Court of Justice (1999) Kasikili/Sedudu Island (Botswana/Namibia), judgment, 13 December, ICJ Reports 1999, p. 1045. Available at: https://www.icj-cij.org/case/98 (Accessed: 20 June 2026).

International Court of Justice (2014) Whaling in the Antarctic (Australia v Japan: New Zealand intervening), judgment, 31 March, ICJ Reports 2014, p. 226. Available at: https://www.icj-cij.org/case/148 (Accessed: 20 June 2026).

International Law Commission (1966) Draft Articles on the Law of Treaties with commentaries, Yearbook of the International Law Commission, 1966, vol. II, UN Doc. A/CN.4/SER.A/1966/Add.1. Available at: https://legal.un.org/ilc/texts/instruments/english/commentaries/1_1_1966.pdf (Accessed: 20 June 2026).

International Law Commission (2011) Draft articles on the effects of armed conflicts on treaties, with commentaries, Report of the International Law Commission on the work of its sixty-third session, UN Doc. A/66/10. Available at: https://legal.un.org/ilc/texts/instruments/english/commentaries/1_10_2011.pdf (Accessed: 20 June 2026).

International Law Commission (2022) Draft conclusions on identification and legal consequences of peremptory norms of general international law (jus cogens), with commentaries, Report of the International Law Commission on the work of its seventy-third session, UN Doc. A/77/10. Available at: https://legal.un.org/ilc/texts/instruments/english/commentaries/1_14_2022.pdf (Accessed: 21 June 2026).

Klabbers, J. (2024) International Law. 4th edn. Cambridge: Cambridge University Press.

Shaw, M.N. (2021) International Law. 9th edn. Cambridge: Cambridge University Press.

Vienna Convention on the Law of Treaties (1969) 23 May 1969, entered into force 27 January 1980, 1155 UNTS 331. Available at: https://legal.un.org/ilc/texts/instruments/english/conventions/1_1_1969.pdf (Accessed: 21 June 2026).

Vienna Convention on the Law of Treaties between States and International Organizations or between International Organizations (1986) 21 March 1986, not yet in force, UN Doc. A/CONF.129/15. Available at: https://legal.un.org/ilc/texts/instruments/english/conventions/1_2_1986.pdf (Accessed: 21 June 2026).