Well it wasn't really due to nuclear war but that's on me for not having a proper explanation.I mean, maybe I misunderstood your map, but isn't it post-apocolyptic due to nuclear war?
I could see Greenland effectively returning to being Terra Nullius.
US went isolationist, russia dominated the eu and china dominated africa into submission, the two became enemies, and then some government scientist discovered a method to create near limitless resources. At which point aliens using Earth as a prison planet (hiding in the Moon) manipulated russia and china into a nuclear war. Then they tower of babel-ed the world by getting rid of all long distance communication and changing a few other things. Maybe they're using earth for entertainment too? But for some reason they let people stay around and let them sort of rebuild society. It was however alot easier in the new world. I'll stick with what I said before and say that the story is just "don't think about it"
So this is the world split into small countries after like 60 years of minor demographic change