Detente period in cold war, means relaxation of tensions; establishment of a working relationship between USSR and U.S usually for trading purposes.
Detente ends w/ Soviet invasion of Afghanistan, ends similarly like U.S in Vietnam.
Gorbachev(leader in power of USSR in 1980's) sees that USSR is in decline, after the cost of both the arms race and the war in Afghanistan. Implements two crucial reform efforts Perestroika and Glasnost. Basically reduced state control of economy and greater freedom of press, criticism of political corruption and so on.
Gorbachev also hoped to reduce superpower tensions, being friendly w/ the West, and negotiates with them. Also loosens USSR grip on Eastern Europe, which soon leads to the freedom of Eastern Europe and the movement's culmination in the fall of the Berlin Wall, ending the symbol of communism in Eastern Europe.
Freedom of Eastern Europe spurs Anti-Soviet nationalism in USSR, encouraged independence of non-Russian parts of the country
Republics formed, wanted formation of a post-Soviet federation.
On christmas day, 1991, Gorbachev resigned as Soviet leader and and end to the USSR.