"If the United States wanted, we could have had almost total world dominance after World War II. However, since we are not like the Romans or Age of Imperialism Brits, we decided to give the land back.

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Major problem when a major nation has a major delusion of power.
I agree with the notion of the Roman empire being one of the most influential of it's times. Considering how vast it got (for it's time, you still had to send men from one end to the other to communicate!) and how much it accomplished before the dark ages began.
However, I'd say ranked up there would also be Natzie germany. This is due to the fact that a little nation like germany managed to pull out of eneconomic debt to well, about half the world, and then become a major threat to the rest of the world that was barely stopped.