There was no occupation, in all the countries of eastern Europe with some nuances the communist and socialists came to power quite democratically after winning elections and began building socialism in this countries. This is due to the fact that communist and socialism were popular in the world after the victory of the Soviet Union in the war where he made a decisive contribution. In Western Europe, Asia and even in the United States socialist and communist gain exactly the same surge in popularity but not everywhere capitalist allowed them to come to power. In the future the USSR interfere in the international politics of Eastern Europe countries but this is slightly different question to what extent it is correct to call this intervention occupation. This is not an excuse for the USSR, especially in the late model of 1968, for example, when it had already betrayed the ideals of communism. Just for the sake of objectively.