Here are some factors that influenced the outcomes in Eastern Europe after the war:
1. The Teheran and Yalta Conferences: At the Teheran and Yalta conferences in 1943 and 1945, respectively, the leaders of the three Allied powers - Roosevelt, Churchill, and Stalin - discussed the plans for the postwar world. Although there was an agreement on the need for a collective security arrangement, the exact shape and boundaries of the postwar order in Eastern Europe were not clearly defined.
2. Soviet Occupation: The Soviet Union had already occupied much of Eastern Europe by the end of World War 2. The presence of Soviet troops gave Stalin a significant advantage in negotiating the postwar arrangements.
3. Western Preoccupations: The Western Allies, primarily the United States and Britain, were focused on defeating Nazi Germany and subsequently rebuilding their economies and societies after the war. This may have diverted some of their attention from Eastern Europe.
4. Realpolitik Considerations: The Allies recognized that the Soviet Union's contribution to the war effort, particularly its vast sacrifices on the Eastern Front, warranted a level of influence in the region. There was also a desire to avoid a direct confrontation with the Soviet Union, which possessed a formidable military power.
5. Lack of Consensus: There was no complete agreement among the Allies about how to deal with Eastern Europe after the war. The Western Allies had different perspectives on the legitimacy of the Soviet-backed governments installed in countries like Poland, Hungary, and Romania.
It is important to note that the domination of Eastern Europe by the Soviet Union was not entirely a consequence of Roosevelt and Churchill's decisions. Instead, it resulted from a combination of factors, including geopolitical realities, military strength, and the competing priorities of the Allies during and after World War 2.