Article 5
The North Atlantic Treaty's collective-defense clause (1949), which commits members to take 'such action as it deems necessary,' including but not limited to armed force, in response to an attack on a member. The unconditional clause is that an attack on one member 'shall be considered an attack against them all'; the discretionary clause — each member deciding what action is 'necessary' — was the specific device that let the treaty clear a two-thirds U.S. Senate ratification vote, since no legislature in 1949 would sign away its own future war-powers discretion.