Article 5 of the Washington Treaty: 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. 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. NATO's Strategic Concept recognises the risks to the Alliance posed by terrorism. What does Article 5 mean? Article 5 is at the basis of a fundamental principle of the North Atlantic Treaty Organisation. It provides that if a NATO Ally is the victim of an armed attack, each and every other member of the Alliance will consider this act of violence as an armed attack against all members and will take the actions it deems necessary to assist the Ally attacked. This is the principle of collective defence. Article 5 and the case of the terrorist attacks against the United States: The United States has been the object of brutal terrorist attacks. It immediately consulted with the other members of the Alliance. The Alliance determined that the US had been the object of an armed attack. The Alliance therefore agreed that if it was determined that this attack was directed from abroad, it would be regarded as covered by Article 5. NATO Secretary General, Lord Robertson, subsequently informed the Secretary-General of the United Nations of the Alliance's decision. Article 5 has thus... more... - Winckel from Bookmarklet

relevant re Turkey's recent request - Winckel

Relevant perhaps but unlikely. - Stephan Planken from iPhone