The the World War II, the Soviet Union had its ambitions of controlling everything west from West Germany, which meant all of Central and Eastern Europe.
Considering the military and political power of the Soviet Union in that period, and the situation in which the countries of Central and Eastern Europe were, it was a relatively easy task. The Soviets managed to install their own people all across the region. Made the communism the most popular doctrine. But also, they were always having military presence, and whenever something was going against their will they were brutal.
Population grew significantly mostly due to reproduction, but there was a significant number of immigrants, Scots-Irish and Germans, coast was more wealthy than west.
