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Great Crusade (ca. 798.M30-005.M31)

The Great Crusade of the Emperor of Mankind began as the Age of Strife came to an end for Humanity. The Warp storms isolating the Human colony worlds for 5,000 Terran years had finally disappeared with the birth of Slaanesh and the Fall of the Aeldari in the 30th Millennium.

The Emperor of Mankind led the Space Marine Legions personally during the Great Crusade in the 30th Millennium.

The man known only as the Emperor, a Perpetual and the greatest psyker in Human history, had emerged mysteriously from the darkness of Old Night as the greatest warlord of the Human homeworld. The Emperor, with the aid of His friend and ally Malcador the Sigillite, had united Terra under His rule during the Unification Wars which had ended in 712.M30 in the final decades of the Age of Strife. He intended to use His genetically-enhanced Space Marines and the Imperial Army to now reunite all of Humanity under His enlightened rule and the rationalist philosophy of the Imperial Truth.

Making a pact known as the Treaty of Mars with the Cult Mechanicus of the Machine God on Mars in 739.M30, the Emperor promised to allow the Tech-priests to continue to practice their faith and maintain the political autonomy of their Mechanicum government if they provided Him with the starships and other advanced technology He needed to reunite the Human-settled galaxy.

The Emperor was a man of enormous skill in the practice of science and the development of new technology, and so in Him many members of the ancient Mechanicum saw the coming of the Omnissiah -- the physical avatar of the Machine God -- that had been prophesied by their faith. In the treaty they signed with the Emperor, the Mechanicum of Mars pledged their support to the creation of a new Imperium of Man in which they would serve as the Imperium's primary purveyors of science and technology in return for first access to any ancient technology, or "archeotech," discovered beyond the bounds of the Sol System.

With the signing of the Treaty of Mars, the symbol of the Imperium changed from the raptor's head and lightning bolt icon used by the Emperor during the Unification Wars on Terra to the double-headed eagle known as the Aquila, to symbolise the union of the empires of Earth and Mars. With the forge factories and industrial output of Mars, the Emperor was able to refit His armies, and more importantly, He now had the use of the Mechanicum's Titan Legions and Knights, giant robotic war machines with the potential to dominate the battlefield.

At first, the newborn Imperium's expansion into interstellar space was slow, since the Imperial Army was still small, and more importantly the Emperor's 20 Space Marine Legions were inhibited by the absence of their primarchs, the genetically-engineered transhuman military commanders the Emperor had created during the Unification Wars to lead His eventual reconquest of the galaxy at the head of the Legiones Astartes, but who had been lost to the Warp before the end of the Unification Wars due to the machinations of the Chaos Gods.

This lack made the time for the creation of a new Space Marine from a normal adolescent Human male much longer. However, this difficulty was to end, as on Cthonia, a planet in a star system not far from Terra, the Emperor for the first time was reunited in 801.M30 with one of His missing primarchs, Horus. Having been discovered at an early age, the Emperor took Horus under His wing and taught him all He knew. Horus and the Emperor had a truly unique bond, that of father and son, and many times they saved each others' lives. But after almost 20 standard years, the Emperor discovered another of the Space Marine primarchs, Leman Russ, on the icy world of Fenris in 819.M30. Although Horus was pleased at the discovery of one of his brothers, he secretly hoped to always remain the Emperor's favourite son.

In time, all the primarchs were found on the worlds they had been sent to by the will of the Ruinous Powers of Chaos, and each was placed in command of their respective Space Marine Legions. The Legions as a result were massively expanded with new recruits drawn from their primarchs' adopted homeworlds, and new Space Marines could now be produced within only a single Terran year. But this acceleration and the genecraft techniques employed to bring it about produced fundamental defects within the psyche of each Space Marine created from certain primarchs' genetic heritages through this process. These defects were later to prove fatal to many of the Emperor's dreams for Humanity.

The Great Crusade lasted for 200 standard years, into the early years of the 31st Millennium, and brought many long-lost star systems into the fold of the newborn Imperium of Man. Under the command of the War Council of the Emperor and His rediscovered primarchs, vast expeditionary fleets comprised of the Imperial Army and the twenty Space Marine Legions fought back aliens, feral Human tribes, petty Human dictators and the Warp-tainted forces that had gained control of large portions of Human-settled space during the Age of Strife.

Once a Human-settled world had been pacified, it was brought into "Imperial Compliance" by a new Imperial planetary governor chosen by the Emperor. Part of this process included the rooting out of any superstitious or religious beliefs of the population with a respect solely for reason and the measured advancement of science and technology, an atheistic belief system known as the "Imperial Truth."

The world was then often occupied by a unit or units of the Imperial Army that were left behind to complete the integration of the planet into the Imperium while the expeditionary fleet, led by the Astartes, moved on to its next target. As the primarchs took control of the Space Marine Legions that had been crafted from their gene-seed, the Great Crusade separated, moving in many directions and reaching far across the galaxy.

After the Imperial Crusade on the world of Ullanor concluded in 000.M30 upon bringing that planet into Imperial Compliance after the defeat of a great force of Orks, the Emperor claimed it as the greatest victory of the Imperium to date, and that Horus should be given all credit. Hailing Horus and his Luna Wolves Legion (later renamed the Sons of Horus), the Emperor stated that He would have to leave the expeditionary fleets behind and return to Terra to begin the next, secret phase of His plan to ensure Humanity's domination of the galaxy.

In His place, the Emperor announced that Horus would be declared Warmaster of the Imperium, the de facto commander of all the Emperor's armies on the Great Crusade. Although Horus was troubled that the Emperor should leave him and his brother primarchs to carry on alone for some unknown reason, with Horus as their new commander, the Imperium's armies and Space Marines continued to expand ever outwards, rediscovering lost Human worlds and bringing them into the enlightened Imperium's fold. This expansion lasted for five more standard years until the outbreak of the terrible Imperial civil war known as the Horus Heresy in 005.M31.