If the invasion goes very poorly and Russia is at risk of losing, Putin will detonate a nuclear weapon over a non-populated area (whether in Ukraine or Russia).

If the invasion goes very poorly and Russia is at risk of losing, Putin will detonate a nuclear weapon over a non-populated area (whether in Ukraine or Russia). It's one of the few ways he can escalate up another level, prior to war with NATO and or nuclear war. When he does it, he'll say it's a message to NATO forces to back off and stop supplying Ukraine (he'll need to blame external actors for his behavior). NATO should immediately begin a no fly zone over the Lviv region (with the possibility of widening it as necessary), in case Zelenskyy needs to leave Kyiv and or the Ukrainians generally need to establish a new base of government. They could shield that region / territory from Russia right now without prompting a Russia v NATO war, as it would require no attack on Russia to maintain near-term. If Russia tries to contest it, they're initiating the war, as they knew the area was protected by NATO forces. It would work as a maneuver. Putin could of course always claim that any move by NATO into Ukraine is an act of war, and or that Russia owns Ukrainian territory so any move by NATO into Ukraine is identical to invading Russia and other blatherings, but he can invent claims at any time to proclaim NATO started a war: the question is, can it be pulled off without sparking a war with Russia, and right now I believe it could be.