"What line?" would I think have been bomb-virgin USA's attitude. "It's just a bigger kaboom".

It's hard to count the living. I think it's almost certain the cold war would have gone nuclear, somewhere, perhaps only a proxy war at first - but I think it would have become a tit-for-tat habit. I think the damage would have been incalculably worse. Not just the usual stuff like climate and radiation, but also the whole experience of a city as a community rather than as ground zero waiting to happen. It would be a loss of subjective security comparable to the collapse of protected trade routes at the fall of Rome - and that was what wiped out their unitary culture. The Americans in the real world lucked out, so to speak - the war ended so soon, they got to feel guilt.

On the other hand... if only the pacific war hadn't started! It was such a stupid, avoidable war. Innocent people (and beautiful cities) should *not* be blasted to ash, even for useful lessons. Circumstances should never have reached that pass!