The core provision of the treaty is Article V, which 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. 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.
This provision was intended so that if a Warsaw Pact member launched an attack against the European allies of the United States, it would be treated as if it was an attack on all member states (including the United States itself), which has the largest military in the alliance and could thus provide the most significant retaliation. However, the feared invasion of Western Europe never came. Instead, the provision was invoked for the first time in the treaty's history on September 12, 2001, in response to the September 11 attacks on the United States the day before.
Greece and Turkey joined the organisation in February 1952. Germany joined as West Germany in 1955 and German reunification in 1990 extended the membership to the areas of the former German Democratic Republic, which was annexed to the Federal Republic of Germany. Spain was admitted on May 30, 1982, and the former Warsaw Pact countries of Poland, Hungary and the Czech Republic made history by becoming members on March 12, 1999.
France is a member of NATO, which retired from the military command in 1966 but rejoined in 1992. Iceland, the sole member of NATO which does not have its own military force (the Icelandic Defence Force being the United States Military contingent permanently stationed in Iceland), joined on the condition that they would not be expected to establish one.
Slovenia, Slovakia, the former Warsaw Pact countries of Bulgaria and Romania, and the former republics of the USSR Estonia, Latvia and Lithuania, officially acceded to NATO on March 29, 2004. They attended their first NATO meeting in April 2004.
The U.S. President, NATO Secretary General, and the Prime Ministers of Slovenia, Lithuania, Slovakia, Romania, Bulgaria and Estonia after a South Lawn ceremony welcoming them into NATO on March 29, 2004.
September 12, 2001: NATO invoked, for the first time in its history, the collective security clause of its charter. Article 5 states that any attack on a member state is considered an attack against the entire alliance. This came in response to the September 11, 2001 Terrorist Attack against the United States.
February 10, 2003: NATO faced a crisis when France and Belgium vetoed the procedure of silent approval concerning the timing of protective measures for Turkey in case of a possible war with Iraq. Germany did not use its right to break the procedure but said it supported the veto.
April 16, 2003: NATO agreed to take command in August of the International Security Assistance Force (ISAF) in Afghanistan. The decision came at the request of Germany and the Netherlands, the two nations leading ISAF at the time of the agreement. All 19 NATO ambassadors approved it unanimously. The handover of control to NATO took place on August 11, and marked the first time in NATO's history that it took charge of a mission outside the north Atlantic area. Canada had originally been slated to take over ISAF by itself on that date.
June 19, 2003: A major restructuring of the NATO military commands began as the Headquarters of the Supreme Allied Commander, Atlantic was abolished and a new command, Allied Command Transformation was established in Norfolk, Virginia, U.S.
March 29, 2004: Bulgaria, Estonia, Latvia, Lithuania, Romania, Slovakia and Slovenia join NATO.
The crumbling of the main "Enemy of the West" in Eastern Europe, as well as dissentions between members about the latest Iraq operations, makes some wonder – in North America as well as in Europe – if NATO has not become obsolete. The presumed terrorist threat could give this institution a new life, but some think also that fighting this new enemy needs a completely different political and military organisation, as well as completely different weapons systems other than those on which NATO was built.
Many also argue that NATO is in conflict with the prospects of deeper European integration in the fields of foreign policy and security within the framework of the EU institutions. Advocates for a strong EU Common Foreign and Security Policy (CFSP) would like to see NATO dismantled and create common defence and foreign policy within the existing EU institutions.
In November 2004 after the re-election of President George W. Bush the Norwegian Prime Minister Kjell Magne Bondevik publicly discussed whether Norway would gain by strengthening her defence relations with the EU. Many Norwegian political analysts consider NATO to be a "politically dead organisation". So do several pundits and political leaders in other member nations. These attitudes will of necessity be reflected in future discussions of NATO expansion.