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.