Describe the fall of the Communist Bloc in Eastern Europe and especially in the Soviet Union.

The "communist bloc in eastern Europe" came into existence
after World War II as the Soviet Union expanded its sphere of influence into the
countries of Bulgaria, Czechoslovakia, East Germany, Hungary, Poland and Romania.  This
expansion of communist governmental and economic philosophies was viewed as
self-protection by the Soviets and as unwarranted aggression by western Europe and its
allies. Over the years, incidents of intense conflict (the construction of the Berlin
Wall, the Cuban missile crisis) occurred, as did periods of comparatively peaceful
coexistence (partial nuclear test ban treaty in 1963, SALT I agreement banning
construction of new ballistic armaments).

As the 1980's
drew to a close, the Soviet Union was facing serious economic challenges within its own
country and was unable to continue sustaining the level of involvement and support it
had been providing to bloc allies. Soviet Prime Minister Mikhail Gorbachev began reform
movements ("perestroika," or reconstruction and "glastnost," or openness) "emphasizing
global interdependence and cooperation and the avoidance of force in the conduct of
foreign policy," with the goal of reviving the Soviet economy and helping the countries
of eastern Europe to find more support from the western world. He negotiated the end to
many of the Soviet Union's former efforts to test, build and maintain weapons arsenals
and put an end to the "Brezhnev Doctrine," which had been used to justify the use of
troops to keep communist governments in power in communist bloc countries. Once this
practice ended, communist bloc countries began to overthrow their communist governments
and broke away from the partnership with the Soviet Union. The varied republics within
the Soviet Union continued to rebellion from the centralized control of the past and
ended the USSR union in 1991, declaring themselves separate and independent
countries.