"The Americans have told the Russians that if you explode a nuke, even if it doesn't kill anybody, we will hit all your targets (positions) in Ukraine with conventional weapons, we’ll destroy all of them," Polish Foreign Minister Radoslaw Sikorski told the Guardian.
The US was clear that a use of just a singular nuclear weapon in Ukraine would result in an overwhelming kinetic response from the US toward Russian invaders in Ukraine. I can easily see other countries, particularly the Baltic states, going much, much harder than that from the outset.
Nukes cannot win the war for Putin, the only thing it can do is force Russia to lose in increasingly spectacular ways.