The Allies began to quarrel almost as soon as the unifying threat of Nazi Germany disappeared, and the interests of the Communist Soviet Union and the capitalist Britain and United States increasingly diverged. The hostility between the Eastern and Western superpowers was the sad but logical outgrowth of military developments, wartime agreements, and long-standing political and ideological differences.

Once the United States entered the war in late 1941, the Allies had made military victory their highest priority, deferring discussion of postwar aims and the shape of the eventual peace settlement. By late 1943, however, negotiations about the postwar settlement could no longer be postponed. The conference that the “Big Three” — Stalin, Roosevelt, and Churchill — held in the Iranian capital of Teheran in November 1943 proved crucial for determining the shape of the postwar world.

At Teheran, the Big Three reaffirmed their determination to crush Germany, followed by tense discussions of Poland’s postwar borders and a strategy to win the war. Stalin asked his allies to relieve his armies by opening a second front in German-occupied France. In the end, this was agreed to, though the date for the invasion was set later than the Soviet leader desired. This decision had momentous implications for the Cold War. While the delay in opening a second front fanned Stalin’s distrust of the Allies, the agreement on a British-U.S. invasion of France also ensured that the American-British and Soviet armies would come together in defeated Germany along a north-south line, and that Soviet troops would play the predominant role in pushing the Germans out of eastern and central Europe. Thus the basic shape of postwar Europe was cast even as the fighting continued.

When the Big Three met again in February 1945 at Yalta, on the Black Sea in southern Russia, advancing Soviet armies had already occupied much of eastern Europe and the Balkans and were within a hundred miles of Berlin. The stalled American-British forces had yet to cross the Rhine into Germany. Moreover, the United States was far from defeating Japan. In short, the U.S.S.R.’s position on the ground was far stronger than that of the United States and Britain, which played to Stalin’s advantage.

The Allies agreed at Yalta that each of the four victorious powers would occupy a separate zone of Germany and that the Germans would pay heavy reparations to the Soviet Union. At American insistence, Stalin agreed to declare war on Japan after Germany’s defeat. As for Poland, the Big Three agreed that the U.S.S.R. would permanently incorporate the eastern Polish territories its army had occupied in 1939 and that Poland would be compensated with German lands to the west. They also agreed in an ambiguous compromise that the new governments in Soviet-occupied Europe would be freely elected but “friendly” to the Soviet Union.

The Yalta compromise over elections in these countries broke down almost immediately. Even before the conference, Communist parties were gaining control in Bulgaria and Poland. Elsewhere, the Soviets formed coalition governments but reserved key government posts for Moscow-trained Communists. At the Potsdam Conference of July 1945, the differences over elections in Soviet-occupied Europe surged to the fore. Roosevelt had died and had been succeeded by President Harry Truman (r. 1945–1953), who demanded immediate free elections throughout central and eastern Europe. Stalin refused.

Stalin, who had lived through two enormously destructive German invasions, was determined to establish a buffer zone of sympathetic states around the U.S.S.R. and at the same time expand the reach of communism and the Soviet state. He believed that only Communists could be dependable allies, and that free elections would result in independent and possibly hostile governments on his western border. With Soviet armies in central and eastern Europe, there was no way short of war for the United States to control the region’s political future, and war was out of the question. The United States, for its part, pushed to maintain democratic capitalism and open access to free markets in western Europe. The Americans quickly showed that they, too, were determined to maintain predominance in their sphere of influence.