The headquarters of NATO, based in Brussels, Belgium, was established as a system of collective defense, as its member states agreed to mutual defense in response to attack by external forces, especially that of the Soviet Union during the Cold War. However, NATO also recognises countries which have adopted a policy of neutrality, such as Sweden
The Treaty of Brussels, signed on March 17, 1949 by France, Luxembourg, Belgium, the UK and the Netherlands, is considered to be the precursor of the NATO agreement. This treaty established a military alliance which later became the Western European Union. During this period, American participation and support was also considered in order to counter the military power of the Soviet Union, which in turn led to the North Atlantic Treaty. The last treaty was signed on April 4, 1949 in Washington D.C. by the five countries of the Treaty of Brussels, along with the U.S., Canada, Portugal, Denmark, Norway, Italy and Iceland. Three years later Greece and Turkey also joined.
On May 30, 1978, the NATO countries officially defined two aims of the Alliance, which included the maintaining of security and the pursuance of détente
(French term for relaxing or easing, used in international politics since the early 1970s). NATO and the Warsaw Pact had reasons to seek relaxation in times of tension with the Soviet Union and the Cold War. The end of the Cold War, along with the dissolution of the Warsaw Pact, removed the main adversary of the Alliance, which resulted in a strategic re-evaluation of NATO’s purpose and tasks, with its gradual expansion to Eastern Europe, and an extension of its activities to areas not formerly NATO concerns. The first post-Cold War expansion came when former East Germany became part of the Federal Republic of Germany and of NATO on October 3, 1990.
In February 1994, the Alliance undertook its first military challenge by shooting down four Bosnian Serb aircrafts which violated a UN-mandated, no-fly zone over Bosnia and Herzegovina. The said no-fly zone was set up the previous year and was to continue until December 1995. NATO air strikes helped bring the Bosnian war to an end. Between 1994 and 1997, various forums concerning regional cooperation between NATO and its neighbors were established. On July 8, 1997, the ex-communist countries Hungary, Poland and the Czech Republic received an invitation to join NATO, which they accepted in 1999.
Expansion of NATO activities and its geographical reach grew further after the September 11, 2001 attacks by al-Qaeda, who crashed two commercial passenger U.S. into the World Trade Center in New York City. The collective security of NATO’s charter was again invoked, which specified that any attack on a NATO member would be considered as an attack against the entire Alliance. Eight official actions were taken by NATO in response to the attacks.