The Warsaw Pact (formally, the Treaty of Friendship, Co-operation, and Mutual Assistance, sometimes, informally WarPac, akin in format to NATO) was a collective defense treaty among Soviet Union and seven Soviet satellite states in Central and Eastern Europe in existence during the Cold War. The Warsaw Pact was the military complement to the Council for Mutual Economic Assistance (CoMEcon), the regional economic organization for the communist states of Central and Eastern Europe. The Warsaw Pact was created in reaction to the integration of West Germany into NATO in 1955 per the Paris Pacts of 1954, but it is also considered to have been motivated by Soviet desires to maintain control over military forces in Central and Eastern Europe. 

While the Warsaw Pact was established as a balance of power or counterweight to NATO, there was no direct confrontation between them. Instead, the conflict was fought on an ideological basis. Both NATO and the Warsaw Pact led to the expansion of military forces and their integration into the respective blocs. The Warsaw Pact's largest military engagement was Warsaw Pact invasion of Czechoslovakia (with the participation of all Pact nations except Romania and Albania). The Pact failed to function when the Revolutions of 1989 spread through Eastern Europe, beginning with the Solidarity movement in Poland and its success in June 1989.
And what did that organization compliment?
The Warsaw Pact