Why was the Warsaw Pact dissolved ?

Back in the late 80's/early 90's, The Soviet Union - at the time the world's leading communist power and one of two superpowers (the other being the U.S.A.) - was evolving from a communist state to a more "westernized" capitalist economy. Because of the negotiations between Mikhail Gorbachev - the last Soviet leader - and President Ronald Reagan, the president of the U.S.A. in the 80's, the Soviet Union dismembered its communist statehood (the process is known as Perestroika). Once the Soviet Union partitioned itself into several new democratic states, several other countries followed suit. The Warsaw Pact, an alliance between Eastern European socialist republics, dissolved with the decline of communism.