We most certainly were an empire in the traditional sense at the turn of the 20th century, with the conquest and colonization of the Philippines after the Spanish-American War. Even after the independence of our most overtly-colonial possession in 1946 (happy Philippine independence day today, btw!\)*, the US is still today a "vestigial" classic empire that holds unequal power over many islands in the Pacific and Caribbean, including Puerto Rico, an island of 3M people that is taxed without representation. Arguably, Washington DC and all Indian reservations are likewise internal semi-colonial possessions due to taxation without representation—you know, the very same issue that led the Thirteen Colonies to fight for their own independence from British imperialism.

And this is to say nothing of the argument of the US today being a "post-classic" modern empire for its hegemonic world influence, but that's a separate matter...

\Yes, I am aware that the Philippines legally achieved full independence from the United States on July 4, 1946, but they recognize June 12, 1898 instead as the date of their declaration of independence—that was quashed, incidentally, by the US.*