[jcranmer]

I don't think Putin would attempt to use nuclear weapons to preserve Russian control over the Donbass. Crimea is less clear (after all, Sevastopol is the main Russian Black Sea naval base), but his sham annexation of the four oblasts makes it less likely in my eyes.

It's hard to see a case where the use of nuclear weapons would improve Russia's situation, especially where Ukraine is seeking to recover territory almost everyone other than Russia agrees is rightfully Ukrainian. The story changes a little more if Ukraine were striking at the heart of Russia, but that is not a realistic option.