First of all, the geographic position is perfect. Otto von Bismarck said: "Americans are  very lucky people. They're bordered to the north and south by weak neighbors, and to the east and west by fish.". Also, The USA is a vast land and they have many resources at their disposal. Finaly, The USA collects the best "brains" from all over the world - India, China, Russia. When put together these 3 things, you will have the most powerfull country.