The US may think they are invulnerable, but they are far from it, the world can crash the US economy in a couple of hours.
Problem is it would hurt the rest of the world too, although not as bad as it would be for USA.
But there needs to be very strong arguments for doing it. Still there has already been hints of the “nuclear” option by EU against USA if USA continues to behave as poorly as they have this past year.
Also China has its own nuclear option in just selling off US treasury bonds. That would make it very hard for USA to finance the federal deficits, and could collapse the American federal government.

The US economy depends on the rest of the world to be the size it is. And without the economy to finance the military, USA will not be able to maintain a military the size they currently have.