The World War II alliance between the United States and Russia was a relationship of mutual interest—but a relationship that was also almost certain to break down once the common enemy was defeated. Since the Bolshevik Revolution of 1917, mistrust between the newly-established Union of Soviet Socialist Republics and the United States had been the defining characteristic of the two countries’ relationship. The Soviet Union was founded on an ideological and geopolitical opposition to the Western powers, and it took the German invasion of the Soviet Union in June 1941 to force the two sides together. Throughout the war, however, that mistrust remained just below the surface of the alliance and only grew with Soviet leader Joseph Stalin’s determination to control as much of Europe and Asia as he could once Nazi Germany and Imperial Japan were defeated. Wartime meetings between the leaders of the alliance—Franklin Roosevelt, Winston Churchill and Stalin, with Harry Truman succeeding Roosevelt towards the end of the war—were characterized by clashing visions of a post-war world. The Cold War had, in effect, begun well before the fall of Berlin.
With Japan’s attack on the United States on December 7, 1941, the latter country was forced once and for all to cast aside its aversion to what Thomas Jefferson referred to as "entangling alliances." By war’s end, former imperial powers Great Britain, France, and Japan were each financially broken and too militarily weakened to continue to hold onto their respective empires. Indeed, in the case of Japan, the destruction wrought by years of American bombing (culminating in the atomic bombings of Hiroshima and Nagasaki) and its complete subjugation to the United States resulted in a very fundamental transformation from militaristic imperialist to subdued democracy. The United States, having transformed its economy during the war into the greatest military-industrial machine in history, was the sole major Western power. The Soviet Union had similarly emerged from the war a major military power, despite the enormity of the physical and emotional destruction it had endured following Germany’s invasion.
The effect of World War II on US foreign policy, then, was to force the United States into the role of leader of what was called "the Free World." America fashioned itself into a bulwark against the perceived threat of Soviet expansionism. The bipolar structure of international relations that defined the world at the end of the war consisted of a US-led alliance facing off against the Soviet Union and its Eastern European satellites (i.e., those countries scuh as Poland, Hungary, Czechoslovakia, and East Germany, which fell under the Russian orbit as a result of wartime agreements and the reality of Red Army occupation of these territories). US foreign policy was a direct outgrowth of the post-war international structure that emerged from the devastation of World War II.