What were the major causes of the domination of the USSR?

The Soviet Union was able to be dominant because its military occupied Eastern Europe immediately after World War II. By brutally ending any dissent by killing political leaders, religious figures, and the intelligentsia, the Soviets were able to kill and intimidate anyone who would stop them. This was seen as early as 1939, when the Soviet Union assassinated Polish officers in a massacre in the Katyn Forest. The Soviet Union also took a lot of the factories of eastern Germany back to the Soviet Union during the postwar occupation. The United States did not speak out against Soviet domination in Eastern Europe at the Yalta Convention, because Roosevelt was willing to do anything to keep the Soviets in the war against imperial Japan. The Soviet Union also took several German scientists in order to assist with the development of the atomic bomb. United States military planners projected that the Soviet Union would have the bomb by 1965, but the Soviets did their first nuclear test in 1949. Soviet military spending and atomic development meant that the United States could not dictate terms to the country without causing World War III. Even though the Soviet domestic economy was in terrible shape and its people were suffering, the country was quite strong in terms of weaponry.