The North Atlantic Treaty Organization (NATO) is a military alliance between the US, Canada, and the majority of Western Europe. The alliance is a collective defense by which if one state of the alliance is attacked, member states must agree to mutual defense in response to the attack.
NATO was formed in 1949 and has since been involved in most conflicts concerning North American and European interests.
The attacks on September 11, 2001 in the United States was the first instance since NATO’s creation Article 5 had been invoked:
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.
After September 11, countering terrorism has been at the top of NATO’s agenda. NATO condemns terrorism and recognizes it poses a threat to the security and safety of all members of the Alliance and destruction of international peace and security.
Since the wars in Afghanistan and Iraq, several of NATO’s European members perceive terrorism as a threat to the US rather than the globe. There has been criticism of NATO members by committing so many resources for fighting another country’s war. Yet, good has come from the Alliance as result of the wars. Increased sharing of information, law and military enforcement has brought members of NATO closer together in the agenda to fight terrorism.
(http://www.nato.int/cps/en/SID-0F672DE7-6265E4E0/natolive/topics_77646.htm?)