North Atlantic Treaty Organization (NATO), regional defense alliance created by the North Atlantic Treaty. NATO’s purpose is to enhance the stability, well-being, and freedom of its members through a system of collective security. Members of the alliance agree to defend one another from attack by other nations or by terrorist groups. NATO has its headquarters in Brussels, Belgium.
The North Atlantic Treaty was signed on April 4, 1949, at the beginning of the Cold War. The original purpose of NATO was to defend Western Europe against possible attack by Communist nations, led by the Union of Soviet Socialist Republics (USSR). The original signatories (signers of treaty) were Belgium, Canada, Denmark, France, Iceland, Italy, Luxembourg, The Netherlands, Norway, Portugal, the United Kingdom, and the United States. Greece and Turkey were admitted to the alliance in 1952, West Germany in 1955, and Spain in 1982. In 1990 the newly unified Germany replaced West Germany as a NATO member.



After the formal end of the Cold War in 1991, NATO reached out to former members of the Warsaw Pact, the Communist military alliance created in 1955 by the USSR to counter NATO. In 1999 former Warsaw Pact members Hungary, Poland, and the Czech Republic became members of NATO, bringing the total membership to 19 nations. In 2002 Russia, once the USSR’s largest republic, became a limited partner in NATO as a member of the NATO-Russia Council. The same year NATO invited the Baltic states of Estonia, Latvia, and Lithuania, formerly part of the USSR, to join, along with Slovenia, formerly part of Communist Yugoslavia, and Bulgaria, Romania, and Slovakia, once part of Czechoslovakia. These countries were formally admitted to NATO in April 2004. Bulgaria, Czechoslovakia, and Romania were all former Warsaw Pact members.
Over the years the existence of NATO has led to closer ties among its members and to a growing community of interests. The treaty itself has provided a model for other collective security agreements. NATO activities are no longer confined only to Europe. In 2003, for the first time in its history, NATO took up peacekeeping activities outside of Europe by deploying troops in Afghanistan.