Along with that adventure in Hawaii, the Spanish-American War was our one shot at European-style imperialism. Rudyard Kipling even wrote us a poem about it at the time. But it didn't work out and we gave the Philippines back to its own people by 1946. (Puerto Rico isn't independent because they don't really want to be, same as Hawaii.)
The continental US west of the original colonies was an interesting case. We only got the bulk of it because Napoleon put the entire Louisiana Territory on fire sale--he wanted to cut his losses in America after losing Haiti and needed the money to try and conquer Europe. We only really wanted the Mississippi River delta to begin with. We did kind of pick a fight with Mexico, but we didn't conquer the whole country, and even paid for the parts we took. Oregon was a disputed claim and we split it down the middle with the British. And Alaska we paid for fair and square. As for the American Indians, you have a point there but frankly smallpox had already depopulated the continent and it was practically empty. If it wasn't, I don't think anyone would have been able to conquer America from the American Indians--nor would any colonial power.
When people call the US an empire, they usually aren't thinking about the Spanish-American War. They're thinking about all the countries we station troops in (most of them asked us to), or all the Coca-Cola we sell (which the world voluntarily buys). We have a big sphere of influence because not too long ago, the world needed us to. We were the only democratic allied power left largely untouched by WWII, and our "imperialism" at the time consisted largely of a Marshall Plan designed to rebuild a continent that destroyed itself twice over, both times somehow roping us into the action.
Can you honestly think of a country the US has dominated against its will since the Philippines? You might be able to make a case for South Vietnam, and you could name maybe a dozen countries we covertly subverted, but outright domination? We didn't hold a candle to the Soviets.