Article 51 is a crucial provision within the UN Charter, which is the foundational treaty of the United Nations, signed on June 26, 1945, and effective from October 24, 1945. It is an exception to the Charter's general prohibition on the use of force, which is enshrined in Article 2(4). The provision was created to solve the problem of a state being left defenseless while the newly formed United Nations Security Council (UNSC) deliberated on a response to an attack.
The article explicitly states that nothing in the Charter shall impair the "inherent right of individual or collective self-defence if an armed attack occurs against a Member of the United Nations". This right exists only until the Security Council has taken the measures necessary to maintain international peace and security. The mechanism requires that any measures taken in the exercise of this right must be immediately reported to the Security Council. Furthermore, the military action must satisfy the customary international law principles of necessity (force must be the only option left) and proportionality (the response should not exceed the scale of the attack).
A significant contemporary debate connects Article 51 to the use of force against Non-State Actors (NSAs), such as terrorist organizations. Traditionally, the right of self-defence applied only to attacks attributable to a state, a stance maintained by the International Court of Justice (ICJ). However, post-9/11, the expansive "unwilling or unable" doctrine has emerged, which is used by some states, including the US, to justify military action against NSAs operating from another state's territory if that host state is unwilling or unable to neutralize the threat. This interpretation has intensified debates over the scope of Article 51, which has been invoked on at least 78 occasions since 2021, marking a significant increase in its use. The core text of Article 51 itself has not been formally amended.