The key terms here are "superpower" and "sphere of influence." America is not an empire, 19th century or otherwise. You have to twist and warp the meaning of the word "empire" to apply it to the United States. If that's OK to do then let's just describe nations and governments based on our personal feelings rather than fact.
And no, Macbeth, Portugal was a colonial empire. They didn't just "empire around" with trade.
And no, Macbeth, Portugal was a colonial empire. They didn't just "empire around" with trade.