Definition
A security commitment under which allies agree to assist a member attacked by an external actor.
TERMINOLOGY NOTE
Distinction: alliance collective defence is a treaty commitment; collective self-defence under UN Charter Article 51 is a legal right following armed attack.
Theoretical Context
NATO Article 5 is a central example of collective defence. Each member determines the assistance it considers necessary, which may include but does not automatically require armed force. This alliance obligation should be distinguished from the legal right of collective self-defence in UN Charter Article 51.
Examples in Practice
01
NATO Article 5 after 9/11
NATO invoked its collective-defence clause for the first time following the attacks in 2001.