I like to say this is the "the enemy of my enemy is my friend" syndrome. During the war, we allied with Russia because we wanted to defeat Hitler. Russia was Hitler's enemy, so that made them no exactly on our side but basically our ally. After the war, we went back to being enemies.
It is interesting that a previous poster pointed out the fact that FDR trusted Stalin more than he trusted Hitler. I would in fact argue that it isn't clear who he trusted more. Hitler certainly thought that the U.S. and the U.K. would eventually come over to join him in his fight against those he considered less than human. He felt they had a shared heritage and there were certainly many in the U.S. that felt positively about the Nazis. (read about Lindberg for one) You might also look at the difference in the way we treated prisoners of war from the German army compared to how we treated Japanese American citizens if you want a clear picture of who we were really friendly towards.
But the U.S. and...
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