Earlier quoted context omitted.
demonstration nuke - it's called tactical nuke, which is different to strategic nuke. Imagine Ukrainian army winning and move towards Russia and Belarus. Putin may decide to fire a tactical nuke to "escalate to de-escalate"
If the Ukrainians push the Russians back to Russia then that is a de-escalation. The Ukrainians are not the aggressors here. I see no reason Ukraine would cross their own borders. If Putin decides to escalate to nukes as a final act of failure then the de-escalation would be the complete annihilation of Russia as a country. In minutes. The idea of getting away with a tactical nuke strike is insane.
to strengthen their negotiation position "We give you back X, you give back Mariupol"
> complete annihilation of Russia as a country
you think US would die for some village in Ukraine? They were plenty of US politicians and generals which were against nuclear umbrella for West Germany.