The purpose of the Warsaw Pact was to provide a counterbalance to NATO and increase the international negotiating power of the Soviet Union. The proximate cause of the Warsaw Pact was the May 9, 1955 decision of the governments of the United States and Western Europe to include West Germany in NATO. The Warsaw Pact lasted from May 14, 1955 to July 1, 1991.
The Warsaw Pact was also part of a Soviet plan to control the militaries of its satellite countries in Eastern Europe more forcefully. Soviet leaders also believed that the treaty would increase their leverage in international negotiations, allowing them to bargain on a deal with the Western countries to ensure mutual peace. The Warsaw Pact's military effectiveness was displayed most forcefully in 1968, when its members mounted an invasion of Czechoslovakia, a member of the Warsaw Pact whose government was liberalizing and attempting to throw off the Soviet yoke. The invasion drew the ire of Albania, which immediately removed itself from the treaty. East Germany removed itself from the Warsaw Pact in 1990, after it reunited with West Germany. The other countries in the Warsaw Pact dissolved the agreement in 1991, a sign of the demise of Soviet hegemony in Eastern Europe and the rise of independent foreign and military policies in the former satellite states. The original members of the Warsaw Pact were the Soviet Union, Albania, Bulgaria, Czechoslovakia, East Germany, Hungary, Poland and Romania.