Russian forces are on their last legs, they can barely fend off under-equipped Ukrainian recruits.
Putin may possibly use a tactical nuke, but if he does, his regime is over as China and India will turn against him. Remember that they have not recognized Donbas or Crimea as Russian territory either.
With Western intervention, Putin would basically be forced to some kind of 'Giant Saving Of Face' withdrawal, which my bet would be something like: "Donbas and Crimea maintain their independence, Russian language and culture is allowed, Russia can have monitors to ensure their are no 'Nazis!' (which is bullshit in the first place), and possibly some citizens can keep their citizenship'
But in reality, it's a Russian collapse as Ukrainian and 'International Soldiers' monitor the areas.
One 'cause' of this war is the 'weakness signal' coming out of DC which Putin gambled mostly correctly on - if he were to have been successfull in taking Kyiv, he would have won. And he almost did.
Washington has been taking the incremental approach and it's mostly prolonged things, we need to step it up and play a few Ace cards.