The military organisation of Eastern Europe signed in Warsaw, Poland in 1955, by Albania, Bulgaria, Czechoslovakia, East Germany, Hungary, Poland, Romania and the Soviet Union. It was a communist counterpart to NATO. Warsaw Pact members were bound to assist each other in the event of an attack on any one of them. Albania withdrew in 1961. The Warsaw Pact collaborated in the invasion of Czechoslovakia in 1968-the only time it took military action. The Pact was ended as a military alliance in 1991, when the demise of communism and the end of the Cold War made it superfluous.