I didn't say it wouldn't change anything. Even one city hit would change everything quite drastically, politically and economically if nothing else.

I believe the parameters Adam had set were "make life on Earth impossible."

A full-out Soviet-era nuclear exchange would have left large areas of both nations dangerously radioactive for some time, but not permanently. The type of ground burst that takes out missile silos and hardened facilities creates a lot of fallout. The type of air burst that takes out cities (like Hiroshima) creates almost none. There were few effects elsewhere in Japan from the two atomic bombs dropped there, but as with Japan, if you're actually on the target, it's a moot point.

There are detailed maps describing places in the U.S. likely to be dangerous after a war, other places marginally dangerous, and many places that would be almost completely safe. Much depends on the wind conditions.

Even then, there are options. Fallout is a relatively short-term phenomenon, and localized. Countries not participating in the war will be relatively unscathed. If China stays out of it, most of what they know will be what they read in the newspapers, and they'll just take over the economic and power niche the U.S. used to occupy.