Fuck off. My words would explain MAD, just like wikipedia did, but I'm too lazy to rewrite wikipedia for someone who has the knowledge of a child and an allergy to reading.
Edit:
If Ukraine got a nuclear weapon and Russia continued to do what it’s doing without using your nuclear weapons.
Stop being ignorant. If Ukraine had nukes, Russia would never had invaded. Your scenario is preposterous and you appear oblivious to even rudimentary geopolitics or military theory. Litteral 12 yo children get this. Why don't you?