... 2001, Article 5 of the North Atlantic Treaty of 1949 was invoked. It states:
The Parties agree that an armed attack against one or more of them in Europe or North America shall be considered an attack against them all and consequently they agree that, if such an armed attack occurs, each of them, in exercise of the right of individual or collective self-defence recognised by Article 51 of the Charter of the United Nations, will assist the Party or Parties so attacked by taking forthwith, individually and in concert with the other Parties, such action as it deems necessary, including the use of armed force, to restore and maintain the security of the North Atlantic area.Invocation meant that if the United States determined "foreign nationals" had committed the terrorist attacks of the previous day, "its 18 NATO partners" would support a military response, according to NATO Secretary-General George Robertson.
Any such armed attack and all measures taken as a result thereof shall immediately be reported to the Security Council. Such measures shall be terminated when the Security Council has taken the measures necessary to restore and maintain international peace and security.
(Prior September 12 posts are here and here.)