id expect totalitarian after totalitarian. Maybe some sort of all military population? Maybe something like Copper Heart
We've gone beyond territorial warfare I feel so the idea of one army marching upon another land seems unrealistic at least among MEDCs because power isn't tied to landmass quite as it was in the past. Other countries can now be controlled through other means - digital warfare, espionage, media, electioneering. That doesn't damage population in the same way as traditional warfare but it does lead to societal erosion which will have effects in other ways.
Humans tend to find ways to survive, but that doesn't mean it would be pretty.
According to Star Trek, WWIII is pretty horrific, but things get a lot better after that... memory-alpha.fando...
We would make it, but there would be big changes (geo-politically, economically, socially), for better or for worse