The Warsaw Pact was a military alliance of the Eastern European Soviet Bloc countries intended to organize the communist block of eastern European countries against the percieved threat from the NATO alliance, established in 1949. The treaty was drafted by Krushchev in 1955 and signed in Warsaw on May 14, 1955; its members were the Soviet Union, Albania, Bulgaria, Romania, East Germany, Hungary, Poland, and Czechoslovakia--all the Communist countries of Eastern Europe except Yugoslavia. The members of the Warsaw Pact were to defend each other if one was attacked.
The Warsaw Pact was dominated by the Soviet Union. Efforts to leave the Warsaw Pact by member countries were crushed, for instance in the Hungarian revolution of 1956. Hungary planned to leave the Warsaw Pact and declare themselves neutral in the Cold War conflict between East and West, but in October 1956 the Red Army entered Hungary and crushed the resistance in two weeks.
Warsaw Pact forces were utilised at times, such as Prague Spring when they invaded Czechoslovakia to put down the discontented uprising. This brought to light a successor to the Warsaw Pact, the Brezhnev Doctorine, that stated "When forces that are hostile to socialism and try to turn the development of some socialist country towards capitalism, it becomes not only a problem of the country concerned, but a common problem and concern of all socialist countries."
NATO and the Warsaw Pact countries never engaged each other in armed conflict, but fought a Cold War for more than 40 years. In December 1988 Mikhail Gorbachev, leader of the Soviet Union at the time, announced that the Warsaw Pact/Brezhnev Doctorine would be abandoned and that the Eastern European countries could do what they liked. When it was clear that the Soviet Union would no longer use force to control the Warsaw Pact countries a series of rapid changes started in Eastern Europe and by 1989 the Warsaw Pact collapsed along with the Soviet Union and Eastern European communism.