What would happen if America withdrew troops globally?

I think it is long past time for a gradual handoff to begin. But we cannot create a power vacuum in our wake.

If we did just up and leave, you would see the Korean peninsula erupt, Taiwan collapse, the South China Sea fall completely under Chinese control, Eastern Europe including Ukraine & likely the Baltics come under Russian control, NATO collapse followed by turmoil in Turkey, and much, much more.

As for what would happen.

In the pacific, two possibilities. Pacific nations such as Singapore, Australia, the Philippines, Japan, South Korea, New Zealand, and others form their own defensive coalition to check Chinese influence in the region. The US loses influence in the region, but China gains little and the independent coalition deals with both powers. If the coalition fails, or is never formed, it's very likely that Chinese hegemony prevails in the Pacific. The smaller nations, many of which already have China as their main trade partner, would also be beholden to China for security. This gives China very powerful influence over the pacific nations, and makes the US the outsider. Pacific trade and politics will be on China's terms.

In the Middle East. Saudi Arabia is already preparing for this contingency. Maybe not to much change there except less favorable oil prices. They will however continue their political and ideological war with Iran. Without US support on SA's side, Iran and likely ally Russia make significant against Saudi Arabia and their regional allies. Continued strife in the region may even erupt into a formal war between nations. In any case, Turkey, Iran, Syria, and possibly Iraq will all move further toward Russia.

In Europe. Western Europe is able to check Russia to some extent, but eastern European nations are on shaky ground. Several of them likely reenter the Russian sphere. The Baltic states may try to align with a Scandinavian pact, but the entire region will find itself in limbo, torn between wealth of Western Europe, and the force of Russian arms.

What does this all mean for the US?

We lose influence around the world. Security treaties, economic bargains, and political leverage all become more difficult for the US. Ever notice how almost every major US military endeavor since WW2 has included Australians? Won't happen anymore if they're busy with their own security, or under the Chinese political sphere. US economic investments abroad become riskier in regions where the US doesn't wield as much influence. Investment in US interests also becomes less attractive for foreign investors, for a similar reason. The US is less capable of advancing it's interests in the global market. Nations don't cooperate with one another out of charity or civility. They do it to advance shared interests. If the US withdrawals from the world (not just militarily, but economically as well), then other nations will have fewer shared interests, and less motivation to cooperate.

I think the world would be more peaceful in most places. The UN should be the body that keeps the peace because they are closer to the consensus of world opinion and have less bias for favoured regimes.

There is a part of me that would like to see this. But I agree with @SaraBee1995, and you for selecting hers as the best answer! A gradual handoff would be good; a power vacuum would not. We learned, or should have, in the lead-in to World War Two what can happen if we adopt an isolationist attitude to the rest of the world... tempting though it is, sometimes, to do so.

North Korea would feel emboldened and invade South Korea, which is still reeling from it's first female President being impeached for corruption charges.

Russia would probably take bigger chunks out of the Ukraine.

China would probably force Taiwan back into the fold and continue to expand into the Pacific.

isolationism is an idea i reject. Simply withdrawing troops from east asia and eastern europe along with other parts of the world without considering any sort of repercussions such as stability in core regions will have dire consequences. I'd much rather see more embassies and diplomacy efforts carried out instead of military bases but that wont happen in the next 4 years seeing that president trump has destroyed the state department budget unfortunately.

a very simple answer for a very complex question......YES!

Yes, please, the sooner the better.

yes I think so. I don't really care what happens to Korea, The Ukraine and so on. Let them bitch and moan over there while killing one another.

America won't withdraw troops cause... money.