The Cold War originated in an accumulation of tensions and conflicting interests between the Soviet Union and the Western powers which developed in the first few years after the end of World War II and which ripened into permanent hostility in the years 1946-1947. Hitler's aggression forced them into a wartime alliance of convenience which worked effectively to defeat fascism and Japanese militarism but which left power vacuums in both Europe and Asia which were largely filled by the new victorious superpowers, the United States and the Soviet Union.
As the Red Army swept West, the Russians installed in Eastern Europe regimes friendly to it, and eventually eliminated non-communist opposition. Despite collaboration between Allied and Soviet military leaders in the closing phases of the war, tensions rapidly developed in 1945-1946 over the domination by the Soviets of governments throughout most of Eastern Europe. Shortly before his death, President Franklin Roosevelt reportedly said: "Averell [Harriman, American Ambassador to the Soviet Union] is right . . . we can't do business with Stalin . . . He has broken every one of his promises he made at Yalta" (Thomas, 1987, p. 121).
FDR's successor, President Harry Truman, was reluctant in 1945 to rupture relations with America's recent Soviet allies and wavered in the face of conflicting advice. According to Chace, "in 1945 and 1946 American foreign policy fluctuate