World War II. This event marked the end of American isolationism and neutralism and the beginning of foreign and defense policy of intense internationalism. ... The U.S. economy was the only major economy that came out of WW II intact. This made the country an instant economic superpower.
Before 1763 the British policy of salutary neglect toward its American colonies was based on on the fact that they considered the colonies to be a direct extension of the Crown.