> If Ukraine somehow takes any Russian territory (defined as post 2014)… it is almost guaranteed nuclear weapons would be used to stop the Ukrainian advance

Nuclear weapons haven't even been used when Ukraine has hit Russias actual territory (in Russia) so why would they use nuclear weapons against Ukraine retaking its own territory?.

> If Ukraine somehow takes any Russian territory (defined as post 2014)…

You mean, pre-2014? They’ve already taken post-2014 “Russian” territory.

I'm not so sure. The whole point of an exercise like this is to demonstrate power, or to acquire resources. Neither tactical nor strategic nuclear weapons would help with those objectives. Ukraine's army isn't isolated to a single spot — tactical nukes wouldn't even be as effective as WW2-era area bombing, and a whole lot more expensive. Strategic weapons would deny resource acquisition to RU. The use of nuclear weapons (of either kind) against civilian centers would be problematic, I think? The US "got away" with its use of nuclear weapons (first use; the US was the only major player left; etc.)

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.