Those are some rather hard questions to answer.
As for the threat posed to the United States?
Probably nobody.
World War 3 will be the US seen as the 'evil' and literally the world against them. there will be a few exceptions of course, but they will be taken
out of the war early, since they are virtually surrounded by the enemies of the US.
China will stay put, simply because the US is that intimidating, and China doesn't want to open that can of worms just yet.

