The NATO response post-9/11 refers to the collective military and political actions taken by the North Atlantic Treaty Organization in the aftermath of the September 11, 2001 terrorist attacks. Following these attacks, NATO invoked Article 5 of its treaty for the first time in its history, which states that an attack on one member is an attack on all. This marked a significant shift in NATO's role and highlighted the alliance's commitment to counterterrorism efforts and collective defense.