The only reason Russia is invading Ukraine is because they feel threatened by the USA and NATO and they need a buffer for their own defense. If USA left NATO and especially if Russia joined after that then Russia would not feel the need to secure a buffer between NATO and Russia.
China probably would not feel as threatened either if the USA were not as involved in Asia as it is currently. I think the world could actually reach a new level of peace if President Trump would withdraw the USA from NATO and isolate the USA from world politics.
Just a little context, as Russian, why we have problem with NATO in Ukraine - not in modern times, but some context before 2014.
Crimea is the base for Russian black sea fleet.
NATO was trying to push Russian black fleet out of there, make military training there (https://www.theguardian.com/world/2006/jun/08/russia.nickpatonwalsh) and visit military ships there (protests against it in 2013 https://www.youtube.com/watch?v=ELNuYLaGV5s).
And Crimea population, being basically Russian population, was greatly against it, and had to stand on it's own against Yuschenko, until Yanukovich came to power.
If Ukraine joined NATO Russia would lose it's black sea fleet.
Oh sweet summer child comments on this video in YT from 10 years ago... mocking this activists, not knowing what will happen in a year.
"Activists like NATO - SERVANT OF THE ANTICHRIST, BANKS, VACCINES, CASHLESS PAYMENT 666."
And some really knowing what's happening:
"Sevastopol will become a hero twice, you will finish playing Yanukovych and co."