Russia still has plenty of nukes, and they could fire them all before anybody has time to react. If that nuke is an ICBM, though, as soon as it leaves the silo the world would know, and the counter barrage of nukes would be firing up before it even lands.

Obviously an ICBM is armageddon. However a tacticsl nuke, one dropped from a plane or something onto Ukraine would be a different story.

The world will be far less inclined to launching ICBMs over that. So it’s just a game of how much they can get away with.