If it actually came down to it, would USA/Russian/China nuke major cities, or is that bluff?

[DaleCooper]

I question your premise.  When has any nuclear power demonstrated, even rhetorically, a willingness to use nukes against civilians?

Other than August 6 and 9th 1945?

they were both military targets.  Hiroshima was the military HQ for all of souther Japan.  Nagasaki was the most import port in the south and held a lot of military industry.  There was other things at play, sure, but there was military justification for the bombings.

<again, I'm admittedly biased>

These types believe that if there are civilians present then it immediately ceases to be a military target. That's why it's so impossible to discuss anything with these people. I wouldn't even be surprised if they'd say Hitler's bunker wasn't a military target because his secretaries were there.