America stands alone

I was watching the intro movie for the game "Freedom Fighters", and was reading the little historical blurbs that popped up on the screen. In this game's universe, the Soviets developed the A-Bomb first, and used it to wipe out Berlin in 1945, forcing a surrender of the Reich to them. By 1953, the Soviets had succeeded in absorbing all of Europe, even Britain, in a more or less peaceful fashion(!). Anyway, my question is: If Europe fell completely into the Soviet camp, what would American policy be? Starting a "war of liberation"? Setting up a new anticommunist league with whoever's left? Or simply self-imposed isolationism?
 
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