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.
In which year did the Warsaw Pact cease to function effectively? 
1989