The reality is that in the Middle East the US has started a lot of wars and due to the vacuum left by the deposing of various dictators, radicals have filled the void.
China is depending of world trade and will ultimately lose trillions of dollars if it started attacking other countries
That wasn't the question of the hypothetical. The question asked what would happen if the U.S. withdrew its military forces from the rest of the world. Of course that asks us to make some assumptions, one of the major ones being that the U.S. would not bother/could be counted on not to intervene in any other conflicts that would inevitably pop up.
What you hypothesize about China is exactly what was said in Europe before WWI kicked off, i.e. "War is too costly and no one would benefit from it, so obviously things will be handled diplomatically from here on out."
Getting into all the possible scenarios is outside the scope of the thread (it would take up an entire series of books), but taking a look at modern China and its stance towards Taiwan (it's now called the Republic of China but "Taiwan" works better for shorthand), what do you think would happen? What about Japan? South Korea? Right now, it is very fair to say that U.S. military might prevents China and its satellite, North Korea, from invading at least Taiwan and SK. The lack of U.S. support would force a strong military alliance between Japan and SK and Japan and Taiwan. It would force Japan to build a military they believed would be capable of defending its nation. Would China wait until Japan re-militarized to that point?
As for Europe, Putin is currently the biggest belligerent there. Without the threat of conflict with the U.S., would he continue to pussyfoot around in Ukraine? Would Poland tolerate an all out incursion by Russia into Ukraine--would the rest of Europe? What about the former Soviet bloc nations?
Every month or so, someone on here posts this same idea, usually with the implication that the world would flower into some peaceful post-U.S. tyranny planet where everyone could finally work together now that America is out of the way. But the notion is silly. For all of America's fuck-ups, of which there are many, it does provide stability for both allies and enemies. And to think that the sudden loss of that stability wouldn't have massive consequences is foolhardy.