re: How will the United States end?

Tough call.

The primary reason earlier civilization lasted that long was the slow development of technology and government systems that were largely based on genetic succession. The large amounts of manpower needed to sustain civilization in the form of slavery and conquered lands also played as a motivator and ultimate detriment.

I severely doubt there will be civil war any time soon. Both parties have too much to lose whether they admit it or not.

I give the US a solid 500 to 600 years. We wont ever be defeated militarily, but will probably divide culturally into multiple different nations. This is a hard right wet dream until you realize that each nation has it's own military, replacing a superpower with 4 to 5 separate weaker nation states.

It will then be a matter of Russia, China, or some other growing power becoming the new superpower. This would be long after the US divided