NATO has invoked Article 5 once, and it turns the right of self-defence into a mutual assistance obligation
Background
NATO describes collective defence as its most fundamental principle.
Article 5 of the North Atlantic Treaty states that an armed attack against one NATO member shall be considered an attack against them all, and it triggers an obligation for each member to come to the assistance of the one attacked.
That assistance may or may not involve the use of armed force, and can include any action the Allies deem necessary to restore and maintain the security of the North Atlantic area.
Article 5 is consistent with Article 51 of the United Nations Charter, which recognises that a State that is the victim of an armed attack has the inherent right to individual or collective self-defence and may request others to come to its assistance.
Within the NATO context, Article 5 translates that right of self-defence into a mutual assistance obligation.
NATO invoked Article 5 for the first and only time in its history after the 9/11 terrorist attacks against the United States.
Though applied only once, Article 5 underpins NATO's wider deterrence and defence work, including military exercises and the deployment of its standing military forces.
In plain words
Article 5 Article 5 of the North Atlantic Treaty — The collective defence clause under which an attack on one member counts as an attack on all.
Article 51 Article 51 of the Charter of the United Nations — The article that recognises the inherent right of individual or collective self-defence against armed attack.