I have a story from Victoria 2 when a massive war erupted in the 1920s between me (Democratic German), France, Italy, Spain and the Ottomans vs Presidential Dicatorship UK (called "Great Britain because of the reactionary revolt), Kingdom of Russia and Austria-Hungary. Not uncommon you might say, but the cause was kinda cool.
Belgium has a Flemish Uprising, creating Flanders in the north. Southern Belgium (Wallonia, I guess) tried to annex Flanders, and Flanders being in my Sphere I decided to stop these shenanigans at once. Since Belgium was in the British Sphere this triggered a series of alliances that resulted in everything exploding at once.
Russia tried to invade me through our polish border - luckily I gained the two regions that form the Russian Bubble under Germany, so a seven province border was just a four province border, and I was able to effectively defend that border. from the pesky Ruskies. Austria-Hungary was kind of screwed from the beggining, since Italy and France basically pumped armies into Southern Austria, though they still put up a fight. On the Ottoman front, Russia were blitzkrieging across Iraq towards my colony in Dumyat (Suez). Normally this wouldn't be a problem, I'd just ship my East Asian troops over to the Middle east and kick them back into Russia, stealing their Vodka and Borscht and declaring victory. However, Britain's anal-fist in India made this impossible. Russia soon reached Istanbul, and the war in the south looked lost.
Then out of no-where, Britain melted. Every single British army revolted and became Jacobin rebels. All of them. Britain's authoritarian state made for high militancy, and this erupted into full scale revolt. Under normal circumstances it would have made a cool Worker's Union, but for some reason Communism was invented really late this game, while Fascism happened really early. Britain was suddenly out of the War. I dumped the entire population of Vietnam on the Russians, and I kicked them out of the Ottoman Empire, who then liberated Kars for good measure.
For the record, we won and Flanders liberated Brussels, and I "liberated" that tiny British island off the coast of Denmark that I apparently had a core at. Might post a picture of Flanders if I can find the save.
Not Hearts of Iron, but I guess it's the same game essentially.