In international law, self-defense is the principal exception that allows a state to use force against another state without breaching the prohibition in Article 2(4) of the UN Charter. Article 51 preserves the "inherent right of individual or collective self-defence if an armed attack occurs against a Member of the United Nations." The interaction between self-defense and the law of state responsibility determines when defensive force is lawful and against whom it may be directed.
Under the International Law Commission's 2001 Articles on Responsibility of States for Internationally Wrongful Acts (ARSIWA), Article 21 provides that the wrongfulness of an act is precluded if the act constitutes a lawful measure of self-defense taken in conformity with the UN Charter. In other words, conduct that would otherwise breach an international obligation (such as violating another state's territorial integrity) is not wrongful when it qualifies as Charter-compliant self-defense.
Two doctrinal questions recur:
- Attribution. When non-state actors carry out an armed attack, can defensive force be used on the territory of a host state? The ICJ's Nicaragua judgment (1986) set a high "effective control" threshold for attributing private conduct to a state, and the Court reiterated a restrictive view in the Wall Advisory Opinion (2004) and Armed Activities (DRC v. Uganda, 2005). State practice after 9/11, including invocations of the "unwilling or unable" doctrine by the United States and others, has pushed in the opposite direction, though it remains contested.
- Necessity and proportionality. Customary law, reaffirmed by the ICJ in Nicaragua and the Oil Platforms case (2003), requires that defensive force be both necessary to repel the attack and proportionate to it. These limits apply independently of Article 51's textual requirements.
Self-defense does not preclude obligations under peremptory norms (jus cogens) or international humanitarian law, and Article 51 requires that measures be reported to the Security Council and cease once the Council acts.
Example
After the September 11, 2001 attacks, the United States invoked Article 51 in a letter to the UN Security Council to justify Operation Enduring Freedom against Al-Qaeda and the Taliban in Afghanistan as self-defense.
Frequently asked questions
In Article 21 of the ILC's 2001 Articles on Responsibility of States for Internationally Wrongful Acts (ARSIWA), which links the defense to lawful measures under the UN Charter.
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