About the "American Empire" thing: First of all: In a different era the USA would not exist at all, the only reason they could exist at all is because of modernization and industrialisation, and because of that they can't hold the territories mentioned before. If the USA would try to hold on to any of the lands mentioned before they wouldn't have been the "sole unquestioned superpower" for 20 years...
That's the problem, yeah they're strong, but they can't take territory unless the territory wants to be taken by the USA... The USA is strong because it has a united people behind it, if it starts conquering unwilling countries they would have an unwilling people behind them and they would fall apart, or give the territories back...
I agree they're a superpower, I agree they're the strongest superpower out there right now, but they're just not an empire. They're strong because their people wants it, there's no way they could expand unless the conquered want it, no matter what's the era.
That being said I like the USA and hate European countries, especially my own, and I'm happy they can protect, and I'm sure they can destroy, but they just can't conquer and that and the fact that it's ruled by the people (which is in fact pretty much the same argument, but who cares :P) means it's in my opinion not an empire.