An association of European and North American states, formed in 1949 for the defence of Europe and the North Atlantic against the perceived threat of Soviet aggression. Dominated by the USA, it identified in 1991 the disintegration of the USSR and instability in Eastern Europe as new dangers. Its headquarters are in Brussels, Belgium. Since the collapse of the USSR, NATO has sought cooperation with Russia and the formerly communist nations of east Europe. In 1997 a treaty with Russia was signed and the Czech Republic, Hungary, and Poland joined NATO in 1999. They were followed in 2004 by Bulgaria, Estonia, Latvia, Lithuania, Romania, Slovakia, and Slovenia; and in 2009 by Albania and Croatia. Despite Russian hostility to the move, Montenegro joined in 2017.

In 1995 NATO took its first aggressive action when it intervened in the war in Bosnia and Herzegovina, bombing Serb positions around the besieged city of Sarajevo; this developed into a peacekeeping operation that lasted until 2004. In 1999 it bombed Serbia in an attempt to end its policy of genocide in Kosovo. NATO has since operated in Afghanistan (2003–14) and carried out air strikes against the Gaddafi regime in Libya (2011). After the Russian annexation of Crimea in 2014, NATO increased its troop deployment in the Baltic states and in eastern Europe.