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.