-Big War (that's what it'll be called) sparked mostly over a scarcity of oil, with a lot of political tension thrown in for good measure.
-I think I will adopt AG's cyborg Undead; they're just too good of an idea!
-Two major European centers of population (one's a city-state, one's really more of a district): the New Holy Reich of Neues Berlin and Novya Moskva (anarchic semi-kleptocracy built in and around the ruins of Moscow).
-Mechs, hell yeah!
-The Followers of Morgenstern, a weird cult-hive of people who aspire to become cyborg-ized by their god-leader, a supercomputer called Morgenstern.
-The Freedom Army, a large, well-armed militia of the French from the Big War which devoted itself to the cause of stopping "American tyranny".
-Siberia is wilderness now; got nuked and then became a sort of bizarre mutant tundra-jungle, with giant carpets of moss and insane fungi, as well as evil-looking pines.
-Japan is hyper-urbanized and technological (think Akira-style).
-Mutants, robots, fallout-savages, cargo cults, and the like.
-Nuke-Winter and Global Warming counteract against each other- huge storms and violent weather (probably not scientific, but hey, who cares?).
IF the U.S. is kaput:
-A community of militant Christian-fanatics in the Southeast (Florida, Georgia, the Virginias maybe) that refer to the time before the Big War as "Babylon" and all others as "Pagans" (Mouthful of Dust tie-in?)
-A military state begun by a stranded U.S. military unit on the last surviving base in Europe; they still cling the hope that somebody is coming to rescue them, even despite being beset by the Freedom Army out of France.
IF the U.S. is (mostly) okay, then radiation-sealed cities policed by robots and ruled by the now-despotic President Campbell (130 years old, sustained by nanotech; Big Brother tendencies); a "democracy" that is really a pluto/aristocracy. Possibly some sort of virtual-reality-consciousness network so that not everybody can see the squalor and decay?
I could probably incorporate the two ideas- perhaps the pseudo-U.S. exists only in the Northeast or West coast of the (former) nation.