is the acronym for the North Atlantic Treaty Organization, established in Washington, DC, on 4 April 1949 by the USA, Canada, UK, France, and other west European countries. This was the culmination of diplomatic efforts by those, including the British government, who saw a defence alliance as vital to safeguard western Europe against possible threats by the USSR. The signatories committed themselves to taking ‘necessary action’ to aid any member facing attack. The Korean War induced the formation of an integrated military command for NATO, which functioned well until the end of the Cold War. Both its length of existence and its role in seeing off the Soviet challenge give NATO a claim to be among the most successful alliances in history. Problems in the 21st cent. suggest however that success can pose its own challenges.