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How can sovereign states be bound to refrain from violence when no global sovereign exists to enforce the prohibition? This question has driven the legal theory of force for centuries. The answer has never been stable: each major framework reopens the problem from a different angle, and the frameworks that dominate today remain in productive disagreement about what counts as a lawful use of force, who decides, and why states comply.
The earliest systematic framework for thinking about force in international law was Natural Law Theory, which dominated European legal thought from roughly 1600 to 1900. Natural lawyers such as Francisco de Vitoria, Hugo Grotius, and Emer de Vattel argued that the use of force was governed by moral principles discoverable through reason. War was permissible only when it met the criteria of just cause, right authority, and right intention. This was not a permissive framework: it imposed substantive moral limits on sovereigns, and it treated the international community as a moral order rather than a mere collection of self-interested states. The just-war tradition that survives today in debates over humanitarian intervention draws directly on this natural law inheritance.
By the nineteenth century, Legal Positivism had largely displaced natural law as the dominant framework for the use of force. Positivists such as John Austin and later Lassa Oppenheim rejected the idea that moral principles could bind states. Instead, they argued that the only valid source of legal obligation was state consent, expressed through treaties and custom. The use of force was lawful unless a state had explicitly consented to a prohibition. This narrowing had a dramatic effect: the nineteenth-century law of war became a permissive system in which sovereigns were free to wage war as an instrument of policy, constrained only by the customary rules of conduct during hostilities. The Hague Conventions of 1899 and 1907 codified this positivist approach, regulating the means and methods of war without challenging the sovereign right to initiate it. Positivism did not disappear; it remains a living tradition, especially in scholarship that insists on strict treaty interpretation and resists expansive readings of the UN Charter.
Marxist Approaches to International Law, emerging after 1917, offered a fundamental challenge to both natural law and positivism. For Marxist theorists, the legal prohibition on force was a bourgeois fiction that masked the economic interests of capitalist states. The use of force was not a matter of legal rules but of class struggle: powerful states used military intervention to protect markets and extract resources, while the law of war served as ideological cover. This framework never aimed to replace positivism as the working doctrine of states; instead, it functioned as a persistent critical undercurrent that exposed the gap between legal rhetoric and material reality. Contemporary Marxist scholars continue to analyze humanitarian intervention and the Responsibility to Protect (R2P) as instruments of hegemonic power, a critique that coexists uneasily with liberal justifications for intervention.
Liberal Internationalism, crystallized after World War I and institutionalized after 1945, was the most ambitious attempt to replace the positivist permissiveness toward war. Its core claim was that the use of force could be legally constrained through multilateral institutions, collective security, and the progressive development of international law. The League of Nations Covenant (1919) and the UN Charter (1945) embodied this vision: Article 2(4) prohibited the threat or use of force against the territorial integrity or political independence of states, and Chapter VII authorized the Security Council to enforce the peace. Liberal internationalists argued that states would comply not merely because of enforcement but because they internalized the norm of non-use of force over time. This framework remains the institutional backbone of the subfield: every subsequent framework must position itself relative to the Charter regime. Its distinctive commitment is to the possibility of legal progress through institutional design, a claim that Legal Realists and Critical scholars have persistently challenged.
Legal Realism, which entered international law debates in the 1930s and remains active, directly contested the liberal faith in rules. Realists such as Hans Morgenthau argued that legal prohibitions on force were indeterminate: the same Charter text could be interpreted to justify or condemn almost any intervention. The real determinants of state behavior were power and national interest, not legal obligation. Legal Realism did not replace Liberal Internationalism; it coexists with it as a skeptical partner, forcing liberal theorists to explain why states comply when enforcement is weak. The realist challenge is especially acute in debates over self-defense: the Caroline test (1837) of necessity and proportionality, though widely cited, remains notoriously open-textured, and realists argue that it provides cover for any use of force a powerful state chooses to justify.
International Legal Process, emerging in the 1960s, shifted the focus from the content of rules to the processes by which rules are made, interpreted, and applied. Scholars such as Abram Chayes and Thomas Franck argued that the use-of-force regime was best understood not as a set of static prohibitions but as a dynamic system of justification, deliberation, and institutional interaction. The UN Security Council, the General Assembly, and the International Court of Justice were not mere enforcement bodies; they were arenas in which states continuously negotiated the meaning of lawful force. This framework absorbed the realist insight about indeterminacy but responded by emphasizing that process itself generates compliance: states care about the legitimacy of their justifications, not just the outcome. International Legal Process remains influential in scholarship on the "responsibility to protect" and the role of the Security Council in authorizing force.
Critical Legal Studies (CLS), which entered international law in the 1970s, radicalized the process-oriented critique. CLS scholars argued that the entire use-of-force framework was internally contradictory: the Charter simultaneously prohibits force and authorizes it through self-defense and Security Council action, and these exceptions can be stretched to swallow the rule. For CLS, the indeterminacy was not a problem to be managed by better process but a structural feature that revealed the law's dependence on political choice. CLS did not offer an alternative doctrine; it functioned as a deconstructive method that exposed the liberal internationalist project as incoherent. This framework remains active as a critical resource, especially in scholarship that challenges the legality of humanitarian intervention and the war on terror.
International Law and Economics, also emerging in the 1970s, offered a very different response to the indeterminacy problem. Drawing on rational-choice theory, scholars such as Jack Goldsmith and Eric Posner argued that the use-of-force regime could be explained as a set of efficient equilibria among self-interested states. States comply with prohibitions on force not because they believe in the rules but because the costs of violation—retaliation, loss of reputation, instability—outweigh the benefits. This framework narrowed the focus to state interests and incentives, deliberately setting aside the normative and institutional commitments of Liberal Internationalism. It coexists with Legal Realism in its skepticism about rules, but it replaces the realist emphasis on power with a more systematic economic analysis of state behavior.
Feminist Approaches to International Law, which gained traction in the 1980s, introduced a different critical lens. Feminist scholars such as Hilary Charlesworth and Christine Chinkin argued that the use-of-force framework was built on masculine assumptions about security, sovereignty, and violence. The prohibition on interstate force left unaddressed the pervasive violence that occurs within states—violence that disproportionately affects women. Feminist approaches did not simply add gender as a variable; they challenged the very categories of the subfield, arguing that the distinction between public and private violence, and between war and peace, was itself gendered. This framework coexists with Marxist and CLS critiques as part of a broader critical project, but it retains a distinctive focus on the embodied experience of violence and the exclusion of women from decision-making about force.
Constructivist International Law, which emerged in the 1990s, offered a direct challenge to both Legal Realism and Law and Economics. Constructivists such as Jutta Brunnée and Stephen Toope argued that state interests were not fixed but were constituted by legal norms. States comply with the prohibition on force not merely because of coercion or calculation but because they have internalized the norm as part of their identity. The ban on force, on this view, is a "shared understanding" that shapes what states perceive as their interests. Constructivism absorbed the liberal internationalist emphasis on institutional progress but grounded it in a sociological account of norm internalization. It remains one of the leading frameworks today, especially in scholarship on the evolution of the R2P norm and the prohibition on torture.
Global Legal Pluralism, also emerging in the 1990s, responded to a different pressure: the fragmentation of international law into specialized regimes. In the use-of-force context, pluralists such as Nico Krisch argued that there was no single legal order but a set of overlapping, sometimes conflicting, normative orders—the UN Charter, regional security arrangements, customary international law, and the laws of war. Pluralism rejected the liberal internationalist aspiration to a unified legal system and instead embraced the productive tension between orders. This framework coexists with Constructivism in its attention to social norms, but it disagrees with International Legal Constitutionalism about whether hierarchy or fragmentation better describes the current landscape.
International Legal Constitutionalism, also dating from the 1990s, took the opposite position. Constitutionalists argued that the UN Charter functioned as a constitution for the international community, establishing a hierarchy of norms—including the prohibition on force as a peremptory norm (jus cogens)—and limiting the sovereignty of states. This framework revived elements of natural law thinking by arguing that certain uses of force were simply illegal regardless of state consent. It remains in direct disagreement with Global Legal Pluralism: constitutionalists see the Charter as a constitutional order with priority over other regimes, while pluralists see a fragmented landscape with no clear hierarchy.
Global Administrative Law (GAL), which emerged around 2000, narrowed the focus to the procedural accountability of institutions that authorize or regulate force. GAL scholars such as Benedict Kingsbury and Richard Stewart argued that the Security Council, the International Criminal Court, and other bodies should be subject to administrative-law principles of transparency, reason-giving, and review. This framework does not directly address the substantive question of when force is lawful; instead, it asks whether the institutions that decide on force are themselves accountable. GAL coexists with International Legal Process in its procedural focus, but it narrows the inquiry to the administrative dimension, leaving the broader normative questions to other frameworks.
Today, the leading frameworks—Liberal Internationalism, Constructivism, Legal Realism, and Global Legal Pluralism—agree on several points: the UN Charter remains the central legal reference point; the prohibition on force is a fundamental norm; and the Security Council has primary responsibility for maintaining peace. But they disagree sharply on why states comply, how much indeterminacy the Charter regime can tolerate, and whether new threats such as cyber operations, drone strikes, and non-state armed groups require new legal categories. Liberal Internationalists and Constructivists argue that the existing framework can adapt through institutional practice and norm evolution. Legal Realists and Global Legal Pluralists are more skeptical, arguing that the Charter regime is too indeterminate to constrain powerful states and that the fragmentation of international law makes unified regulation impossible. These disagreements are not signs of weakness; they are the engine of the subfield, ensuring that the question of lawful force remains open to contestation and refinement.