Declaration of War in International Law: Is It Still Legally Required?

Introduction

A declaration of war is no longer a general legal prerequisite for armed conflict between states. Under contemporary international law, military hostilities may constitute an international armed conflict even when neither government formally declares war. Nevertheless, Hague Convention III of 1907 remains in force and requires prior, explicit warning before hostilities begin between its contracting parties. The distinction is significant because the absence of a declaration does not determine whether military action is lawful or whether the laws of armed conflict apply.

Historically, a declaration of war marked a formal change in relations between states. It announced the existence of a state of war and could affect neutrality, diplomatic relations, commercial activity, and the treatment of enemy nationals. The United Nations Charter of 1945 transformed the legal position by restricting the threat or use of force, while the Geneva Conventions of 1949 ensured that humanitarian protections would apply regardless of whether governments acknowledged a state of war.

Contemporary confrontations involving Iran, Israel, and the United States illustrate the continuing distinction between formal declarations and armed hostilities. Governments may describe military attacks as declarations of war without issuing a formal declaration themselves. Determining the legal consequences requires consideration of Hague Convention III, the UN Charter, international humanitarian law, and the treaty obligations of the states concerned.

1. What Constitutes a Declaration of War?

A declaration of war is a formal unilateral act through which a state explicitly affirms the existence or commencement of a state of war with another state. Traditionally, it identifies the opposing state, communicates the declaring government's intention, and establishes the point at which the legal relationship between the parties changes. The declaration is issued by an authority competent under the state's constitutional system and communicated through an official channel (Brunner, 2018).

International law distinguishes such an act from ordinary political statements. An ultimatum may threaten military action unless specified demands are satisfied, while a declaration of war formally affirms a state of war. Military mobilization, the recall of ambassadors, the suspension of diplomatic relations, and statements condemning an adversary's aggression do not automatically constitute declarations. Their legal significance depends on their content and circumstances.

The distinction received consideration in the Eritrea–Ethiopia Claims Commission's Partial Award on Jus Ad Bellum of December 19, 2005. Eritrea argued that Ethiopia had declared war through a resolution adopted by its Council of Ministers and Parliament on May 13, 1998, following Eritrean military operations in disputed border territory. The resolution condemned the invasion and demanded the immediate withdrawal of Eritrean forces.

The Commission rejected Eritrea's characterization. In paragraph 17, it identified the essential characteristic of a declaration of war as an explicit affirmation that a state of war exists between belligerents. Ethiopia's resolution expressed its determination to resist Eritrean military action, but it did not constitute such a declaration. The Commission also noted the subsequent continuation of diplomatic and some economic relations, which appeared inconsistent with a formal declaration of war (Eritrea–Ethiopia Claims Commission, 2005, para. 17).

The award was an arbitral determination concerning the legality of resorting to force in a particular interstate dispute. It provides substantial authority for distinguishing a formal declaration from an announcement of defensive intentions, although it did not determine every question concerning the contemporary operation of Hague Convention III.

A government may consequently recognize an armed attack, announce defensive measures, or condemn military aggression without formally declaring war. Equally, the existence of hostilities does not require the government to adopt the terminology of war. The formal legal act and the factual circumstances of the conflict remain separate.

2. Hague Convention III of 1907 and the Duty to Warn

The Convention relative to the Opening of Hostilities, commonly known as Hague Convention III of 1907, was adopted on October 18, 1907, at the Second Hague Peace Conference. It entered into force on January 26, 1910. Its principal purpose was to establish an obligation of prior warning before the commencement of hostilities between contracting powers.

The Convention emerged partly from the Russo-Japanese War of 1904–1905, which began without a formal declaration of war. That conflict strengthened diplomatic support for regulating the opening of military hostilities. The Convention formed part of the wider Hague Conventions, through which states sought to codify rules concerning warfare and peaceful international relations.

Hague Convention III contains eight articles, but its principal substantive provisions are Articles 1, 2, and 3. They establish a prior-warning requirement, regulate notification to neutral powers, and define the states between which those obligations operate. The Convention was adopted when international law still permitted states considerably greater freedom to resort to war than exists under the UN Charter (Hague Convention III, 1907).

2.1 Article 1: Declaration of War or Conditional Ultimatum

Article 1 establishes that hostilities between contracting powers must not commence without previous and explicit warning. The warning may take the form of a declaration of war giving reasons or an ultimatum containing a conditional declaration of war. The provision creates a binding procedural obligation for states to which it applies, rather than merely recommending a diplomatic courtesy.

A declaration of war communicates that a state of war exists or is being initiated. A conditional ultimatum operates differently: it specifies demands and announces that war will follow if the opposing state does not comply. An ordinary diplomatic warning does not necessarily satisfy Article 1 unless its terms meet the Convention's requirements.

The Convention does not prescribe a minimum waiting period between the warning and the commencement of hostilities. Its requirement concerns the existence of previous and explicit notice. Nevertheless, an announcement made only after military operations have begun cannot retroactively satisfy the obligation to warn before initiating them (Brunner, 2018).

Article 1 also requires a declaration of war to provide reasons. This provision does not mean that the stated reasons establish the legality of military action. A government may assert that its conduct is defensive or necessary, but that assertion remains subject to the applicable rules governing the use of force. Compliance with Hague Convention III cannot independently justify an otherwise unlawful war.

2.2 Article 3: Treaty Participation and Customary Law

Article 3 limits the application of Article 1 to hostilities between two or more contracting powers. This provision is essential to the Convention's legal operation. A state that has ratified or acceded to the Convention is not automatically entitled to invoke its Article 1 obligations against every other state, irrespective of the latter's treaty status.

Signature and ratification must also be distinguished. Signature alone does not ordinarily establish consent to be bound where a treaty requires ratification. Hague Convention III expressly provides for ratification in Article 4 and accession in Article 5. Its obligations consequently depend on the applicable treaty relationships, rather than on historical participation in the 1907 conference.

The Convention's continuing force raises a separate question concerning customary international law. A treaty obligation may remain binding on its parties even when comparable conduct is not required of all states under customary law. Establishing a customary obligation would require sufficiently general state practice accompanied by acceptance of that practice as law.

The historical evidence does not clearly establish a universal customary obligation to declare war before hostilities. Formal declarations occurred during particular periods, but interstate conflicts also began without them. The practice became increasingly inconsistent during the twentieth century, especially after 1945 (Brunner, 2018).

Widespread noncompliance does not, by itself, terminate an existing treaty. Nor does the decline of declarations prove that an identical customary obligation has survived independently. The legally defensible position is narrower: Hague Convention III continues to impose its treaty-based requirements within their stated scope, while the existence of a corresponding universal customary rule remains insufficiently established.

The practical consequences of violating Article 1 are less clearly settled. The Convention does not establish a dedicated enforcement mechanism or specify a penalty for commencing hostilities without the required warning. A breach may engage the general law of state responsibility, but the existence and extent of any resulting remedy must be assessed separately. Noncompliance does not prevent the Geneva Conventions from applying or independently determine whether the resort to force violates the UN Charter.

2.3 Article 2: Notification of Neutral Powers

Article 2 establishes a distinct obligation concerning neutral powers. It requires the existence of a state of war to be communicated to them without delay and provides that the state of war does not take effect in relation to neutral powers until notification has been received. The provision expressly permits notification by telegraph, reflecting the communications technology available when the Convention was adopted.

A neutral state cannot rely on the absence of formal notification when it is clearly established that it actually knew of the state of war. The provision recognizes the legal significance of actual knowledge while retaining the obligation of notification. It was designed to protect states outside a conflict from uncertainty concerning their legal position.

Article 3 makes Article 2 binding between a belligerent contracting power and neutral powers that are also parties to the Convention. This differs from Article 1, which governs prior warning between opposing contracting powers. The absence of a treaty relationship between two belligerents does not necessarily dispose of every question concerning notification owed by a contracting belligerent to neutral contracting states.

The provision must nevertheless be interpreted within its historical and legal context. Article 2 refers expressly to the existence of a state of war, while contemporary international law frequently determines the existence of armed conflict through objective facts rather than formal recognition. Its application to particular undeclared hostilities requires attention to that distinction. The wider rights and duties of neutral states are governed by additional treaty and customary rules and do not depend entirely upon receipt of a notification under Hague Convention III.

3. The UN Charter and the Legality of Declaring War

The modern law governing interstate force developed through significant changes during the twentieth century. The 1928 General Treaty for Renunciation of War as an Instrument of National Policy, commonly known as the Kellogg–Briand Pact, sought to renounce war as an instrument of national policy. The United Nations Charter subsequently established a broader prohibition addressing both the threat and use of force.

Article 2(4) of the Charter requires UN members to refrain from the threat or use of force against the territorial integrity or political independence of another state, or in any other manner inconsistent with the purposes of the United Nations. This prohibition changed the legal significance of formally declaring war. A state cannot obtain a right to attack another state simply by announcing its intention to do so.

The distinction is central to the contemporary prohibition on the use of force under Article 2(4). A formal declaration may satisfy an applicable procedural obligation under Hague Convention III while the contemplated military action still violates the Charter. Conversely, the absence of a declaration does not automatically establish that military force is unlawful.

Article 51 preserves the inherent right of individual or collective self-defense if an armed attack occurs. The exercise of that right remains subject to the applicable conditions of international law, including necessity and proportionality. Chapter VII also empowers the Security Council to authorize enforcement measures, including military action under the relevant Charter provisions (United Nations, 1945, arts. 39–42, 51).

Neither source of legal authority depends on a formal declaration of war. Article 51 instead requires measures taken in self-defense to be reported immediately to the Security Council. Such notification informs the Council of measures undertaken on a claimed legal basis. It serves a different function from Hague Convention III's obligation to warn an opposing contracting state before commencing hostilities.

The relationship between prior warning and immediate self-defense requires particular care. Article 1 of Hague Convention III addresses the commencement of hostilities, whereas Article 51 concerns defensive action following an armed attack. A state already subjected to military hostilities is not necessarily in the same position as a state initiating them. The earlier treaty cannot be interpreted in isolation from the Charter's recognition of self-defense, but neither should an automatic exemption from Hague III be assumed without examining the circumstances and applicable obligations.

A declaration may itself raise a Charter question if it communicates a threat to use unlawful force. Its legal character depends on the proposed conduct and surrounding circumstances. The International Court of Justice has treated the customary prohibition on force and the rules governing self-defense as distinct, though closely related, legal principles in Military and Paramilitary Activities in and against Nicaragua (ICJ, 1986, paras. 188–201).

The law governing resort to force, traditionally described as jus ad bellum, must also be distinguished from jus in bello, which regulates conduct during armed conflict regardless of the legality of its commencement. These regimes address different legal questions. A lawful defensive operation remains subject to humanitarian restrictions, while compliance with those restrictions cannot legalize an otherwise unlawful resort to force.

4. Armed Conflict Without a Formal Declaration

The Geneva Conventions of August 12, 1949, changed the significance of formal declarations for the application of humanitarian law. Their common Article 2 provides that the Conventions apply to declared war and to other international armed conflicts between contracting states, even when a party does not recognize the existence of a state of war.

The provision prevents governments from avoiding humanitarian obligations through terminology. A state cannot escape the rules protecting wounded soldiers, prisoners of war, or civilians simply by describing military operations as limited strikes, security measures, or defensive operations. The legal classification depends primarily on the circumstances of the conflict (ICRC, 2016, commentary on common Article 2, paras. 201–218).

This approach is central to the Geneva Conventions and the protection of war victims. International humanitarian law regulates armed conflict independently of the legality of resorting to force. A state acting lawfully in self-defense remains bound by humanitarian rules, while an opposing state accused of aggression does not lose the protections that those rules establish.

4.1 When Hostilities Trigger International Humanitarian Law

An international armed conflict ordinarily arises when one or more states resort to armed force against another state. Under common Article 2, the threshold does not generally require prolonged fighting, substantial casualties, or a formal acknowledgment of war. The International Committee of the Red Cross considers the factual resort to armed force between states sufficient to trigger the applicable international armed conflict framework (ICRC, 2016, para. 218).

This interpretation reflects the approach adopted by the International Criminal Tribunal for the former Yugoslavia in the Tadić jurisdiction decision of October 2, 1995. The Appeals Chamber identified the resort to armed force between states as a basis for determining the existence of an armed conflict. The criterion directs attention to the conduct of the parties rather than to their diplomatic descriptions of that conduct (ICTY, 1995, para. 70).

The test differs from the criteria commonly associated with non-international armed conflicts, where the organization of the parties and the intensity of violence are central considerations. A limited interstate missile strike or military engagement may engage international humanitarian law even when the participating governments deny that they are at war. Questions involving genuinely accidental or exceptionally minor incidents require closer factual assessment.

Common Article 2 also addresses occupation. The Geneva Conventions apply to partial or total occupation of another contracting party's territory even when the occupation encounters no armed resistance. The absence of battlefield fighting does not eliminate the legal obligations arising from occupation once the relevant conditions are satisfied.

The consequences are substantial. Civilian protection, the treatment of captured combatants, medical care, and restrictions on methods of warfare do not wait for a diplomatic declaration. Humanitarian law operates because its legal conditions are satisfied, not because a government chooses to acknowledge them.

4.2 A Declared State of War Without Actual Fighting

Common Article 2 also preserves the independent legal significance of declared war. A formal declaration may bring the Geneva Conventions into operation even if the declaring state and its designated adversary do not engage in armed fighting. The treaty expressly includes declared war as an alternative basis for application.

The ICRC's 2016 Commentary to Geneva Convention I explains that declared war and international armed conflict are complementary concepts. The former relies upon a formal announcement, while the latter ordinarily rests on objective facts. A declaration can consequently trigger the application of the Geneva Conventions without accompanying military hostilities (ICRC, 2016, paras. 201–206).

Such consequences are particularly relevant to persons who find themselves under the authority of an opposing state. Geneva Convention IV establishes protections concerning civilians, including certain enemy nationals present in belligerent territory. However, its Article 4 defines the category of protected persons by reference to nationality, the authority exercising control, and specified exclusions. A national of an opposing state does not qualify automatically under every provision merely because war has been declared.

The ICRC's 2025 Commentary on Article 4 confirms that the Convention's personal scope depends on the circumstances of the individual and the applicable exceptions. Those distinctions remain relevant where a formal declaration precedes actual combat. The declaration may trigger the Convention's application, but the protections available to particular persons must still be determined under its substantive provisions (ICRC, 2025, commentary on Article 4).

The existence of humanitarian obligations does not determine whether the declaration or an accompanying threat of force complies with the UN Charter. The same formal act may produce consequences under the law of armed conflict while raising separate questions concerning the legality of threatened military action.

5. The Decline of Formal Declarations After 1945

Declarations of war were prominent in nineteenth- and early twentieth-century diplomacy, although states did not consistently declare war before commencing military operations. Their use became particularly visible during the First World War, when several states formally entered a state of war with opposing powers even without participating directly in major fighting.

Austria-Hungary declared war on Serbia on July 28, 1914, following its earlier ultimatum. Other declarations accompanied the widening conflict. Some states subsequently declared war against the Central Powers without conducting substantial military operations against them. The distinction between a formally established state of war and actual hostilities was already evident (Brunner, 2018).

The Second World War demonstrated the limits of formal declarations. Germany's invasion of Poland in September 1939 began without a prior declaration of war, while Britain and France subsequently issued declarations against Germany. Japan's attack on Pearl Harbor on December 7, 1941, preceded the United States' formal declaration of war on Japan the following day.

The United States also declared war against Germany and Italy on December 11, 1941. Its final formal declarations during the conflict were adopted on June 4, 1942, against Bulgaria, Hungary, and Romania. The United States Senate's historical records identify eleven congressional declarations of war across five conflicts, with no subsequent formal declaration after the Second World War (United States Senate, n.d.).

Since 1945, states have undertaken numerous military operations without employing traditional declarations of war. The Korean War, the 1982 Falklands War, and Russia's 2022 invasion of Ukraine illustrate the prevalence of armed hostilities undertaken or confronted without the old formal procedure. Governments have increasingly preferred expressions such as military operation, intervention, and armed conflict.

The Falklands War provides a revealing example. Following Argentina's invasion of the islands in April 1982, the United Kingdom conducted military operations without formally declaring war. The British government considered the question of a declaration, while contemporary public statements distinguished the military campaign from a formally declared war. Historical legal commentary has identified potential consequences for Argentine nationals and commercial relations as considerations relevant to such a decision (Brunner, 2018).

The abandonment of formal declarations as a common diplomatic practice does not resolve whether every relevant legal obligation has disappeared. State practice helps explain their diminished use, but the continuing force of Hague Convention III must be considered separately. Modern international law has changed the legal significance of the institution without eliminating the distinction between formal war and actual armed conflict.

6. The Legal Consequences of a State of War

A formal declaration can continue to produce legal consequences even though it is no longer generally necessary for armed hostilities to exist. Traditionally, the declaration marked a change from peaceful relations to a state of war, affecting the position of the belligerents and states remaining outside the conflict. Modern law assigns many comparable consequences to the factual existence of international armed conflict.

One significant area concerns neutrality in international law. Hague Convention V of 1907 regulates the rights and duties of neutral powers and persons in land warfare, including the inviolability of neutral territory. Hague Convention XIII addresses neutrality in naval war. A formal declaration may clarify when the parties consider a state of war to exist, but the operation of neutrality law cannot be determined solely by whether such a declaration has been issued.

The interaction between neutrality and the UN Charter also requires care. Traditional neutrality rules developed within a legal order that recognized a broader freedom to wage war. Under the Charter, those rules must operate alongside the prohibition on force and the collective security system. Notification under Hague Convention III may affect the legal position of neutral contracting states, but it is not the sole basis of their rights and duties.

Maritime warfare presents further consequences. Traditional prize law governs certain belligerent rights concerning the capture and adjudication of enemy and neutral vessels and cargo. A state of war could also affect commercial restrictions and the legal position of enemy property. The precise rules depend on the applicable treaty obligations, customary law, and domestic legislation rather than on an unrestricted power to seize property after declaring war.

Diplomatic relations may be interrupted during warfare, although severance is not an automatic consequence of every armed conflict. Governments may recall representatives, close missions, or communicate through third states. Even where diplomatic relations are severed, Article 45 of the Vienna Convention on Diplomatic Relations requires the receiving state to respect and protect the premises of the mission, together with its property and archives. A declaration of war does not extinguish these obligations (Vienna Convention on Diplomatic Relations, 1961, art. 45).

The effect of armed conflict on treaties also requires a separate legal assessment. In its 2011 Draft Articles on the Effects of Armed Conflicts on Treaties, the International Law Commission rejected the proposition that the existence of armed conflict automatically terminates or suspends treaties between the states involved. Article 3 states the general principle, while subsequent provisions identify factors relevant to the continued operation of particular agreements (ILC, 2011, arts. 3–7).

Those articles were adopted as a draft text, not as a universally binding convention. The Commission did not establish an unconditional presumption that all treaties continue unchanged during hostilities. Its approach instead rejects automatic termination or suspension and calls for examination of the particular treaty and surrounding circumstances. The traditional assumption that declaring war automatically terminates every agreement between belligerents is inconsistent with that more differentiated position.

Domestic constitutional law creates another distinction. In the United States, Article I, Section 8 of the Constitution assigns Congress the power to declare war, while Article II identifies the President as Commander in Chief. Congress may authorize military operations without adopting a formal declaration. The constitutional powers of Congress and the President, especially regarding military operations undertaken without specific congressional authorization, remain the subject of legal disagreement.

The War Powers Resolution of 1973 establishes statutory reporting and procedural requirements concerning the introduction of American forces into hostilities. It does not convert every military engagement into a formally declared war. Nor does compliance with domestic authorization procedures independently establish compliance with international law. A military operation can raise distinct questions under constitutional law, the UN Charter, and international humanitarian law (Congressional Research Service, 2014).

7. Iran and the Legal Meaning of Declaring War

Military confrontations involving Iran illustrate how easily political descriptions of warfare can be confused with formal declarations. Governments frequently characterize attacks against their territory as acts of war or declarations of war. Such statements may express allegations of aggression, announce defensive intentions, or seek international support, but their legal character depends on their content and the circumstances in which they are made.

The distinction became particularly visible during the Israeli military strikes against Iran in June 2025 and the subsequent United States–Israeli military operations beginning in February 2026. Both episodes involved interstate force and formal communications to the United Nations. Their legal significance cannot be determined simply by asking whether a government used the language of declaring war.

The application of Hague Convention III adds another consideration. Israel, Iran, and the United States do not possess identical treaty relationships under the Convention. Its rules governing warning and notification must be examined in light of the relevant participation records, while the UN Charter and Geneva Conventions operate according to their own conditions.

7.1 Iran's Characterization of the June 2025 Attacks

On June 13, 2025, Israel launched coordinated military strikes against targets across Iran, including military installations and nuclear facilities. At an emergency meeting of the UN Security Council, Under-Secretary-General Rosemary DiCarlo reported attacks against several locations, including residential areas in Tehran. Israel characterized its operation as preventive action against threats associated with Iran's nuclear and military capabilities (UN Security Council, 2025, S/PV.9936).

Iran rejected Israel's justification and characterized the strikes as unlawful aggression. Its Permanent Representative, Amir Saeid Iravani, stated during the Security Council meeting that the actions amounted to a declaration of war. The Iranian position was associated with an assertion of the right of self-defense under Article 51 of the UN Charter.

That characterization did not establish that Israel had issued a formal declaration of war. Iran was describing the legal and political significance it attributed to Israel's military conduct. The distinction resembles the one examined by the Eritrea–Ethiopia Claims Commission: a statement concerning the gravity of military action is not necessarily an explicit formal affirmation of a state of war by the state alleged to have initiated it.

Iran's communication to the Security Council also served a different function from Hague Convention III's prior-warning procedure. The June 13 letter transmitted by Iran's Permanent Representative, registered as S/2025/379, alleged violations of sovereignty and the prohibition on force. It requested Security Council action rather than constituting an Iranian declaration initiating war (United Nations, 2025, S/2025/379).

The existence of an international armed conflict between Iran and Israel depended on the factual use of armed force between the states, not on the language adopted by either government. Whether particular operations complied with the UN Charter remained a separate question involving the legal grounds asserted and the relevant factual circumstances.

7.2 The 2026 Conflict and Hague III's Treaty Limits

On February 28, 2026, the United States and Israel began military operations against Iran. Iran subsequently carried out strikes against Israeli targets and American military facilities in the region. The UN Security Council convened an emergency meeting that day, reflecting the seriousness of the military escalation and competing legal positions advanced by the states involved.

In a letter dated March 10, 2026, transmitted as UN document S/2026/161, the United States informed the Security Council that it had commenced combat operations against Iran on February 28. The American government characterized its actions as measures of self-defense under Article 51 of the Charter. That communication presented the United States' asserted legal justification; it did not constitute an independent determination that the conditions for lawful self-defense had been satisfied (United Nations, 2026, S/2026/161).

Iran advanced its own legal position. In a letter dated March 3, 2026, registered as S/2026/121, its Permanent Representative informed the United Nations that Iran had undertaken defensive operations against American and Israeli military facilities in exercise of its claimed right of self-defense. These communications demonstrate that the parties invoked established Charter terminology while contesting the legality of military conduct (United Nations, 2026, S/2026/121).

An Article 51 notification is not equivalent to a formal declaration of war. It presents measures as an exercise of the right of self-defense and informs the Security Council of the state's legal position. Filing such a communication does not itself establish that the conditions for lawful self-defense have been satisfied. The larger controversy concerning the USA–Iran war and the use of force under international law involves separate questions of armed attack, necessity, proportionality, and the Charter's prohibition on force.

The hostilities continued beyond the opening phase. The United States and Iran agreed to a ceasefire in April 2026 and signed a memorandum of understanding in June intended to facilitate a negotiated settlement. The agreement subsequently collapsed, and limited military exchanges resumed. The Congressional Research Service reported continued intermittent hostilities in September, while the Council on Foreign Relations described the conflict as unresolved in its October 2026 assessment (Congressional Research Service, 2026; Council on Foreign Relations, 2026).

These developments also demonstrate why the existence of a state of war or armed conflict cannot be determined solely through diplomatic terminology. A ceasefire may suspend fighting without conclusively terminating an international armed conflict, particularly where the parties resume hostilities. The legal significance of a ceasefire or political agreement depends on its terms, implementation, and the circumstances of the conflict.

The treaty position under Hague Convention III is more specific. According to the Netherlands Treaty Database, Iran signed the Convention on October 18, 1907, but no Iranian ratification or entry into force is recorded. The United States ratified the Convention, while Israel is not listed among its parties. Article 1's treaty requirement consequently cannot automatically be imposed on the United States–Iran or Israel–Iran confrontations as though the opposing states were all contracting powers (Netherlands Treaty Database, n.d.).

This conclusion concerns the Article 1 obligation between belligerents. Article 2 presents a distinct question because it regulates notification between a belligerent contracting power and neutral contracting powers. The United States' treaty status may accordingly remain relevant to its relations with neutral states, even though Iran's non-party status prevents the straightforward application of Article 1 between those adversaries. The precise application of Article 2 depends on the circumstances and the provision's treatment of a state of war.

The absence of a formal declaration also does not prevent the Geneva Conventions from applying. Interstate military attacks can establish an international armed conflict regardless of whether a state declares war. Nor does the absence of a declaration determine whether the initiating force was justified. The legal classification of the 2026 hostilities and the legality of the operations must be assessed under the appropriate bodies of international law, not through the terminology used in diplomatic exchanges.

Conclusion

International law no longer makes a formal declaration of war a general condition for the existence of interstate armed conflict. The Geneva Conventions apply to declared war and to other international armed conflicts, ensuring that humanitarian protections are not dependent on the formal acknowledgment of hostilities. The UN Charter separately governs the legality of threatened or actual military force, which cannot be established simply by issuing a declaration.

Hague Convention III of 1907 nonetheless retains legal significance. Its Article 1 requires previous and explicit warning between contracting powers, while Article 2 establishes separate notification obligations concerning neutral contracting states. These obligations must be interpreted within their treaty limits and distinguished from any alleged universal customary requirement. A formal declaration may also produce consequences relating to neutrality, humanitarian protection, diplomatic relations, and domestic law.

The confrontations involving Iran demonstrate the continuing importance of legal classification. A government may describe an attack as a declaration of war without the opposing state having issued one, and military hostilities may create an international armed conflict without either party formally declaring war. The decisive legal questions concern the applicable treaty obligations, the facts establishing armed conflict, and the independent justification for using force. Formal declarations have become uncommon, but their declining use has not made their legal consequences or the treaty rules governing them entirely obsolete.

References

Brunner, M. (2018) ‘Declaration of War’, Max Planck Encyclopedia of Public International Law. Oxford: Oxford University Press.

Charter of the United Nations (1945) Signed at San Francisco, 26 June 1945; entered into force 24 October 1945. Available at: https://www.un.org/en/about-us/un-charter/full-text (Accessed: 15 October 2026).

Congressional Research Service (2014) Declarations of War and Authorizations for the Use of Military Force: Historical Background and Legal Implications. Report RL31133, 18 April. Washington, DC: Congressional Research Service. Available at: https://www.congress.gov/crs-product/RL31133 (Accessed: 15 October 2026).

Congressional Research Service (2026) U.S. Conflict with Iran. Report R48887, updated 18 September. Washington, DC: Congressional Research Service. Available at: https://www.congress.gov/crs_external_products/R/PDF/R48887/R48887.3.pdf (Accessed: 15 October 2026).

Constitution of the United States (1787) Signed 17 September 1787; federal government commenced operations under the Constitution on 4 March 1789. Available at: https://www.archives.gov/founding-docs/constitution-transcript (Accessed: 15 October 2026).

Council on Foreign Relations (2026) Conflict With Iran, Global Conflict Tracker [online]. Available at: https://www.cfr.org/global-conflict-tracker/conflict/confrontation-between-united-states-and---iran (Accessed: 15 October 2026).

Eritrea–Ethiopia Claims Commission (2005) Partial Award: Jus Ad Bellum—Ethiopia’s Claims 1–8, 19 December. Reports of International Arbitral Awards, Vol. XXVI, pp. 457–469. Available at: https://legal.un.org/riaa/cases/vol_XXVI/457-469.pdf (Accessed: 18 October 2026).

General Treaty for Renunciation of War as an Instrument of National Policy (1928) Signed at Paris, 27 August 1928; entered into force 24 July 1929. Available at: https://avalon.law.yale.edu/20th_century/kbpact.asp (Accessed: 18 October 2026).

Geneva Convention (I) for the Amelioration of the Condition of the Wounded and Sick in Armed Forces in the Field (1949) Adopted 12 August 1949; entered into force 21 October 1950. 75 UNTS 31. Available at: https://ihl-databases.icrc.org/en/ihl-treaties/gci-1949 (Accessed: 18 October 2026).

Geneva Convention (II) for the Amelioration of the Condition of Wounded, Sick and Shipwrecked Members of Armed Forces at Sea (1949) Adopted 12 August 1949; entered into force 21 October 1950. 75 UNTS 85. Available at: https://ihl-databases.icrc.org/en/ihl-treaties/gcii-1949 (Accessed: 18 October 2026).

Geneva Convention (III) relative to the Treatment of Prisoners of War (1949) Adopted 12 August 1949; entered into force 21 October 1950. 75 UNTS 135. Available at: https://ihl-databases.icrc.org/en/ihl-treaties/gciii-1949 (Accessed: 18 October 2026).

Geneva Convention (IV) relative to the Protection of Civilian Persons in Time of War (1949) Adopted 12 August 1949; entered into force 21 October 1950. 75 UNTS 287. Available at: https://ihl-databases.icrc.org/en/ihl-treaties/gciv-1949 (Accessed: 1 November 2026).

Hague Convention (III) relative to the Opening of Hostilities (1907) Adopted at The Hague, 18 October 1907; entered into force 26 January 1910. 205 CTS 263. Available at: https://ihl-databases.icrc.org/en/ihl-treaties/hague-conv-iii-1907 (Accessed: 1 November 2026).

Hague Convention (V) respecting the Rights and Duties of Neutral Powers and Persons in Case of War on Land (1907) Adopted at The Hague, 18 October 1907; entered into force 26 January 1910. Available at: https://ihl-databases.icrc.org/assets/treaties/200-IHL-20-EN.pdf (Accessed: 1 November 2026).

Hague Convention (XIII) concerning the Rights and Duties of Neutral Powers in Naval War (1907) Adopted at The Hague, 18 October 1907; entered into force 26 January 1910. Available at: https://ihl-databases.icrc.org/assets/treaties/240-IHL-28-EN.pdf (Accessed: 1 November 2026).

International Committee of the Red Cross (2016) Commentary on the First Geneva Convention: Article 2—Application of the Convention [online]. Geneva: ICRC. Available at: https://ihl-databases.icrc.org/en/ihl-treaties/gci-1949/article-2/commentary/2016 (Accessed: 1 November 2026).

International Committee of the Red Cross (2025) Commentary on the Fourth Geneva Convention: Article 4—Definition of Protected Persons [online]. Geneva: ICRC. Available at: https://ihl-databases.icrc.org/en/ihl-treaties/gciv-1949/article-4/commentary/2025 (Accessed: 1 November 2026).

International Court of Justice (1986) Military and Paramilitary Activities in and against Nicaragua (Nicaragua v. United States of America), Merits, Judgment of 27 June 1986. ICJ Reports 1986, p. 14. Available at: https://api.icj-cij.org/sites/default/files/case-related/70/070-19860627-JUD-01-00-EN.pdf (Accessed: 1 November 2026).

International Criminal Tribunal for the Former Yugoslavia (1995) Prosecutor v. Duško Tadić, Decision on the Defence Motion for Interlocutory Appeal on Jurisdiction, Case No. IT-94-1-AR72, Appeals Chamber, 2 October 1995. Available at: https://www.icty.org/x/cases/tadic/acdec/en/51002.htm (Accessed: 1 November 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, Chapter VI. Available at: https://legal.un.org/ilc/reports/2011/ (Accessed: 1 November 2026).

Netherlands Treaty Database (n.d.) Convention relative to the Opening of Hostilities, Treaty No. 003318 [online]. Government of the Netherlands. Available at: https://treatydatabase.overheid.nl/en/Treaty/Details/003318 (Accessed: 1 November 2026).

United Nations Security Council (2025a) Letter dated 13 June 2025 from the Permanent Representative of the Islamic Republic of Iran to the United Nations addressed to the Secretary-General and the President of the Security Council. UN Doc. S/2025/379, 13 June. Available at: https://digitallibrary.un.org/record/4083834 (Accessed: 1 November 2026).

United Nations Security Council (2025b) Threats to International Peace and Security. Official Records, 9936th Meeting, 13 June 2025. UN Doc. S/PV.9936. Available at: https://digitallibrary.un.org/record/4084351/files/S_PV.9936-EN.pdf (Accessed: 1 November 2026).

United Nations Security Council (2026a) Letter dated 3 March 2026 from the Permanent Representative of the Islamic Republic of Iran to the United Nations addressed to the Secretary-General. UN Doc. S/2026/121, 3 March. Available at: https://digitallibrary.un.org/record/4104928 (Accessed: 1 November 2026).

United Nations Security Council (2026b) Letter dated 10 March 2026 from the Permanent Representative of the United States of America to the United Nations addressed to the President of the Security Council. UN Doc. S/2026/161, 10 March. Available at: https://digitallibrary.un.org/record/4105908 (Accessed: 1 November 2026).

United States Senate (n.d.) About Declarations of War by Congress [online]. Senate Historical Office. Available at: https://www.senate.gov/about/powers-procedures/declarations-of-war.htm (Accessed: 1 November 2026).

Vienna Convention on Diplomatic Relations (1961) Adopted at Vienna, 18 April 1961; entered into force 24 April 1964. 500 UNTS 95. Available at: https://legal.un.org/ilc/texts/instruments/english/conventions/9_1_1961.pdf (Accessed: 1 November 2026).

War Powers Resolution (1973) Public Law 93-148, 7 November 1973. 87 Stat. 555. Available at: https://www.govinfo.gov/content/pkg/STATUTE-87/pdf/STATUTE-87-Pg555.pdf (Accessed: 1 November 2026).