Who would support that? Not only is Berlin not bad now (because nobody lives there except a few stubborn old people) but the Germans would raise absolute hell; they see the ruins as practically a monument if not holy ground. Berlin was where the truce formally ending the civil war was finally signed and where von Schleicher's "unity government" was formed. Berlin was where the Germans made their final stand against the invading Allied Powers, and their first tactical victory since the monarchists first crossed the border into Luxembourg; it's where they started rolling back the invading Allied Powers. I'm still a bit surprised they didn't make the final peace treaty be signed there; I suppose the piles upon piles of rotting corpses from almost a decade of war outweighed any other political concerns. Nobody's going to drop a fission bomb on Berlin; if anything, somebody might hit Munich to finally clear all the anthrax out.
You do remember that there was a European theater in WWII, right? Sorry if that comes across as insulting, but the war wasn't just "eastern empires and western empires trade colonies until the Hongxian Emperor finally dies and everyone goes home." As exciting as that was, there was enough going on in the rest of the world to justify calling it a World War.
Though speaking of the Pacific/SEA theater, that might be a good way to partially meet the challenge. If China decides to invade Chinese claims in Russia, we could see "Communist" warlords still occupying parts of Russia after they collapse; Uighurstan did have its "anarchist" warlords, after all. They won't be able to occupy all of Russia - I'd be generous in saying they could manage to occupy parts of it for three months, if the Russians' internal problems flare up early - but maybe they're remembered fondly for whatever reason when the Green Revolution finally rolls around.
OOC: I think that resolved everything, like how a Germany in a three-way civil war can be its own "side" in WWII