NATO's Article 5 treats an armed attack on one ally as an attack on all, and has been invoked only once

Background

Standing background on collective defence. Article 5 of the North Atlantic Treaty treats an armed attack on one NATO member as an attack on all. Each ally must act by taking forthwith, individually and in concert with the other Parties, such action as it deems necessary. Assistance may or may not involve armed force, so there is no automatic military response. The North Atlantic Council meets to assess whether the conditions apply. Article 5 has been triggered once, after the terrorist attacks on the United States, leading to Operation Eagle Assist with NATO AWACS aircraft.