Answer:
Stalin felt the Soviets Union needed the Eastern European nations as satellites to protect their own interests. The fact that Nazi Germany had invaded Germany in World War II and millions of Soviet lives were lost provided Stalin's justification for loyal states along the Soviet border.
Historical context:
US president Franklin Roosevelt, British prime minister Winston Churchill, and Soviet premier Joseph Stalin, the leaders of the Allies in World War II, met at Yalta in February, 1945.
Churchill in particular (along with Roosevelt) pushed strongly for Stalin to allow free elections to take place in the nations of Europe after the war. At that time Stalin agreed, but there was a strong feeling by the other leaders that he might renege on that promise. The Soviets never did allow those free elections to occur. Later, Winston Churchill wrote, ""Our hopeful assumptions were soon to be falsified." A line of countries in Eastern Europe came into line with the USSR and communism. Churchill later would say an "iron curtain" had fallen between Western and Eastern Europe.
