Article 5 of the North Atlantic Treaty, also known as the principle of collective defense, states that an armed attack against one or more member countries is considered an attack against all members. This means that if a NATO member is attacked, the other members are obligated to assist in defense, which can include military action. This article was first invoked after the September 11 attacks in 2001, marking a significant moment in NATO's history. It emphasizes the importance of solidarity among member nations, ensuring that they stand together in the face of threats to their security.