Answer:
United States were and are psychically divided from the rest of the world. That is why for a long period of time they were not engage in any conflict on their territory. Even when they waged wars, it was on the territory of some other country.
Step-by-step explanation:
Physical Isolation, helped with politics of isolationism enabled United States to develop as a leading force in the world. By using their resources and constantly spreading their territory to the West, they strengthened themselves from the inside, without being disturbed for a longer period of time by any other country.