A lot of US expansionism in the modern era is a holdover from the Cold War. Back then, politicians believed in "domino theory"- if you didn't stop communism on all fronts, the world will slowly collapse into a communist hellhole. We sent money to many nations because they were poor/under threat of commie invasion/functionally retarded/whatever. However, when the Soviet Union collapsed in 1989, we had the chance to pull back. We didn't take it, because the corrupt powers that be liked the arrangement.
So the USA became the world's abused parent: constantly insulted, put down, taunted, and mocked on the world stage by all and sundry, before the world demanded its allowance. And for thirty years, we gave it to them.
I'm reminded, in a way, of the world of Atlas Shrugged. In that book, the world had collapsed into socialism and existed only on handouts from America. Meanwhile, they were dismissive of the USA, and corrupt Americans were saying, "Yeah, look at the world! It'll have us beat in ten years if we don't appease them!"