Kellogg–Briand Pact 1928

1. Introduction

The Kellogg–Briand Pact marked a decisive rupture in the legal treatment of war in international law. Adopted in 1928, the treaty represented the first multilateral and legally binding commitment by states to renounce war as a legitimate instrument of national policy. Its objective was not to regulate how wars were fought, but to challenge the assumption—deeply embedded in classical international law—that sovereign states were legally entitled to wage war to pursue political or territorial aims. By doing so, the pact sought to redefine the boundary between lawful and unlawful state behaviour at the most fundamental level.

Before 1928, international law largely accepted war as a lawful means of enforcing rights, subject mainly to procedural requirements and rules governing conduct during hostilities. The devastation of the First World War fundamentally destabilised that framework. Mass civilian casualties, industrialised violence, and the collapse of European security exposed the inadequacy of a legal order that treated war as a neutral policy choice. The treaty emerged from this crisis as an attempt to convert moral revulsion into legal obligation, transforming anti-war sentiment into a formal rule of international law (Oppenheim, 1952; Hathaway and Shapiro, 2017).

The legal ambition of the pact exceeded its institutional design. It contains no enforcement mechanisms, no sanctions for breach, and no permanent body tasked with supervision. These limitations have frequently led commentators to dismiss it as symbolic or ineffective. Such critiques overlook how international law develops in practice. Legal significance does not depend solely on coercive enforcement but also on the capacity of norms to reshape expectations, constrain justification, and influence later legal instruments. The pact accomplished this by delegitimising aggressive war as a lawful policy option, even when states continued to violate the rule in practice (Brownlie, 1963).

Its influence became visible in subsequent legal developments. The prohibition of the use of force in the United Nations Charter did not arise in a vacuum but was built upon the conceptual shift initiated in 1928. Similarly, the prosecution of crimes against peace at the Nuremberg Tribunal relied on the premise that aggressive war was already prohibited under international law, a premise grounded directly in the pact’s normative framework (Cassese, 2008). These developments demonstrate that the treaty’s impact must be assessed through its doctrinal legacy rather than its immediate political outcomes.

This article analyses the pact as a legal turning point in the evolution of jus ad bellum. It explains the historical context in which the treaty was negotiated, examines its core provisions and interpretive challenges, and evaluates its role in shaping modern international law on the use of force. The analysis is grounded in treaty text, state practice, jurisprudence, and scholarly commentary to show how a brief and structurally weak agreement produced durable legal consequences that continue to inform contemporary debates on aggression, self-defence, and international responsibility.

2. Pre-1928 Legal and Political Setting

2.1 The Pre-Pact Legal Status of War

Before 1928, international law did not prohibit war as such. Classical doctrine treated war as a lawful instrument of state sovereignty, provided certain formal conditions were met. The distinction between war and peace was legally meaningful: war was a recognised legal status that triggered specific rights and duties, including neutrality obligations, belligerent rights, and the application of the laws of war. States could declare war to pursue political, territorial, or economic objectives without violating international law, so long as they complied with procedural norms and humanitarian restraints governing conduct during hostilities (Oppenheim, 1952).

This framework reflected a conception of sovereignty rooted in voluntarism and decentralisation. Without a central enforcement authority, international law relied on self-help, and war functioned as an ultimate means of enforcing claims. Legal scholarship of the nineteenth and early twentieth centuries focused on regulating warfare rather than questioning its legality, reinforcing the idea that war was a legitimate, if regrettable, policy option (Grewe, 2000).

The First World War profoundly undermined this model. The scale and intensity of the conflict exposed the human and economic costs of treating war as a normal instrument of policy. Industrialised violence, mass civilian casualties, and the collapse of diplomatic restraint generated widespread scepticism toward a legal order that appeared incapable of preventing catastrophe. The war also demonstrated that procedural restraints and humanitarian rules could not compensate for the absence of substantive limits on recourse to force. As a result, legal and political discourse increasingly shifted toward the idea that war itself should be delegitimised, giving rise to movements advocating its legal prohibition rather than mere regulation (Hathaway and Shapiro, 2017).

2.2 Interwar Peace Architecture as the Pact’s Backdrop

The interwar period saw multiple attempts to stabilise international relations through law and institutions. The League of Nations embodied this effort by promoting collective security, compulsory dispute settlement, and gradual disarmament. Its Covenant imposed procedural obligations on states to submit disputes to arbitration or judicial settlement and to observe cooling-off periods before resorting to force. These mechanisms, however, stopped short of an outright ban on war. States retained the legal possibility of using force after fulfilling procedural requirements, preserving the underlying permissibility of war in international law (Henkin, 1979).

Disarmament initiatives faced similar limitations. Conferences and treaties aimed at reducing armaments struggled against mistrust, asymmetric security concerns, and the absence of effective verification. Many states viewed disarmament as desirable but unrealistic without stronger legal guarantees against aggression. Within this context, a short and declaratory treaty renouncing war appealed precisely because it avoided complex institutional commitments. Its moral clarity and legal simplicity allowed states to express collective condemnation of war without surrendering control to supranational bodies or enforcement regimes (Simpson, 2004).

The appeal of such an instrument also lay in its universality. Unlike League mechanisms, which were tied to membership and institutional procedures, a general treaty renouncing war could attract participation from states with divergent political systems and strategic interests. This universality was seen as essential for reshaping expectations about lawful state conduct, even in the absence of immediate enforcement.

2.3 United States and French Strategic Motives

The origins of the pact are inseparable from the strategic calculations of France and the United States. France emerged from the First World War deeply concerned about its security, particularly in relation to Germany. Legal guarantees against aggression were viewed as a means of reinforcing a fragile post-war settlement and compensating for demographic and strategic vulnerabilities. French policymakers saw legal commitments renouncing war as a tool to constrain potential aggressors and strengthen diplomatic leverage (Marks, 1976).

The United States approached the initiative from a different angle. Strong domestic resistance to military alliances and permanent security commitments shaped American foreign policy throughout the interwar period. Political leaders sought ways to promote peace without entangling the country in collective defence obligations. A multilateral treaty renouncing war offered an attractive solution: it aligned with public anti-war sentiment while avoiding institutional or military commitments that might require congressional approval or future intervention (Schlesinger, 1957).

Multilateralisation became central to reconciling these interests. By expanding the agreement beyond a bilateral Franco-American arrangement, the United States avoided the appearance of a special security guarantee to France, while France gained broader normative support for the principle of outlawing war. This convergence of motives explains both the rapid expansion of the treaty’s membership and its deliberately restrained legal design, which prioritised widespread acceptance over detailed enforcement.

3. Political and Diplomatic Origins of the Pact

3.1 French Security Concerns and Briand’s Initiative

France’s role in initiating the pact was shaped by acute post-war insecurity. Despite emerging on the victorious side of the First World War, France faced a structurally fragile strategic position. Demographic losses, economic strain, and geographic exposure along the Rhine contributed to a persistent fear of German recovery and rearmament. Military guarantees obtained through the Versailles system appeared increasingly unreliable during the 1920s, particularly as enforcement mechanisms weakened and international attention shifted toward reconciliation and economic stability (Marks, 1976).

Aristide Briand, serving as French Foreign Minister, sought to supplement traditional security arrangements with legal commitments. His approach reflected a broader French strategy of embedding security concerns within international legal frameworks, thereby internationalising potential threats. Briand’s initial proposal in 1927 envisioned a bilateral agreement with the United States, renouncing war between the two states. The logic was both symbolic and strategic: an explicit American commitment to peaceful relations would strengthen France’s diplomatic position and contribute to the wider delegitimation of aggressive war (Boemeke, Feldman and Glaser, 1998).

Briand did not view law as a substitute for power but as a constraint on its use. The proposal aimed to create a normative barrier against aggression by transforming peace from a political aspiration into a legal obligation. For France, the appeal lay in using legal commitments to restrain future German policy indirectly, without provoking immediate confrontation or relying solely on military deterrence.

3.2 The United States’ Role and Kellogg’s Conditions

The American response to Briand’s proposal was shaped by domestic political realities. Strong isolationist sentiment, reinforced by the perceived costs of European entanglement during the First World War, constrained U.S. foreign policy throughout the 1920s. The Senate’s rejection of the Treaty of Versailles and refusal to join the League of Nations underscored deep resistance to binding security commitments. Any agreement that resembled a bilateral alliance risked political failure at home (Schlesinger, 1957).

Secretary of State Frank B. Kellogg, therefore, conditioned American participation on a fundamental redesign of the proposal. The United States insisted that any renunciation of war take a multilateral form and apply equally to all participating states. This approach avoided the appearance of a special relationship with France and reduced the risk that the agreement would be interpreted as a security guarantee or mutual defence pact. Multilateralisation allowed American policymakers to frame the initiative as a general moral and legal statement rather than a strategic commitment (Hathaway and Shapiro, 2017).

Domestic considerations also influenced the treaty’s legal form. The administration sought a simple, declaratory text that would be politically palatable and unlikely to trigger constitutional controversy. Detailed enforcement provisions, arbitration mechanisms, or collective security clauses were deliberately excluded. The resulting design reflected a balance between international ambition and domestic constraint, prioritising ratifiability over institutional depth.

3.3 From Bilateral Proposal to Global Treaty

The transformation of Briand’s bilateral initiative into a near-universal treaty was the product of diplomatic compromise. France accepted multilateralisation in exchange for broad endorsement of the principle it sought to advance: the renunciation of war as national policy. Other states were invited to accede on equal terms, reinforcing the perception that the pact expressed a collective normative commitment rather than a regional or strategic arrangement (Simpson, 2004).

This expansion altered the treaty’s function. What began as a targeted security gesture became a general statement about lawful state behaviour. The inclusion of diverse states with varying interests required deliberate ambiguity in drafting, particularly on issues such as self-defence and enforcement. That ambiguity facilitated widespread accession while leaving interpretive space for states to reconcile the pact with existing security doctrines.

By 1929, the agreement had attracted participation from most major powers and many smaller states, giving it a level of acceptance unmatched by earlier peace initiatives. The diplomatic success of this transformation lay not in creating an enforceable security system but in establishing a shared legal premise: that aggressive war no longer belonged within the category of legitimate state action. This premise would later prove decisive in the evolution of international law governing the use of force.

4. Structure and Legal Nature of the Kellogg–Briand Pact

4.1 Form, Length, and Drafting Philosophy

The pact is notable for its extreme brevity and restrained structure. Comprising a short preamble and only three substantive articles, it departs sharply from earlier efforts to regulate war through complex procedural frameworks. This drafting choice was deliberate. The negotiators sought a text that expressed a clear legal principle while minimising technical detail that could provoke political resistance or impede ratification. Simplicity functioned as a tool of universality rather than as a sign of legal weakness.

The declaratory character of the instrument reflects a conscious trade-off between legal precision and political acceptability. A more detailed treaty, defining aggression, enumerating exceptions, or establishing enforcement mechanisms, would have required agreement on highly contested issues and likely fractured consensus. By contrast, a concise formulation allowed states with divergent strategic interests to commit to a shared normative position without resolving every downstream implication at the moment of signature (Simpson, 2004).

This approach also aligned with the intellectual climate of the interwar peace movement. Advocates of the “outlawry of war” believed that legal transformation could begin with categorical condemnation, leaving interpretation and application to evolve through state practice and legal argument. The pact, therefore, prioritised clarity of purpose over doctrinal completeness. Its language was intended to be accessible, universal, and symbolically powerful, reinforcing the idea that war as state policy had become incompatible with the emerging international legal order (Hathaway and Shapiro, 2017).

The cost of this strategy was ambiguity. Key terms such as “war” and “national policy” were left undefined, and no guidance was provided on permissible uses of force short of war. These omissions created space for divergent interpretations and later circumvention. At the same time, they enabled widespread accession and ensured that the core principle could take root across different legal systems. The pact’s form thus reflects a calculated decision to privilege normative reach over immediate operability.

4.2 Binding Force and Treaty Classification

Despite its brevity and lack of enforcement provisions, the pact constitutes a legally binding treaty under international law. It was concluded through formal consent, ratified by participating states, and intended to create obligations governed by international law. The absence of sanctions or institutional oversight does not negate its binding character. International law has long recognised that legal obligations may exist independently of enforcement mechanisms, particularly in a decentralised legal system (Brownlie, 1963).

The pact imposes two core obligations: the renunciation of war as an instrument of national policy and the commitment to settle disputes exclusively by peaceful means. These obligations are expressed in mandatory language and framed as undertakings owed by each state to all others. Their collective nature reinforces the pact’s legal status, situating it within the category of multilateral law-making treaties rather than political declarations (McNair, 1961).

The relationship between moral condemnation and legal obligation is central to understanding the pact’s nature. While the treaty’s language is infused with ethical judgment, condemning war as contrary to the interests of humanity, this moral framing does not diminish its legal effect. On the contrary, the fusion of moral purpose with legal form strengthened the normative force of the obligation by signalling a shift in collective legal conscience. The pact transformed what had previously been a matter of political discretion into a subject of legal evaluation and potential responsibility (Cassese, 2008).

This dual character explains both the pact’s immediate limitations and its long-term influence. It lacked the means to prevent violations in the short term, yet it altered the criteria by which state conduct was judged. Once war was formally renounced as policy, its justification required legal argument rather than mere assertion of sovereign right. This shift laid the conceptual groundwork for later developments, including the prohibition of the use of force in the United Nations Charter and the emergence of individual criminal responsibility for aggression.

5. Article I Explained: Renunciation of War

5.1 “Condemnation” versus Prohibition

Article I opens with the parties’ agreement to “condemn recourse to war for the solution of international controversies.” The choice of the verb “condemn” was legally significant. In classical treaty practice, condemnation signals collective disapproval elevated into a normative judgment shared by all parties. It does not merely express political regret; it establishes a common evaluative standard against which conduct is judged. In this context, condemnation functions as the normative foundation for the operative commitment that follows, rather than as a substitute for legal obligation.

The provision then moves beyond moral language by coupling condemnation with renunciation. The structure of the article links the ethical assessment of war to a binding undertaking not to employ it as a policy tool. This sequencing reflects a deliberate drafting strategy. The condemnation clause frames war as incompatible with the values and purposes of the international community, while the renunciation clause translates that assessment into a concrete obligation (McNair, 1961).

Legal commentators at the time recognised that the pact did not merely restate existing principles but introduced a new standard of legality. War ceased to be a neutral legal condition and became an act requiring justification against a background of prohibition. The absence of explicit sanctions did not deprive the condemnation of legal relevance; rather, it established a benchmark for responsibility and breach that could operate through diplomatic, reputational, and later judicial mechanisms (Brownlie, 1963).

5.2 “War as an Instrument of National Policy”

The core operative phrase of Article I is the renunciation of war “as an instrument of national policy.” This formulation narrows the scope of the prohibition to wars undertaken to advance state interests through deliberate policy choice. It targets aggressive, expansionist, or coercive wars aimed at altering the status quo, acquiring territory, enforcing claims, or reshaping political arrangements by force.

The focus on policy-driven war distinguishes the prohibited conduct from other forms of violence that states historically claimed as lawful. In classical doctrine, war was a means of self-help available to sovereigns. Article I rejects that conception by denying states the legal entitlement to wage war for discretionary political objectives. The prohibition thus operates at the level of purpose rather than method, addressing why force is used rather than how it is conducted (Oppenheim, 1952).

At the same time, the article does not equate “war” with every use of armed force. The interwar legal vocabulary treated war as a formal state of hostilities, often accompanied by declaration and recognised legal consequences. By using the term “war” rather than “force,” the provision left unresolved questions about military actions falling short of full-scale war. This distinction later proved consequential, as states sought to characterise uses of force as limited, defensive, or non-war measures to avoid falling within the scope of the renunciation (Simpson, 2004).

5.3 What Article I Does Not Prohibit

Article I does not purport to abolish all uses of force. Most notably, it does not explicitly address self-defence. During negotiations and ratification, many states made clear that they did not interpret the pact as requiring passivity in the face of armed attack. The prevailing understanding was that defensive force, employed to repel aggression rather than to pursue policy objectives, lay outside the prohibition. This interpretation, while not spelled out in the text, became widely accepted in state practice and legal commentary (Henkin, 1979).

Other categories of force were also treated as falling outside the scope of the article. Police actions, including measures to maintain internal order or suppress rebellion, were generally regarded as matters of domestic jurisdiction rather than international war. Colonial enforcement actions were similarly excluded by several imperial powers, which maintained that the pact applied only to relations between sovereign states and not to the administration of dependent territories. These exclusions reveal the political limits of the interwar legal imagination and the persistence of hierarchical assumptions within international law (Anghie, 2004).

The absence of clear textual boundaries created enduring interpretive tension. While flexibility facilitated widespread adherence, it also enabled states to stretch the concept of self-defence or deny the existence of “war” altogether when engaging in coercive conduct. These ambiguities weakened the pact’s immediate restraining effect but did not negate its legal significance. By removing aggressive war from the category of lawful policy choices, Article I forced states to justify force in legal terms rather than invoking sovereign discretion. That requirement marked a fundamental shift in the structure of international legal argument, one that would later be formalised and narrowed under the United Nations Charter regime (Cassese, 2008).

6. Article II Explained: Obligation of Pacific Settlement

6.1 Scope of the Obligation

Article II complements the renunciation of war by imposing a positive duty on the parties to resolve disputes “only by pacific means.” This obligation is broad in scope. It applies to all disputes or conflicts “of whatever nature or of whatever origin,” leaving no category of international disagreement outside its reach. The provision rejects the classical idea that certain disputes—particularly those touching vital interests or national honour—could justify recourse to war if peaceful avenues failed.

The legal effect of this formulation is to universalise peaceful settlement as the sole lawful method of dispute resolution between states. Arbitration, judicial settlement, negotiation, mediation, and conciliation are all implicitly endorsed, while coercive methods are excluded as legitimate options. The obligation operates independently of the availability or effectiveness of specific procedures. States are required to pursue peaceful means in good faith, not merely to exhaust formal steps before reverting to force (McNair, 1961).

Importantly, Article II does not prescribe a hierarchy among peaceful methods or mandate acceptance of compulsory adjudication. States retain discretion as to the choice of means, provided those means are non-violent. This flexibility was central to securing widespread acceptance, particularly among states wary of binding jurisdiction. The legal innovation lies not in procedural compulsion but in the categorical exclusion of war as a fallback mechanism when peaceful efforts prove politically inconvenient.

6.2 Legal Consequences of Breach

Although Article II contains no enforcement provisions, its breach engages international responsibility. Under general principles of international law, the violation of a treaty obligation constitutes an internationally wrongful act, regardless of whether sanctions are specified. Responsibility arises from the failure to comply with the duty itself, not from the presence of an enforcement regime (Brownlie, 1963).

A state that resorts to war or force instead of peaceful settlement breaches both the negative obligation in Article I and the positive obligation in Article II. This dual breach strengthens the legal characterisation of the conduct as unlawful. Even in the absence of immediate consequences, the breach alters the legal position of the acting state, exposing it to claims of responsibility, loss of legal justification, and collective condemnation. The pact thereby transforms the legal narrative surrounding conflict, shifting attention from procedural compliance to substantive unlawfulness.

The absence of sanctions does not render the obligation ineffective. International law frequently relies on decentralised mechanisms such as diplomatic protest, reputational harm, and reciprocal measures to give effect to legal norms. Article II contributes to this framework by clarifying that violent dispute settlement lacks legal legitimacy, narrowing the range of acceptable justifications available to states that resort to force (Henkin, 1979).

6.3 Relationship to Later Dispute-Settlement Regimes

Article II exerted a significant influence on the development of later international dispute-settlement regimes. Its insistence on peaceful means anticipated and informed the structure of the United Nations Charter, particularly the obligation under Article 2(3) requiring states to settle disputes by peaceful means so as not to endanger international peace and security. The Charter system institutionalised this principle by linking peaceful settlement to collective security and, in certain cases, compulsory procedures.

While the Charter introduced more detailed mechanisms, including Security Council involvement and judicial settlement through the International Court of Justice, the normative foundation was already established in 1928. The shift from permissive war to obligatory peaceful settlement did not originate with the United Nations but was consolidated there. Article II represents the moment at which peaceful dispute resolution ceased to be a preferred option and became a legal requirement of general application (Cassese, 2008).

The influence of this obligation is also visible in the gradual expansion of compulsory dispute-settlement clauses in multilateral treaties and regional systems. Although states continue to resist universal compulsory jurisdiction, the expectation that disputes must be addressed without force has become a baseline assumption of the international legal order. Article II thus occupies a critical place in the transition from a system tolerant of war to one structured around legalised and institutionalised peaceful settlement.

7. Article III Explained: Ratification and Universality

7.1 Open Accession and Rapid Global Adoption

Article III governs ratification and accession, providing that the treaty would enter into force upon ratification by the original signatories and remain open for accession by other states. This provision was crucial to the pact’s political and legal ambition. By avoiding restrictive membership criteria or institutional affiliation, the agreement positioned itself as a general normative instrument rather than a regional or alliance-based arrangement.

The response was unusually rapid and expansive. Within a short period, most major powers and a significant number of smaller states became parties. This near-universal participation mattered symbolically because it conveyed a collective rejection of war as legitimate policy, cutting across political systems, geographic regions, and levels of power. In a period marked by ideological diversity and strategic mistrust, such convergence on a core legal principle was exceptional (Simpson, 2004).

Legally, widespread ratification strengthened the pact’s authority beyond its formal obligations. High levels of participation reduced the plausibility of claims that the renunciation of war represented a parochial or culturally contingent norm. Instead, the treaty presented itself as a general statement of lawful conduct applicable to the international community as a whole. This universality also reinforced the reciprocity of obligations: each state’s commitment was owed not to a single counterpart but to all other parties, amplifying the reputational and legal consequences of breach (McNair, 1961).

7.2 Legal Significance of Widespread Consent

The breadth of consent achieved through Article III had implications extending beyond treaty law. When a large and representative group of states accepts a legal principle in a formal and public manner, that principle gains relevance for the formation of customary international law. The pact provided clear evidence of opinio juris, demonstrating that states regarded the renunciation of aggressive war as a legal obligation rather than a purely political aspiration (Brownlie, 1963).

State practice in the interwar period did not consistently conform to this obligation, but customary law does not require perfect compliance. What matters is whether violations are treated as breaches rather than as exercises of lawful discretion. After 1928, states increasingly sought to justify uses of force in defensive or exceptional terms, indicating an implicit acceptance that aggressive war required a legal excuse. This pattern of justificatory behaviour supports the view that the pact contributed to altering the legal baseline governing recourse to war (Hathaway and Shapiro, 2017).

The legal significance of this development became clearer in later decades. International tribunals and legal commentators drew upon the pact to support the proposition that aggressive war had already been outlawed before the adoption of the United Nations Charter. The widespread consent embodied in Article III thus enabled the treaty to function as more than a contractual arrangement. It operated as a catalyst for a broader normative transformation, embedding the illegitimacy of aggressive war into the structure of general international law and shaping the legal environment in which subsequent rules on the use of force emerged.

8. Interpretive Problems and Structural Weaknesses

8.1 Absence of Enforcement Mechanisms

One of the most frequently cited weaknesses of the pact is the complete absence of enforcement machinery. The treaty establishes obligations but provides no institutional framework for monitoring compliance, determining breach, or imposing sanctions. This design reflects the political realities of the interwar period. States were unwilling to accept external authority capable of judging their security decisions or compelling compliance through coercive means. As a result, the pact relies entirely on good faith performance and the indirect effects of political and legal consequences (McNair, 1961).

In practical terms, compliance depended on reputational costs, diplomatic pressure, and reciprocal expectations rather than on formal enforcement. A state that violated the pact risked loss of legitimacy and moral standing but faced no automatic legal penalty. This reliance on decentralised enforcement limited the treaty’s immediate restraining effect, particularly in an international environment characterised by power asymmetries and weak collective institutions (Henkin, 1979).

Yet this structural choice was not accidental. The absence of enforcement was the price of universality. Introducing sanctions or compulsory adjudication would likely have prevented broad participation and undermined the treaty’s normative ambition. The pact thus embodies a classic tension in international law between effectiveness and acceptance, opting for wide consent at the expense of immediate coercive capacity.

8.2 Undefined Concepts: “War” and “Self-Defence”

A second structural weakness lies in the treaty’s use of undefined and contested concepts. The term “war” was left without clarification, despite its central role in Article I. In the interwar legal vocabulary, war referred to a formal state of hostilities, often marked by declaration and recognised legal consequences. This understanding allowed states to argue that military actions falling short of declared war did not trigger the pact’s prohibition (Simpson, 2004).

Similarly, self-defence is not mentioned explicitly in the text. While most states assumed that defensive force remained permissible, the absence of a definition created space for expansive interpretations. States could claim defensive necessity in circumstances that blurred the line between genuine self-protection and preventive or retaliatory action. This ambiguity weakened the pact’s capacity to constrain behaviour in the short term, as legal arguments could be tailored to justify contested uses of force.

At the same time, these gaps enabled later reinterpretation. As international law evolved, the concept of “war” gave way to a broader focus on the use of force, and self-defence became subject to more articulated legal criteria. The pact’s open-textured language allowed it to be read compatibly with these developments, supporting its continued relevance rather than confining it to an obsolete legal vocabulary (Cassese, 2008).

8.3 Abuse and Circumvention in the 1930s

The structural and interpretive weaknesses of the pact were exposed most clearly during the 1930s. States engaged in aggressive conduct while denying that their actions constituted war or violated the treaty. Japan’s actions in Manchuria were framed as measures of self-defence and regional stabilisation rather than as aggressive war. Italy characterised its invasion of Ethiopia as a civilising mission and a response to alleged provocations. Germany employed legalistic rhetoric to present territorial expansion as the rectification of injustices or the fulfilment of self-determination claims (Marks, 1976).

These justifications did not reflect good-faith interpretation but strategic recharacterisation. States sought to operate within the formal language of the pact while emptying its core obligation of practical effect. The ease with which such arguments were advanced highlights the limitations of a treaty lacking precise definitions and enforcement mechanisms.

Despite these failures, the pattern of circumvention itself is legally revealing. Aggressive acts were no longer openly defended as lawful exercises of sovereign discretion. Instead, they were masked in defensive or exceptional terms, implicitly acknowledging that aggressive war had become legally problematic. This shift in justificatory behaviour supports the view that the pact succeeded in altering the legal grammar of international relations, even as it failed to prevent the descent into renewed global conflict.

9. The Pact in State Practice Between 1928 and 1945

9.1 Japanese, Italian, and German Violations

State practice in the period between 1928 and the end of the Second World War exposed the pact’s severe practical limitations. The most consequential violations were committed by Japan, Italy, and Germany, whose actions demonstrated how a legal prohibition lacking enforcement mechanisms could be overridden by strategic ambition.

Japan’s occupation of Manchuria in 1931 marked the first major test. Rather than declaring war, Japanese authorities characterised the operation as a defensive response to alleged instability and threats to national interests. By avoiding the formal label of war, Japan sought to place its conduct outside the scope of the treaty. This approach exploited the pact’s reliance on the traditional concept of war and revealed how formalism could be used to evade substantive obligations (Simpson, 2004).

Italy’s invasion of Ethiopia in 1935 followed a similar pattern. Italian justifications framed the campaign as a response to border incidents and as a civilising mission rather than an act of aggressive war. The absence of an effective collective reaction underscored the weakness of legal condemnation unsupported by coercive measures. Although the invasion plainly contradicted the pact’s renunciation of war as national policy, the lack of decisive consequences demonstrated the fragility of interwar legal constraints when confronted with determined violators (Cassese, 2008).

Germany’s actions were the most systematic and destabilising. Territorial expansion through rearmament, annexation, and invasion was consistently accompanied by legal rhetoric portraying these acts as defensive, corrective, or consensual. The remilitarisation of the Rhineland, the Anschluss with Austria, and subsequent aggressions were presented as restoring sovereignty or protecting ethnic populations. These narratives aimed to obscure the policy-driven nature of the use of force and avoid explicit repudiation of the treaty’s core principle (Marks, 1976).

Collectively, these violations demonstrated that the pact lacked the capacity to restrain powerful states willing to disregard legal obligations. They also revealed how ambiguity in treaty language could be manipulated to justify aggression while maintaining a façade of legality.

9.2 Why Violation Did Not Nullify Legal Significance

The widespread breach of the pact has often been cited as proof of its failure. This conclusion rests on an overly narrow understanding of legal effectiveness. In international law, the persistence of violations does not necessarily negate the existence or significance of a norm. The relevant question is not whether the rule was obeyed in every instance, but whether it altered the legal framework within which conduct was assessed.

The distinction between ineffective enforcement and normative failure is central here. The pact failed as a mechanism of prevention, but it succeeded in changing the legal status of aggressive war. Before 1928, war could be justified as a lawful policy choice. After the pact, states engaging in aggression no longer claimed a legal right to do so. Instead, they resorted to defensive narratives and exceptional justifications, implicitly acknowledging that aggressive war required a legal excuse rather than sovereign assertion (Brownlie, 1963).

This shift had lasting consequences. It enabled post-war tribunals to treat aggressive war as unlawful under pre-existing international law, rather than as a retroactively criminalised act. It also influenced the drafting of the United Nations Charter, which transformed the pact’s normative condemnation into a more structured prohibition supported by institutional mechanisms. The legal legacy of the pact, therefore, survived its political collapse.

The interwar experience illustrates a broader truth about international law: norms often mature through contestation rather than compliance. The pact’s inability to prevent catastrophe does not erase its role in delegitimising war as policy. Instead, the contrast between its aspirations and the realities of the 1930s highlights why later legal systems sought to preserve its core principle while addressing its structural weaknesses.

10. The Kellogg–Briand Pact and International Criminal Responsibility

10.1 Use at Nuremberg

The pact played a decisive role in the legal reasoning of the International Military Tribunal at Nuremberg. When the Tribunal addressed the charge of crimes against peace, it confronted the question of whether aggressive war had been prohibited under international law before the outbreak of the Second World War. The defence argued that no clear legal rule criminalised the resort to war at the time the alleged acts were committed. The prosecution responded by grounding the prohibition in existing treaty law, with the pact occupying a central position.

The Tribunal accepted this argument. It treated the renunciation of war as an instrument of national policy as evidence that aggressive war had already been outlawed by international law prior to 1939. The pact was cited as a multilateral commitment through which states had collectively rejected war as a lawful means of pursuing policy objectives. On this basis, the Tribunal concluded that the initiation of aggressive war constituted a violation of international law and could serve as the foundation for criminal liability (IMT, 1947; Cassese, 2008).

This reasoning was significant because it rejected the idea that the prohibition of aggressive war emerged only after the war ended. Instead, it affirmed that the legal transformation had already occurred during the interwar period. The pact thus functioned as a bridge between treaty-based obligations of states and the post-war effort to impose accountability for their breach.

10.2 From Treaty Obligation to Individual Criminal Liability

The move from state responsibility to individual criminal liability represented a major doctrinal development. Traditional international law addressed breaches of obligations through inter-state responsibility, reparations, and diplomatic consequences. The pact itself operates within this framework, imposing duties on states rather than individuals. Nothing in its text refers to criminal sanctions or personal accountability.

The Nuremberg Tribunal nonetheless held that individuals could be held responsible for violations of international law, including the initiation of aggressive war. This conclusion rested on the premise that international law binds not only abstract entities but also the persons who act on their behalf. Leaders who plan, prepare, initiate, or wage aggressive war were therefore treated as personally responsible for conduct that violated obligations already binding on their states (Kelsen, 1947).

The pact was essential to this reasoning because it established the unlawfulness of aggressive war independently of criminal law. Once aggressive war was recognised as illegal, the further step of attaching individual responsibility became a matter of legal technique rather than normative invention. The Tribunal emphasised that criminal liability did not arise from the treaty itself but from general principles of law, recognising that serious violations of international obligations could entail personal accountability.

10.3 “Crimes against Peace” as a Legal Innovation

The category of crimes against peace was one of the most innovative aspects of the Nuremberg framework. It transformed the initiation of aggressive war from a matter of state conduct into an offence attracting individual criminal punishment. The pact’s contribution lay in supplying the substantive illegality required to support this transformation. Without a prior legal prohibition on aggressive war, the charge would have risked appearing retroactive.

Although the treaty contained no criminal provisions, its normative content enabled the Tribunal to characterise aggressive war as the “supreme international crime,” distinguished by the fact that it contained within itself the accumulated evil of subsequent atrocities (IMT, 1947). This characterisation relied on the idea that war itself had been placed outside the bounds of lawful state action well before 1939.

The importance of the pact in this context lies not in its enforcement capacity but in its doctrinal function. It demonstrated that international law could impose substantive limits on sovereign choice in matters of war and peace. By doing so, it made possible the later development of international criminal law relating to aggression. The legacy of crimes against peace thus confirms that even a treaty lacking penal mechanisms can generate far-reaching legal consequences when its core norm reshapes the structure of legal responsibility.

11. The Pact’s Role in the Development of the UN Charter

11.1 Continuity with Article 2(4) of the UN Charter

The prohibition on the use of force enshrined in Article 2(4) of the United Nations Charter represents a direct doctrinal continuation of the legal transformation initiated in 1928. The pact introduced the foundational idea that war could no longer be treated as a lawful instrument of state policy. The Charter took that premise and recast it in more precise and operational legal terms, shifting the focus from “war” to the broader concept of force in international relations.

This evolution addressed one of the principal weaknesses of the earlier treaty. By prohibiting not only war but the threat or use of force against the territorial integrity or political independence of any state, the Charter closed the formalistic gap that had allowed states to evade legal responsibility by denying that their actions constituted war. The moral condemnation articulated in 1928 was thus converted into a general legal rule applicable to all coercive uses of force, regardless of form or intensity (Brownlie, 1963).

The continuity between the two instruments is also evident in their shared normative orientation. Both express a collective judgment that recourse to force undermines international order and must be excluded from lawful policy choices. The Charter did not repudiate the earlier treaty but absorbed its core principle into a more comprehensive legal framework. In doing so, it confirmed that the pact represented an early stage in the development of a general prohibition rather than an isolated or failed experiment (Cassese, 2008).

11.2 What the Charter Adds—and What It Inherits

While the Charter inherits the normative foundation laid in 1928, it adds elements that the pact conspicuously lacked. Most importantly, it establishes an institutional system for enforcement and collective security. The Security Council is endowed with authority to determine threats to peace, breaches of peace, and acts of aggression, and to authorise collective measures in response. This institutionalisation addressed the pact’s reliance on decentralised and political enforcement, which had proven insufficient in the interwar period (Henkin, 1979).

The Charter also clarifies and confines exceptions to the prohibition. Self-defence is expressly recognised but is subject to conditions and procedural requirements, including necessity, proportionality, and reporting obligations. This articulation narrowed the interpretive space that had allowed expansive defensive claims under the earlier treaty. By defining both the rule and its exceptions, the Charter strengthened legal certainty and reduced opportunities for abuse.

At the same time, the Charter inherits key features of the pact. It preserves the idea that peace is a legal obligation rather than a discretionary objective and maintains the centrality of peaceful dispute settlement as a core duty of states. The pact’s emphasis on normative transformation is reflected in the Charter’s aspirational language and its framing of peace as a collective interest.

The relationship between the two instruments illustrates a process of legal maturation rather than replacement. The pact supplied the initial normative rupture by denying the legality of war as policy. The Charter built upon that rupture by embedding the prohibition within a structured legal order capable of responding to violations. Together, they mark the transition from a system tolerant of war to one organised around the legal regulation and institutional control of force in international relations.

12. Status of the Kellogg–Briand Pact in Contemporary International Law

12.1 Is the Pact Still Legally in Force?

The treaty remains legally in force for the states that ratified it and have not withdrawn. Its continued validity is not displaced by the adoption of the United Nations Charter. Under general principles of treaty law, later treaties do not automatically terminate earlier ones unless they are incompatible or expressly supersede them. The obligations contained in the pact are compatible with the Charter’s framework, as both instruments are directed toward the same objective: excluding aggressive war from lawful state conduct (McNair, 1961).

The Charter introduced a more comprehensive and institutionalised system governing the use of force, but it did not repeal earlier commitments renouncing war. Instead, the two instruments operate in parallel, with the Charter providing the primary operational regime and the earlier treaty retaining residual legal relevance. This coexistence reflects a layered approach to international legal development, in which foundational norms persist even as new structures are built around them (Brownlie, 1963).

In practice, contemporary legal analysis rarely relies on the treaty as a standalone source of obligation. Its continued force is nonetheless significant because it confirms that the renunciation of aggressive war did not begin in 1945. The prohibition articulated in the Charter rests on a longer normative trajectory, strengthening its claim to general and settled legal status.

12.2 Independent Relevance Today

Although seldom invoked explicitly in modern litigation or diplomatic exchanges, the treaty retains independent relevance in specific contexts. One such context is historical legal assessment. When evaluating the legality of conduct occurring between 1928 and 1945, the treaty provides a critical reference point for determining whether aggressive war was already prohibited at the time. This relevance was central to post-war criminal proceedings and remains important in historical accountability debates (Cassese, 2008).

The treaty also plays a supplementary role in interpretive reasoning. Where questions arise about the scope or meaning of the prohibition on the use of force, reference to earlier commitments renouncing war helps illuminate the underlying purpose of the legal regime. The treaty reinforces the understanding that the modern prohibition is not merely a technical rule but part of a broader effort to eliminate force as a policy choice in international relations (Hathaway and Shapiro, 2017).

In addition, the pact retains symbolic and pedagogical value within international law. It serves as a reminder that normative change often precedes institutional capacity. The absence of frequent citation does not equate to irrelevance; instead, it reflects the degree to which the treaty’s core principle has been absorbed into the general structure of the law governing international peace and security.

12.3 Contribution to Customary International Law

The treaty’s most enduring contribution lies in its role in shaping customary international law. Through widespread ratification and public commitment, states articulated a shared legal belief that aggressive war was no longer acceptable. This expression of opinio juris was reinforced by subsequent patterns of justification, in which states increasingly framed uses of force as defensive, exceptional, or legally compelled rather than as discretionary policy choices (Brownlie, 1963).

Customary law does not require uniform compliance. What matters is whether departures from the rule are treated as breaches requiring explanation. After 1928, overt claims of a legal right to wage aggressive war largely disappeared from serious legal discourse. This shift supports the conclusion that the treaty contributed to altering the baseline of legality, even though violations continued to occur (Henkin, 1979).

The survival of the treaty’s principles within customary law explains why explicit reference has become unnecessary. Norms that achieve general acceptance often recede into the background, operating as assumptions rather than as cited authorities. The renunciation of aggressive war has reached this stage. Its roots lie in the interwar legal transformation initiated by the pact, whose influence persists through the customary and treaty-based rules that now govern the use of force in international law.

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13. Concrete Application Example

13.1 Aggression and Legal Argumentation

In contemporary debates on aggression, the logic introduced in 1928 continues to operate beneath the surface of legal argumentation. Modern disputes concerning the legality of cross-border military action are rarely framed as assertions of a sovereign right to use force. Instead, states consistently seek to situate their conduct within recognised exceptions, most commonly self-defence. This argumentative structure reflects an inherited legal baseline: aggressive war is presumed unlawful unless justified within narrowly defined categories.

The prohibition on aggression under contemporary international law is often traced formally to the United Nations Charter, yet the argumentative posture adopted by states reveals a deeper lineage. Legal advisers emphasise necessity, proportionality, immediacy, and defensive purpose precisely because the initiation of force as a policy choice is no longer considered legally acceptable. This presumption did not originate in 1945. It emerged earlier, when war was stripped of its status as a lawful instrument of national policy and recast as conduct requiring legal justification (Brownlie, 1963).

In practice, this can be observed in how states deny aggressive intent. Even large-scale military operations are presented as compelled responses to external threats, protection of nationals, or enforcement of international obligations. The fact that such narratives are deemed necessary indicates that the core idea introduced in 1928 remains operative: force is presumptively illegal when pursued as policy, and legal argument must therefore neutralise or displace that presumption. This mode of reasoning mirrors the structure established by the earlier treaty, which transformed war from a permissible option into a legally suspect act requiring defence rather than assertion.

13.2 Normative Framing in International Litigation and Diplomacy

The concept of outlawry introduced in 1928 continues to shape the normative framing of international disputes, particularly in litigation and diplomatic exchanges. International courts and tribunals assess the legality of force by examining whether conduct aligns with recognised legal justifications, rather than by balancing sovereign interests. This approach presupposes that the default position is prohibition, a presupposition that owes its historical origins to the interwar rejection of war as lawful policy (Cassese, 2008).

Diplomatic practice reflects the same logic. Condemnations issued by states and international organisations rarely accuse an actor simply of acting imprudently or destabilising order. Instead, they characterise aggressive conduct as unlawful, illegitimate, or contrary to fundamental principles of international law. This language reinforces the idea that peace is a legal obligation, not merely a political preference. The rhetorical force of such condemnations depends on the prior acceptance of a legal norm excluding aggressive war from the realm of legitimate state action.

Legal advisers continue to rely on the outlawry concept because it structures both advocacy and assessment. It narrows the range of acceptable arguments, disciplines justificatory narratives, and frames accountability discussions. Even when the earlier treaty is not cited explicitly, its normative contribution persists through the legal architecture that governs contemporary discourse on aggression. The continued reliance on this framework demonstrates that the pact’s most durable achievement was not enforcement, but the reconfiguration of how international law speaks about war and responsibility.

14. Critical Assessment

14.1 Was the Kellogg–Briand Pact a Failure?

If the metric is prevention, the answer is straightforward: it did not stop the return of large-scale interstate violence. The 1930s exposed how a rule without enforcement can be ignored by revisionist powers, especially when other states lack the willingness or capacity to impose costs. The treaty’s design offered no compulsory dispute settlement, no enforcement body, no automatic sanctions, and no agreed definition of the prohibited conduct. These gaps made it easy to violate and difficult to operationalise as a tool of crisis response (Henkin, 1979).

That conclusion, however, conflates enforcement failure with normative failure. In international law, a rule can be legally meaningful even if frequently breached, provided that the breach is treated as a violation requiring justification rather than as an exercise of lawful discretion. The central test is the “language of legality” that follows. After 1928, major aggressors typically avoided claiming a legal right to wage aggressive war. Instead, they relied on defensive narratives, necessity claims, provocation arguments, or semantic strategies denying that their acts amounted to “war.” The need to provide such justifications indicates that the legal baseline had shifted. Aggressive war had become harder to defend as a lawful policy (Brownlie, 1963).

The pact, therefore, failed as a short-term security mechanism but succeeded in changing what counts as a legally acceptable rationale for force. That shift matters because international law often works by constraining justificatory space. Once a category of conduct becomes presumptively unlawful, the burden moves to the actor to defend legality, and that burden can influence diplomacy, coalition-building, reputational cost, and later institutional responses. On those terms, it is misleading to label the pact simply a failure (Hathaway and Shapiro, 2017).

14.2 Why the Pact Succeeded Legally Despite Political Collapse

The treaty succeeded legally because it altered the conceptual structure of jus ad bellum. It broke with the classical view that war was a lawful tool of sovereignty and established a new premise: war pursued as policy is illegitimate. Even though the treaty used the language of “war” rather than “force,” the normative move was broader. It changed the default assumption underpinning legal argument, pushing the system away from permissive sovereignty and toward prohibition (McNair, 1961).

This transformation had three durable legal effects.

First, it provided a foundation for the criminalisation of aggressive war after 1945. Nuremberg’s treatment of crimes against peace relied on the idea that aggressive war was already unlawful under international law before the Charter system fully institutionalised the prohibition. The treaty supplied a key part of that pre-Charter legal basis, making accountability arguments more credible and less vulnerable to charges of retroactivity (Cassese, 2008).

Second, it shaped the architecture of the United Nations Charter. The Charter did not invent the idea that force as policy is unlawful; it operationalised it by extending the prohibition to the “threat or use of force,” defining institutional roles, and structuring enforcement through collective security. The earlier treaty is best understood as the normative precursor; the Charter is the institutional consolidation (Henkin, 1979).

Third, it changed how states justify military action. Contemporary state practice is saturated with legal argument about self-defence, necessity, proportionality, and attribution. This argumentative environment exists because the default position is prohibition. That default is the long-term achievement of the interwar outlawry project. The treaty did not create a stable peace, but it helped create a legal world in which aggressive war cannot be presented as normal statecraft (Hathaway and Shapiro, 2017).

The political order that produced the treaty collapsed, but the norm it articulated survived and matured. Its lasting contribution is the transformation of war from a legally tolerated option into conduct that demands exceptional legal justification and triggers responsibility when that justification fails.

15. Conclusion

The central legal achievement of the Kellogg–Briand Pact lies in its decisive break with the classical assumption that war was a lawful instrument of state policy. By formally renouncing war as a means of pursuing national objectives, the treaty removed aggressive war from the category of legally permissible state action. This shift did not depend on enforcement capacity or institutional strength. It operated at a deeper level, redefining what international law considers justifiable conduct in relations between states.

That transformation reshaped the structure of jus ad bellum. Before 1928, legal analysis focused on how wars were fought and how belligerent rights were exercised. After the pact, attention increasingly turned to whether the resort to war itself could be justified at all. This reorientation laid the groundwork for later legal developments, including the prohibition of the use of force in the United Nations Charter and the recognition of aggressive war as a matter of legal responsibility rather than sovereign discretion (Brownlie, 1963; Cassese, 2008).

The treaty’s legacy is therefore structural rather than operational. It did not prevent conflict in the short term, but it altered the legal grammar through which conflict is assessed. States no longer claim a general right to wage war. They argue self-defence, necessity, or exceptional circumstance precisely because war as a policy has lost legal legitimacy. This argumentative constraint is one of the most enduring effects of the interwar outlawry project (Hathaway and Shapiro, 2017).

For these reasons, the pact cannot be dismissed as obsolete. Its principles have been absorbed into the core of contemporary international law, often without explicit citation, because they have become foundational assumptions rather than contested claims. The treaty marks the point at which international law crossed a normative threshold, replacing tolerance of war with prohibition as the default rule. In that sense, its relevance has not diminished with time. It endures as a cornerstone of the modern legal order governing the use of force.

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