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The international law on the use of force governs when states may lawfully resort to military action against other states. It is one of the most contested and politically charged areas of international law, because it sits at the intersection of state sovereignty, collective security, and the raw reality of armed conflict. The field is often summarized by the question: When, if ever, may one state use military force against another? Its central texts are the Charter of the United Nations (1945), customary international law, and decades of state practice and jurisprudence interpreting both.
The modern law of the use of force is built on the UN Charter, which was drafted in the aftermath of World War II. The Charter’s foundational rule is found in Article 2(4), which prohibits states from the "threat or use of force against the territorial integrity or political independence of any state, or in any other manner inconsistent with the Purposes of the United Nations." This prohibition is widely regarded as a rule of customary international law, binding on all states, not just UN members.
The Charter creates two, and only two, exceptions to this prohibition. The first is self-defense, recognized in Article 51. The second is military action authorized by the UN Security Council under Chapter VII of the Charter, which empowers the Council to take action to maintain or restore international peace and security. This structure—a general prohibition with narrow exceptions—is the doctrinal core of the field. Much of the subsequent history of the law of the use of force is a series of arguments about the scope of these exceptions.
The field’s enduring questions all concern the boundaries of the Charter’s framework. The most fundamental is the meaning of "force" itself. Does it include economic coercion, cyber operations, or support for rebel groups? The dominant view is that "force" in Article 2(4) refers primarily to armed force, but the application of this to new technologies and indirect forms of intervention remains unsettled.
A second cluster of questions concerns self-defense. Article 51 preserves the "inherent right" of self-defense "if an armed attack occurs." But what counts as an armed attack? Does it include attacks by non-state actors, such as terrorist groups operating from another state’s territory? May a state respond in self-defense against an imminent attack before it has actually occurred, and if so, what counts as "imminent"? May self-defense be exercised against a state that harbors but does not direct such groups? These questions have generated enormous controversy, especially since the 2001 attacks of September 11 and the subsequent "war on terror."
A third set of questions concerns the Security Council’s role. The Council has the power to authorize force, but its permanent members hold vetoes, and the Council has often been deadlocked. This has led to debates about whether force can be used without Council authorization in cases of humanitarian catastrophe (the "responsibility to protect" debate), or to enforce Council resolutions that do not explicitly authorize force. The 1999 NATO intervention in Kosovo, undertaken without Council authorization, and the 2003 invasion of Iraq, which was justified by some states as enforcing earlier Council resolutions, are the two most frequently cited examples of this problem.
The prohibition on the use of force is a relatively recent achievement. For most of the history of international law, war was considered a sovereign right of states. The classical international law of the nineteenth century distinguished between "just" and "unjust" wars in moral and theological terms, but legal doctrine generally treated war as a legitimate instrument of state policy. The first serious legal restrictions on the right to go to war came in the early twentieth century. The League of Nations Covenant (1919) imposed procedural limits on war, requiring states to submit disputes to arbitration or the League Council before resorting to force. The Kellogg-Briand Pact (1928) went further, renouncing war as an instrument of national policy, but it lacked enforcement mechanisms and did not prevent the aggression of the 1930s.
The UN Charter represented a decisive break. Its drafters, reacting to the failure of the League, sought not merely to regulate war but to prohibit it, with the exception of self-defense and collective action. The Charter’s prohibition was reinforced by the Nuremberg Tribunal, which held that waging aggressive war was a crime under international law. The Cold War, however, prevented the Security Council from functioning as the Charter’s architects had intended. The Council was paralyzed by the rivalry between the United States and the Soviet Union, and the law of the use of force developed largely through state practice, often in ways that tested the Charter’s limits.
The field is not organized into rival "schools" in the way that, say, economics or philosophy might be. Instead, it is characterized by a persistent tension between two broad interpretive tendencies, which are best understood as poles on a spectrum rather than fixed camps.
The first tendency is restrictivist or Charter-literalist. Restrictivists argue that the Charter’s text should be read narrowly. The prohibition on force is broad; the exceptions are narrow. Self-defense, in this view, is limited to responses to actual armed attacks, and anticipatory self-defense is unlawful. The Security Council is the only legitimate authorizer of non-defensive force. This approach emphasizes the Charter’s purpose of minimizing the unilateral use of force and protecting state sovereignty. It is associated with many international lawyers, particularly in the Global South, who fear that broad readings of self-defense or humanitarian intervention will be used by powerful states to justify aggression.
The second tendency is permissive or dynamic. Permissivists argue that the Charter must be interpreted in light of changing circumstances, including the rise of non-state terrorism, the proliferation of weapons of mass destruction, and the failure of the Security Council to act in humanitarian crises. They tend to favor broader readings of self-defense, including the right to act against imminent threats and against non-state actors, and they are more open to the idea that humanitarian intervention may be lawful even without Council authorization. This approach is often associated with the practice of Western states, particularly the United States and its allies.
It is important to note that these are not formal schools with named adherents. Individual states and lawyers shift positions depending on the case. A state that takes a permissive view of self-defense in one context may take a restrictive view in another. The distinction is nonetheless analytically useful because it captures the fundamental fault line in the field.
A third, more recent approach is the practice-based or empirical tradition. Rather than starting from the Charter’s text, scholars in this tradition examine what states actually do and say when they use force. They ask whether state practice has modified the Charter’s rules, and they pay close attention to the legal justifications states offer for their actions. This approach has been influential in debates about humanitarian intervention and the use of drones, where the gap between legal doctrine and state behavior is particularly wide. Its limitation is that state practice is often ambiguous, and it can be difficult to distinguish between a state’s violation of a rule and its attempt to change the rule.
The International Court of Justice (ICJ) has played a significant role in interpreting the law on the use of force, but its jurisprudence is not comprehensive. The Court’s most important decision is the 1986 Nicaragua case, in which it held that the United States’ support for the Contras violated the prohibition on the use of force, and that collective self-defense could only be exercised at the request of the victim state. The Court also drew a distinction between an "armed attack" (which justifies self-defense) and a "mere frontier incident" (which does not). The Nicaragua decision remains the leading authority on many aspects of the law, but it has been criticized for being too restrictive and for not addressing the problem of non-state actors.
The ICJ’s 2003–2005 Oil Platforms case and its 2005 Armed Activities case (between the Democratic Republic of the Congo and Uganda) further refined the law, but they did not resolve the central controversies. The Court has never ruled on the legality of humanitarian intervention, the use of force against non-state actors in another state’s territory, or the meaning of "imminence" in the age of terrorism.
Because the ICJ’s role is limited, much of the law is shaped by the practice of states and the opinions of legal advisers within foreign ministries. This has led to a field in which legal argument is often intertwined with political advocacy. States that use force typically offer legal justifications, and other states either accept, reject, or remain silent on those justifications. Over time, this process of claim and response can harden into customary law. The difficulty is that the process is slow, and the law can remain unsettled for decades.
The current landscape of the field is marked by several unresolved controversies. The most significant is the lawfulness of self-defense against non-state actors. Since 2001, many states, including the United States, the United Kingdom, and others, have used force against terrorist groups in states that did not consent to the intervention (e.g., in Syria, Pakistan, and Somalia). They have argued that a state that is unwilling or unable to prevent attacks from its territory cannot invoke sovereignty to shield the attackers. This "unwilling or unable" doctrine is contested. Some states and scholars accept it as a reasonable adaptation of self-defense law; others argue that it has no basis in the Charter and that it dangerously expands the right to use force.
A second controversy concerns anticipatory self-defense. The classic formulation is the Caroline test, dating from the 1837–1842 border disputes between the United States and Canada, which held that self-defense is justified only when the threat is "instant, overwhelming, and leaving no choice of means, and no moment for deliberation." Modern permissivists argue that this test is outdated in an age of ballistic missiles and terrorist networks, and that a state may act against a threat that is not imminent in the traditional sense. Restrictivists respond that loosening the imminence requirement would allow states to use force based on speculation and fear, undermining the Charter’s prohibition.
A third controversy is humanitarian intervention. The 1999 NATO intervention in Kosovo, undertaken without Security Council authorization, remains the most prominent example. Some argue that the intervention was lawful because it was necessary to prevent a humanitarian catastrophe and was supported by a broad coalition of states. Others argue that it was a violation of the Charter, and that the law should not be bent to accommodate even morally appealing uses of force. The "responsibility to protect" (R2P) doctrine, adopted by the UN General Assembly in 2005, holds that states have a responsibility to protect their populations from mass atrocities, and that the international community may take collective action if they fail. But R2P explicitly requires Security Council authorization for coercive action, and it has not resolved the underlying legal question.
A fourth area of active debate is the application of the law to cyber operations and autonomous weapons. The UN Charter’s prohibition on force was drafted in an era of physical, kinetic warfare. Whether a cyber operation that causes physical damage (e.g., disabling a nuclear facility) constitutes an "armed attack" is now the subject of extensive scholarly and governmental analysis. The Tallinn Manual, a set of expert studies, has attempted to apply existing law to cyber operations, but its conclusions are not binding and remain contested.
A final, overarching theme is the relationship between law and politics in this field. The law on the use of force is often criticized as being too vague to constrain powerful states, or too rigid to address real-world security threats. Defenders of the law respond that it provides a common language for states to justify their actions and to criticize the actions of others, and that even violations of the law are usually framed in its terms, which is itself a form of compliance.
The field is also marked by a persistent gap between the law as written and the law as practiced. States frequently use force in ways that are of doubtful legality, and the international community often responds with condemnation, but rarely with enforcement. This has led some scholars to argue that the law is best understood not as a set of binding rules but as a framework for argument and legitimation. Others maintain that the law has real constraining power, and that the gap between doctrine and practice is narrower than it appears.
For the educated newcomer, the most useful way to approach the field is to understand its central tension: the Charter’s prohibition on force is clear, but its exceptions are ambiguous, and the ambiguity is not an accident. It reflects the fundamental difficulty of creating a legal order that can both prevent aggression and respond to genuine threats. The law on the use of force is not a settled code but an ongoing argument, and the argument is the field.