United States by virtue of its status as a superpower after the end of the war, and many exiles United States European intellectual, naturally became the cradle of realism.
With the superpower status of the United States in the war after the end of their exile in the United States and many European intellectuals, naturally become a hotbed realist theory.
And at the end of the war American by virtue of its superpower status and many exiled USA European intellectuals, logical to become a source of realism theory.