If we tried to take over say Canada and Mexico for instance, being the easiest targets, the rest of the world will rise up against us.
Sure, we have a huge military and a lot of nukes, but they have nukes also and a hell of a lot more money and manpower.
The only way we can win would be to launch a preemptive nuclear strike against the eastern side of the planet and assuming that we don't get nuked back to the Stone age in return immediately launch a strike against Canada and Mexico while the rest of the world is cleaning up after the shit we started.
9 times out of 10, almost everyone in America dies, and the rest get to spend 150 years paying back for the damages.
1 time out of 10, we become king shit of a hell hole of our own creation.