“with all the trouble Russia is having with Ukraine how they going to fight a world war?”
ICBMs and hypersonic reentry vehicles.
I don’t think they would start a conventional war that could result in a nuclear war if they are likely to be defeated in a conventional war. So getting their butt really kicked in by Ukraine would just make that possibility more unlikely. Also beating Russia would make it more unlikely that China would invade Taiwan or even North Korea would be discouraged from invading the North.