The NATO nations have agreed that in case of an attack against a member nation what happeds?

If a NATO member nation is attacked, the principle of collective defense under Article 5 of the North Atlantic Treaty is invoked. This means that an attack against one member is considered an attack against all members, obligating them to respond collectively. Member nations can choose how to respond, which may include military action, support, or other measures, depending on the situation and the consensus among allies. This principle has only been invoked once, following the September 11, 2001 attacks in the United States.