The Use of Force in International Law

Introduction

The regulation of the use of force in international law is one of the most fundamental and contested areas of the international legal order. The UN Charter, adopted in 1945, established a comprehensive legal framework prohibiting the threat or use of force while authorizing limited exceptions for self-defense and Security Council-authorized enforcement action. The Charter system represented a radical break with the pre-1945 legal order, in which the right to go to war (jus ad bellum) was largely unrestricted. The post-Cold War era has witnessed new challenges to the Charter framework: humanitarian intervention, the Responsibility to Protect (R2P), preemptive self-defense, drone strikes, cyber operations, and the use of force against non-state actors. The legal regulation of force remains at the center of contemporary international legal debate.

Article 2(4): The Prohibition on the Use of Force

Article 2(4) of the UN Charter provides the foundational prohibition: “All Members shall refrain in their international relations 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.” The prohibition covers both actual uses of force and threats of force. It applies to all states, whether UN members or not, and is widely accepted as customary international law and as a jus cogens norm — a peremptory norm from which no derogation is permitted.

The scope of Article 2(4) has generated extensive debate. The prohibition covers armed force; economic coercion is not included, though the Declaration on Principles of International Law concerning Friendly Relations (1970) condemns economic and political coercion. The phrase “against the territorial integrity or political independence” was argued by some states (particularly during the Cold War) to permit humanitarian intervention or force that does not permanently alter borders or subjugate a government, but the dominant view is that the prohibition is comprehensive and admits only the two exceptions expressly recognized in the Charter.

Self-Defense Under Article 51

Article 51 of the UN Charter recognizes the inherent right of individual or collective self-defense if an armed attack occurs, until the Security Council has taken measures necessary to maintain international peace and security. Self-defense is a customary right codified in the Charter, not created by it. The right arises only when an armed attack occurs — the most restrictive interpretation, requiring an actual military attack. The International Court of Justice in Nicaragua v. United States (1986) distinguished between the most grave forms of the use of force (armed attack) and less grave forms, holding that the right of self-defense arises only in response to armed attacks.

The Caroline test, derived from the 1837 Caroline incident and the diplomatic correspondence between the United States and Britain, provides the customary law standard for self-defense: the use of force must be necessary (no reasonable alternative) and proportionate (the force used must be proportionate to the threat). The Caroline test has been endorsed by the International Court of Justice and is widely accepted as reflecting customary international law.

Anticipatory and Preemptive Self-Defense

The most contentious issue in the law of self-defense is whether force may be used in anticipation of an armed attack that has not yet occurred. The traditional Caroline test arguably permits anticipatory self-defense when an attack is imminent — the “necessity of self-defense is instant, overwhelming, leaving no choice of means, and no moment for deliberation.” The UK Attorney-General’s legal advice on the 2003 Iraq War invoked a doctrine of preemptive self-defense against threats that are not imminent but are grave and potentially catastrophic.

The 2002 U.S. National Security Strategy (the Bush Doctrine) asserted a right of preemptive self-defense against emerging threats, particularly terrorist groups and rogue states developing weapons of mass destruction. The majority of international legal scholars and states reject the doctrine as inconsistent with Article 51 and the Caroline test. The International Court of Justice has not directly ruled on anticipatory self-defense, but its Nicaragua and Oil Platforms judgments suggest a strict reading requiring an actual armed attack. The 9/11 attacks and the subsequent “war on terror” have prompted renewed debate about the scope of self-defense against non-state actors and the imminence requirement in the age of terrorism.

Security Council Authorization Under Chapter VII

The UN Security Council has primary responsibility for the maintenance of international peace and security (Article 24). Under Chapter VII, the Security Council may determine the existence of any threat to the peace, breach of the peace, or act of aggression and may authorize enforcement action (Articles 39, 42) or impose sanctions (Article 41). The Council’s powers under Article 42 are comprehensive: it may take such action by air, sea, or land forces as may be necessary to maintain or restore international peace and security.

The Security Council has authorized the use of force in numerous situations: the Korean War (1950, through a recommendation under Chapter VII), the authorization to use “all necessary means” to liberate Kuwait (Resolution 678, 1990), the authorization of force in Somalia (Resolution 794, 1992), the authorization to restore peace and security in Haiti (Resolution 940, 1994), the authorization of force to protect civilians in Libya (Resolution 1973, 2011), and various peace enforcement missions. The Council has also authorized member states to use force to protect civilians in South Sudan, the Central African Republic, and elsewhere.

The Prohibition of Aggression

Aggression is the most serious form of the unlawful use of force. The UN General Assembly’s Definition of Aggression (Resolution 3314, 1974) defines aggression as the use of armed force by a state against the sovereignty, territorial integrity, or political independence of another state. The Definition lists acts constituting aggression: invasion, bombardment, blockade, attack on military forces, and the sending of armed bands. The Rome Statute of the International Criminal Court includes aggression as a crime, defined by the Kampala Amendment (2010). The International Court of Justice has addressed aggression in Nicaragua (1986), Congo v. Uganda (2005), and Georgia v. Russia (2011), though the ICJ has never found a state responsible for aggression.

Humanitarian Intervention and R2P

Humanitarian intervention — the use of force by a state or group of states to protect populations from mass atrocities without Security Council authorization — is one of the most contested issues in the law on the use of force. NATO’s intervention in Kosovo (1999) was the paradigmatic case: NATO bombed Yugoslavia to halt ethnic cleansing in Kosovo without Security Council authorization (Russia threatened a veto). The intervention was condemned by some states as a violation of Article 2(4) but defended by others as morally and legally justified.

The Responsibility to Protect (R2P), endorsed by the UN General Assembly at the 2005 World Summit, seeks to reconcile humanitarian protection with legal constraint. R2P has three pillars: the primary responsibility of each state to protect its population from genocide, war crimes, ethnic cleansing, and crimes against humanity; the international community’s responsibility to assist states in fulfilling this responsibility; and the international community’s responsibility to take collective action, through the Security Council, when a state manifestly fails to protect its population. R2P applies only to the four atrocity crimes, requires exhaustion of peaceful means, and must be authorized by the Security Council. The intervention in Libya (2011) was the first Security Council-authorized R2P mission but was criticized for exceeding its mandate by pursuing regime change. The failure to intervene in Syria (after 2011) demonstrated the limits of R2P when the Security Council is divided.

Contemporary Challenges

The Charter framework faces significant contemporary challenges. Drone strikes by the United States and other states against terrorist targets in the territory of non-consenting states raise questions about the scope of self-defense against non-state actors, the definition of imminence, and the applicable legal framework (international humanitarian law or human rights law). Cyber operations targeting critical infrastructure may constitute uses of force or armed attacks, but the legal framework for determining when a cyber operation crosses the threshold is still developing. The use of force against non-state actors — ISIS in Syria, armed groups in the Sahel — stretches the traditional interstate framework. The erosion of Security Council authority, demonstrated by Russia’s veto of resolutions on Ukraine and the Council’s paralysis on Syria, has led some states to use force without authorization. The future of the law on the use of force depends on whether states can adapt the Charter framework to these new challenges while maintaining the prohibition on force as the foundation of the international legal order.