It has been invoked only once in NATO history : by the United States after the September 11 attacks in 2001 . The invocation was confirmed on 4 October 2001 , when NATO determined that the attacks were indeed eligible under the terms of the North Atlantic Treaty . The eight official actions taken by NATO in response to the 9/11 attacks included Operation '' Eagle Assist '' and Operation '' Active Endeavour '' , a naval operation in the Mediterranean which was designed to prevent the movement of terrorists or weapons of mass destruction , as well as enhancing the security of shipping in general . Active Endeavour began on 4 October 2001
September 11 attacks
September 11 attacks
North Atlantic Treaty