Nobody would call the US an "empire" if all it did was keep in its own territory and stay out of other country's internal affairs. Nobody calls Canada an empire for example. But the US keeps deciding to outright attack other countries or uses the CIA to subvert internal politics (and sometimes to overthrow democratically elected governments e.g. Iran).
The US deserves to be accused of having imperial intent, even if just based on their interest in other country's oil.
The word you're looking for is "imperial." "Empirical" means something else entirely.
http://www.amazon.co.uk/Dangerous-Nation-Americas-Earliest-T...
America has had the most powerful military on earth for 70 years, along with a large population. List all of the vast amounts of territory it has annexed using that extraordinary military power. There are endless opportunities to do so, and the US is the sole military with global projection.
Some people point to military bases. Which doesn't work in any regard. A few critical differences being: taxing power on the local population, or vast plunder. Neither of which America is known for. And US military bases around the globe overwhelmingly exist by permission.
Controlling territory but not extracting wealth is pointless. The U.S. doesn't extract wealth from its foreign bases; in fact it injects wealth because the bases are paid for by U.S. dollars but the downstream spending goes into the local economy.
And you might think "well the U.S. extracts the wealth through trade." But trade is mutually beneficial (unlike taxes) and the U.S. runs a foreign trade deficit anyway.
The things that the U.S. gets for its projected power are peace and stability. These are things that benefit any nation, though.
