Any direct fight between NATO and the RF would be so overwhelmingly one-sided that the Russians could easily view it as existential — even if it was not on their territory — and their probably panicked response could be nuclear. I'm pretty sure a lot of the caution around what weapons to send to Ukraine has come out of this, particularly the ones that could strike deep into Russia.

The other side of this is that a direct confrontation with NATO — and subsequent loss — might actually be preferable for Putin. They think they’re a superpower. Losing to another superpower is likely less perilous to Putin than losing to what they’ve been declaring not-really-a-country and who they’ve been declaring not-really-a-people.