The USA could, if extremely motivated, kill pretty much
EVERYBODY in China. There are certain elements that could
be packaged with the nukes that would produce insane,
literally glow-in-the-dark, levels of radioactivity in
the fallout for a few days. There are also a few that
can produce lethal radioactivity for a thousand years
or more - territory-denial weapons. Look up cobalt-60.
This would be the extreme case, but alas it's not
entirely a fictional scenerio these days given Xi's
ambitions and the weakened state of the western
powers. We cannot resist China with conventional
arms anymore so it'd rapidly turn into a nukefest.
Note that the Chinese can also produce fallout-enhanced
weapons. While their inventory is smaller it could still
do absolutely horrendous damage to the USA/Canada/EU.
It's the war nobody but nobody wins.
In short, the term "civilization-ending" comes into play.
Back to the darkest of dark ages. Those weren't fun.
The fruits of civilization - knowledge, technology,
philosophy - are far more volatile than most people
think. They require the energy of civilization to
perpetuate, to preserve against entropy. Even a "short"
gap and it all goes away. Warlords and bandit-kings
and fanatics rule.
Oh well, a few thousand years and they'll start building
sun pyramids - filled with cut-out hearts - again ....
Amazingly (and barely) we survived the old cold war
without getting to this debacle. Bad attack-detection
systems brought us REALLY close though. One Russian
officer was demoted for NOT launching a total retaliation
after his system detected a US missile attack (it was
reflections off of clouds in Montana). The HUMAN
intuited that the picture didn't look quite right ...
but humans are increasingly being replaced by "AI".