Nazi Germany was an Empire, because the term refers to a form of government and state structure, Nazi Germany, although it had no emperor, had an equally powerful figure as "Führer", and tried by all means will return a quasi-feudal system but with greater freedom economy.
The United States is not an empire, it is a federal constitutional republic. If you mean that the United States is the first world power and the dominant power, I give you the reason, in the bad and in the good that this implies.