If Ukrainians continue to put up a good fight and maintain strategically significant control, the risk of a nuclear war increases. With Western weapons flowing into Ukraine, Putin might do something extreme as a scare tactic. I doubt there's much increased risk of a nuclear volley; if Putin did do the unthinkable, the West would almost certainly cease the flow of arms. But that still constitutes a nuclear war, leaving behind a far bleaker and far more unpredictable legacy.