What led the United States and the Soviet Union to go from being allies to bitter Cold War rivals?

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The Soviet Union had proved a useful ally to the United States during World War II. But once the war was over, the old tensions between the world's largest capitalist and Communist powers resurfaced. In the immediate post-war period, the Soviet Union found itself in control of vast swathes of...

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The Soviet Union had proved a useful ally to the United States during World War II. But once the war was over, the old tensions between the world's largest capitalist and Communist powers resurfaced. In the immediate post-war period, the Soviet Union found itself in control of vast swathes of territory in eastern Europe. This alarmed the Americans, who felt that the spread of Communism represented a real danger to the stability of Europe.

There were long-established, well-organized Communist parties in western European countries such as France and Italy. These parties routinely toed the Soviet line and were potential agents of subversion in the countries in which they operated. The United States was worried that what had happened in eastern Europe could just as easily happen in the West. American concerns at growing Soviet influence in Europe were the main impetus behind the Marshall Plan.

The differences between the United States and the USSR were largely ideological. Yet so long as they shared a common enemy, as they had done during World War II, some kind of accommodation was possible. But with Hitler and the Nazis out of the way, it was inevitable that, sooner or later, the two superpowers would revert to seeing each other as serious threats to their respective spheres of influence.

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The US and the Soviet Union were only allies in WWII because they were both so worried about Nazi Germany.  It was not as if the two countries had any natural reason to be allies.  That means that we should not be surprised that they went back to being rivals after WWII.

The two countries were "natural" enemies because their systems were opposed to one another.  Communist theory held that communism was inevitably going to take over the world and the Soviets wanted to help that process along.  The US, meanwhile, thought that communism was evil and that democracy was the best system.  The US's fear of communism and its expansionist tendencies had led to bad relations between the two countries from the time the USSR was established until the early 1930s.  This enmity simply came back to the fore when the threat of Nazi Germany was gone.

Overall, then, these two countries became rivals because of their political/economic systems.  The differences between their systems overcame any inclination they might have had to be friendly because of their wartime alliance.

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