Russian President Vladimir Putin has warned the West that Russia is technically prepared for a nuclear war, and that should the US send troops to Ukraine, it would be regarded as a considerable escalation of the war.
Does he know how fast he would be turned into radioactive dust if he starts a nuclear escalation with the US?
IRC a more realistic scenario is that they do an airburst strike (limits fallout) above an unoccupied (no civlians left) part of Ukraine as a show of force. Official Russian doctrine:
They think this will bring their enemies to the table.
I honestly don’t think it will, after all the atrocities they’ve committed. More likely, they’ll be fully ostracized, the west will give Ukraine even more weapons, Europe will enter the war in Ukraine, and the US will come with a conventional but devastating response.