North Atlantic Treaty Organization

The North Atlantic Treaty Organization was founded in the aftermath of the Second World War with the signing of the North Atlantic Treaty in 1949, establishing a system of collective security and deterrence among its member states in Western Europe and North America, chiefly directed against the Soviet Union during the Cold War. Its founding principle, that an armed attack against one member is an attack against all, has been formally invoked only once, by the United States after the 11 September 2001 attacks. The alliance expanded eastward after the end of the Cold War to include many former Warsaw Pact and Soviet states, and it remains headquartered in Brussels.