Article 5 is a key provision of the North Atlantic Treaty Organization (NATO) that states an armed attack against one or more NATO members is considered an attack against all members. Invoked only once in NATO's history, after the September 11, 2001 attacks, Article 5 commits each member country to respond to an attack on any ally, which may include military force or other actions as deemed necessary. This collective defense principle is central to NATO's purpose, aiming to deter aggression and ensure the security of its member states.