Article 51 of the UN Charter is a foundational provision of international law that recognizes the inherent right of individual or collective self-defence for a Member State. It serves as the primary exception to the general prohibition on the use of force outlined in Article 2(4) of the Charter. The provision was created as part of the UN Charter, which was signed on June 26, 1945, and came into force on October 24, 1945, following the devastation of World War II. Its purpose was to balance the new collective security framework with the practical necessity of allowing states to respond immediately to aggression.
The mechanism of Article 51 is strictly conditional: the right to self-defence is triggered only if an armed attack occurs against a Member State. This right is temporary, existing until the Security Council has taken measures necessary to maintain international peace and security. A procedural requirement mandates that any measures taken in self-defence must be immediately reported to the Security Council. Furthermore, the exercise of this right is governed by customary international law principles of necessity and proportionality.
This provision is a critical part of Chapter VII of the UN Charter, which deals with threats to peace. The concept of collective self-defence under Article 51 provides the legal basis for military alliances like NATO. While the text of the Article remains unchanged, its interpretation has evolved, particularly concerning attacks by non-state actors (NSAs). Post-9/11, the controversial "unwilling or unable" doctrine emerged, which some states use to justify strikes against NSAs in another state's territory if that state cannot control them. However, the International Court of Justice (ICJ) has historically maintained a more restrictive view, requiring state attribution for the right of self-defence to be invoked. The invocation of Article 51 has seen a marked increase in recent years, with at least 78 occasions cited since 2021.