A random thought:
If Putin strikes Ukraine with a nuke, or nukes, how sure are we that this would be a suicidal move?
Many people I've talked to are absolutely convinced this would lead to an American nuclear strike on Russia - but is that really so certain? I don't think so.
If Ukraine gets hit with nukes, that's just Ukraine destroyed, while NATO and other nuclear armed states are relatively safe. Whereas if they retaliate it's MAD.
Agreed, nuclear strikes in Ukraine would not automatically mean a nuclear exchange between Russia and the West.
But: NATO officials have repeatedly stated that in the case of NATO countries being affected by Russian use of WMDs this would force them to rethink their stance of military non-interference in Ukraine. It might even trigger Article 5:
If Russia Uses a WMD in Ukraine, the Fallout Could Trigger a NATO Response, Key Lawmaker Says
Imagine a nuke gets dropped on a nuclear power plant. This would practically force NATO to directly intervene (conventionally) which in turn could further escalate to WW3 and eventually to nuclear war. IMO dropping a nuke on a big city could have the same result.
This thought alone, and I think if it makes sense that none would retaliate for Ukraine, might make a nuclear strike more realistic than we currently think.
I'm with @STURM here - not very realistic. What would that mean for Putin / Russia in the long term?
- In large parts of the world, Putin would probably be regarded as the worst war criminal in history (beside Hitler). I highly doubt that this is what he wants his legacy to be.
- Total trade embargo (for decades) by the West and many other countries would be practically inevitable
- It could well mean that Russia alienates its remaining friends (China and India especially).
- It would inevitably lead to a new conventional arms race, which Russia cannot afford. According to some scholars, inability of keeping up in the arms race with the West was a key factor in the collapse of the Soviet Union. And this was the mighty USSR - now it's Russia alone.
- It would drastically increase the likelyhood of the creation of a common NATO or EU nuclear strike force
- Would nuclear strikes really cause Ukraine to immediately give in or even capitulate? Considering the Ukrainian doggedness, I have my doubts about that.