The World War II-era alliance between the United States, Great Britain and the Soviet Union stood on a very fragile foundation. Mistrustful of each other before the war, the United States and Russia in particular eyed each other warily once Germany's invasion of the Soviet Union in June 1941 solidified the alliance against a common enemy. The democratic, free-market-oriented United States and the totalitarian, Marxist-Leninist colossus known as the Soviet Union had little in common save for their mutual interest in the defeat of the Axis powers (the U.S.S.R., Japan, and Italy). Serious differences between the two future superpowers over the strategy to defeat Germany, the aforementioned history of hostility, and concern, once Germany's eventual defeat was assured, regarding each nation's post-war ambitions for Europe and Asia guaranteed that the relationship between the U.S. and the U.S.S.R. remained tense. One thing was certain to the Americans, led by President Franklin Roosevelt: The enormity of suffering endured by Russia during the war and the now-extensive military power of the Soviet Union and its enormous role in defeating Germany meant that the wishes of Soviet dictator Joseph Stalin could not be ignored. Additionally, unsure just yet of the future of the war against Japan, Roosevelt continued to push Stalin to help out in the Far East. Just as apparent to Roosevelt, and Winston Churchill, however, was that a Europe dominated by Stalin would be intolerable. In short, the U.S. and Great Britain recognized that Stalin would have considerable influence over the structure of the post-war continent.
It was with these thoughts in mind that the three leaders -- Roosevelt, Churchill and Stalin -- met at the Russian town of Yalta from February 4-11, 1945. It was at this conference that the three leaders agreed in principle that the nation-states that were already falling under Soviet sway as the Red Army advanced towards Berlin would remain under Soviet domination after the war. Roosevelt and Churchill pushed, at least ostensibly, for guarantees that those East European nation-states would enjoy the opportunities of democracy, but Stalin was under no real obligation, given Soviet military domination of those regions, to abide by those agreements, and he would prove unwilling to do so. No sooner had Germany officially surrendered, then communist regimes were imposed throughout what would become known as the Soviet Bloc.
It has been widely speculated that Roosevelt's failing health contributed to his inability to negotiate terms at Yalta more favorable to the Western powers, and his death soon after the conference (he died in April) certainly lent credence to that speculation. As importantly, however, was his failure to keep his vice president at the time, Harry Truman, fully informed of the details of his negotiations with Stalin. At the Potsdam Conference later that year (July 18), an unprepared Truman, together with an equally unprepared British Prime Minister Clement Atlee (who had just replaced the politically-defeated Churchill), took the principles established at Yalta and formalized them, thereby institutionalizing Soviet control over Eastern Europe.
In conclusion, the nation-states of Eastern Europe fell under Soviet influence because the Red Army had a de facto hold on those territories in the course of Russia's war against Germany, because Roosevelt understood the depths of Stalin's paranoia regarding a future threat from a resurgent Germany (hence, that nation's division among the four victorious countries), and because Russia had suffered enormously under German occupation. That Soviet control over these countries would prove enormously repressive was an unfortunate reality that the U.S.S.R.'s growing military strength helped ensure.