This is coming off as deliberately obtuse.
If you have a nuke, and someone wants to take your shit or kill you, they will be deterred from doing so. Will anyone ever use a nuke in this situation? Who knows, but it would probably cause a world war. Millions, if not billions could die. Governments could collapse. Entire species could be wiped out. The results are nearly unfathomable. I don't think anyone wants to be the cause of that.
If Ukraine nuked Russia, or vice versa, it would likely result in the destruction of both countries regardless of who the nuke hit, hence the previous commenters wiki link to mutually assured destruction.
It's uncharted territory for humanity, and no one wants to fuck around a find out.