Using it as his justification for pulling out of NATO comes to mind.
Leaving NATO would mean closing US-bases in Europe overnight, not getting valuable intel from partners in NATO, jeopardizing US defense deals, and a million other things.
As always, it's grandstanding from Trump to get some extra bucks from his allies.
US pulling out of NATO would likely embolden China to make a move on Taiwan. Seeing how much of the US economy revolves around technology, I really don't think there's any other option than to defend Taiwan, as it stands. Sure - Europe also depends on chips from Taiwan, but they'd also be swamped in the Ukraine/Russia conflict.
* People who say "Trump isn't going to...!" and then how they act like it was all part of the plan when he actually does it.
* People who say "Trump is going to...!" and the how they quietly stop mentioning it and move on to the next (bad) thing he's going to do when he doesn't do it.
The size of that list in his prior term is a lot larger than many people are comfortable with, and the purge of insufficiently loyal members from the party as well as loyalty tests for appointees suggests much of that list is now back in play.
They can withdraw based on treaty itself or based on law of treaties (art 42)
What happens then of course only depends on what the sovereigns will want to do... In this case I'd presume it would not mean restarting WW2 after zillion years have passed. :-)))