First off Ukraine and the Balkan nations and most of eastern Europe don't have nuclear weapons. They by themselves won't launch an invasion of Russia Russia is the aggressor, they may or may not raid. But any invasion or attempted occupation will get crushed, it's like Mexico or Canada invading America they aren't in the same league. Second if you are surrounded on multiple sides then you are fucked and will lose no matter what unless you are led by a military genius. So you may as well concentrate your forces to try and break at least one of your enemies instead of keeping them weak and seperate so they can slowly be defeated. Three yes nukes would be involved, this is the whole point of MAD. If the Soviet Union managed to conquer all of Europe and beat the US Navy and launched an invasion of CONUS are you saying America would not launch because "the soviets would then level us with nukes also!" The point of MAD is that it prevents nations with nukes from having wars to the knife. Russia and China may fight each other using proxies in central Asia. But China invading actual Russian territory, unless those troops pull the fuck out of there, nukes will fly. What does it matter to Russia whether they get their nation destroyed conventionally by regular bombs and soldiers or by nuclear weapons. Nuclear weapons prevent full scale wars because they allow the side that is losing conventionally to flip the chessboard over and kill everyone.