If the Russians were to use nuclear weapons, the most likely scenario is that they would use them in Ukraine. And Ukraine does not have nuclear weapons of its own. So the Ukrainians would not be able to retaliate against the Russians with their own nuclear weapons. That weakens deterrence. Furthermore, if the Russians use nuclear weapons in Ukraine, the West,. and you were talking mainly about the United States, is not going to retaliate with nuclear weapons against Russia. If we were to do that, we would then have a great power war. NATO would be at war against Russia.