That was brilliant and very nearly what I was saying. The threshold for actual nuclear war is so high that a belligerent actor could theoretically push quite far by exploiting the natural tendency of the adversary to not desire mutual destruction.

I'm not sure our leaders know what the actual "line" is because there isn't one. Will America MAD to save Europe? I do think it's possible that full-on WW3 engagement with non-nuclear forces could be possible with MAD serving to deter mutual usage and perhaps deter invasion of Russian/American mainland. That's wild speculation though. I base it primarily on the assumption that neither side wants MAD and will act to avoid it. The line for nuclear engagement (as opposed to chest-beating) reduces all the way back to a line drawn around the vital cities/industry/nuclear force deployments of a given country. Everything else might possibly be given up to avoid MAD which is an end-state. Maybe a game theorist can chime in.