NATO and Global Military Alliances

The North Atlantic Treaty Organization was founded on April 4, 1949, when the United States, Canada, and ten Western European states signed the Washington Treaty establishing a system of collective defense against the perceived threat of Soviet expansion in postwar Europe. Article 5 of the treaty provides that an armed attack against one member shall be considered an attack against all, obligating other members to assist "as it deems necessary," including the use of armed force, though the clause has been formally invoked only once in the alliance's history — by the United States itself, following the September 11, 2001 terrorist attacks, leading NATO allies to contribute forces to the International Security Assistance Force in Afghanistan. The Soviet Union and its Eastern European satellites responded to West Germany's 1955 NATO accession by forming the rival Warsaw Pact that same year; the Pact dissolved in 1991 alongside the broader collapse of Soviet-aligned communist governance in Eastern Europe.