I think America's presense as an imperial power tends to wax and wane. It probably reached a zenith during the Vietnam conflict, and is coming back somewhat now (also acknowledging that we gave France money in the 50's via the Marshall Plan to prop up its military efforts in Cambodia and Vietnam). What makes America a unique hegemon is that our culture seems to be what is most offensive to other countries, our McDonalds and Nikes more threatening to them than bayonets and missiles.
Whereas England occupied various territories, America has no such comparable history; Panama, Grenada, and eventually Afghanistan will stand historically as short-term occupations where we did not extract significant levels of a countries resources. But the conversion and domination via the export of one's culture is probably the most threatening aspect of America to the world today. Cultures want to retain what makes them unique.
Overall, I don't see our country as an overbearing imperial power.
