Is the United States an imperialist country? If one conflates the United States sovereign with those who own the politicians who fill the legal positions created by the sovereign's organic documents (which it is perfectly reasonable and understandable to do), then the answer is yes. The United States in an imperialist country.
However, if one considers only the aspirational intent laid out in those same documents, and looks only what the United States is supposed to be, then the answer is no. The Unites States is not an imperialist country.
An example might help. If Joe Schmo goes over seas and runs afoul of another sovereign's laws (or the chosen implementation of those laws by those acting under color of law), regardless of how far those laws or the implementation may stray from traditional notions of fair play and substantial justice, Joe is fucked. He might get a token condolence, a tip of the hat, or a phone call from the U.S. Embassy; or his name might even come up at a wine-and-cheese-cocktail- party at some dignitaries house. Someone might even visit his family's home here in the U.S. and offer condolences and promises that everything under the sun is being done. But in the final analysis, the U.S. doesn't give a shit and he's fucked.
Compare: If large corporation X has a little problem in some foreign country (like some uppity peasant has actually persuaded the local U.S. proxy government to consider letting him have the land he has been on since Christ was a Corporal, and the oil under it) then gun-boat diplomacy is on the table. I know, I know; before all you historians try to correct me with your irrelevant BS, try considering the gist of what I am saying. Nothing but the names and the sophistication and nuanced methodologies have changed between 1905 and today.
If Americans were to ever themselves get uppity and back that peasant's (or his government's) hand, we would then learn the difference between what we aspire to be and what we really are. And we'd have to listen to a metric shit-ton of wailing and whining and crying about "socialism" and "communism" from a bunch of anti-intellectual, uneducated, conservative Republican stupid fucks who wouldn't know socialism, communism, or representative democracy if it hit them over their pointed little heads.