The Great Crusade was the sweeping campaign of conquest through which the Emperor of Mankind sought to reunite the scattered worlds of humanity under a single dominion. Launched from Terra after the Emperor emerged to lead His species out of a long age of isolation and darkness, it aimed to reclaim every lost human colony and forge a galaxy-spanning civilization guided by reason.
At the head of this endeavor marched the newly created Space Marine Legions and their rediscovered Primarchs, alongside vast mortal armies, fleets, and the forces of the Mechanicus. World after world was brought into compliance, whether by diplomacy, demonstration of overwhelming might, or outright war. The Emperor envisioned a secular empire freed from superstition, and He suppressed religious worship even of Himself, insisting that humanity's future lay in enlightenment rather than faith.
For roughly two centuries the Crusade blazed across the stars, liberating human populations from alien overlords and terrible isolation. Yet its very success sowed the seeds of catastrophe. Ambition, resentment, and the corrupting whispers of the warp gradually turned brother against brother, culminating in the ruinous civil war that ended the Crusade forever. What began as humanity's brightest hope became the prelude to its greatest tragedy, and the galaxy the Emperor dreamed of uniting was instead left broken and eternally at war.