Great Crusade
The Great Crusade was an important event in the history of Warhammer 40,000, seeing the formation of the Imperium of Man and reunification of humanity.
The Emperor Emerges
For much of the 20,000's, Terra and the other human colonies were riven by war after the rebellion of the Iron Men. The techno-barbarians (UNN-TSS! UNN-TSS!) ruled little fiefdoms on once-proud Terra, and all seemed lost.
Then, the Emperor of Mankind appeared in M28, and started conquering Terra, piece by piece. His army consisted of the first generation of Space Marines, using powered armor that wasn't even void-sealed (although they still had massive pauldrons). This was approximately the time that he decided that religion was bad for humanity and implemented the Imperial Truth.
The Next Step
Of course, Terra wasn't enough for the Emperor; he wanted to rule over all of Humanity. He had already arranged for the Void Dragon to be sealed on Mars so that, by this time in history, he would have the Adeptus Mechanicus to build awesome weapons for his armies, and now he needed some armies to wield those weapons. And to lead those armies, he needed generals, so he started the Primarch Project in M29, creating twenty super-human warriors, his sons, and his generals. The Gods of Chaos perceived this project to be a threat to them, so they scattered the Primarchs throughout the galaxy, but the Emperor had enough data to create his Space Marine Legions based on their DNA, what would later be called the First Founding. He sent the Legions out to find their Primarchs, and to re-unite the human colonies as they went.
The Age of the Imperium
Over the course of the next millennium or so, the Space Marine Legions found their Primarchs, and the Imperium of Man grew, world by world, until it spanned almost the whole galaxy. Many Xenos were exterminated along the way, as the Emperor believed them to be a threat to humanity's dominance. Between this directive and the influence of his Slaaneshi daemon blade, Fulgrim wiped out many Eldar worlds, which resulted in mutual animosity between the Imperium and the Eldar for millennia to come.
After the Ullanor Crusade, the Emperor announced that he was returning to Terra, saying that his sons were to prove that they were the leaders he had made them to be, and appointed Horus, Primarch of the Luna Wolves, as Warmaster of the Great Crusade, and withdrew to the Imperial Palace. In truth, the Emperor was ready to begin his next phase of star travel, creating a Webway portal under the Palace using the Golden Throne. Unfortunately, the Emperor's detachment from humanity created a great deal of rivalry and resentment among his sons, and his failure to fully explain matters to his sons only exacerbated the situation. The Ruinous Powers were quick to take advantage of these weak spots, and slowly but steadily began to corrupt some of the Primarchs.
This culminated with the Horus Heresy.
See Also
- The Last Church, a short story taking place towards the end of the Emperor's conquest of Terra.
- Horus Heresy, which brought the Great Crusade to a sudden stop.