When Mikhail Gorbachev became General Secretary of the Soviet Union in 1985, the USSR saw severe economic stagnation, technological backwardness, and growing public disillusionment. Gorbachev introduced two major reform programs: glasnost (openness) and perestroika (restructuring).
Glasnost relaxed censorship, permitted discussion of previously taboo topics like Stalin's crimes & Soviet history, and enabled independent media and public discourse. Perestroika introduced economic reforms (limited market mechanisms, reduced central planning, and allowed some private enterprise) as well as political reforms (competitive elections and anti corruption campaigns)
Between 1989-1991, Eastern European communist regimes collapsed, the Berlin Wall fell, and Soviet republics declared independence. After a failed hardliner coup in August 1991, the USSR formally dissolved on December 25, 1991.