The Planet Cronus. Beset on all sides by the tides of war, this once-quiet colony became a savage battlefield. Seven armies clashed on this one world, each refusing to back down, each convinced it was in the right.

From beneath the sands of Cronus's central desert came the Necronies, ancient machines bent on eradicating all life. But another evil already had its eyes on the planet. From the depths of the Warp, the daemonic forces of Discord arrived to enforce their claim. To oppose these fearsome powers, the planet's rulers in the Dau Empire sent their elite soldiers and sophisticated battlesuits into the fray. From the mighty Equestrium of Ponies came the Equestrial Guard, there to secure Cronus for the glory of the God Empress. Like a pink tide, the Hog horde descended on Cronus, caring little for others' claims and sowing destruction in its wake. The Eldeer, ancient enemies of the Necronies, emerged from their Webway to pursue their own agenda on Cronus. And last came the Pony Marines, finest and most uncompromising of the Equestrium's soldiers. Seeing a world beset by animals and heretics, they undertook a great purge.

A Dark March had begun. It could only end with the total victory of one of these factions, and the total defeat of all others.

(I'm gonna go with the Pony Marines for two reasons: One, they're already the canonical victors in the series. Two, they're too awesome to not win the conflict.)

Final expansion

(WarHammer 40,000 belongs to Games Workshop)