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Part of the Emperor's vision of human unification was the [[Primarch Project]], meant to be the creation of 20 superhumans to lead the [[Emperor]]'s forces to reclaim the [[galaxy]] in his name. It also saw the creation of the [[Space Marine Legions]] from the [[gene-seed]] samples that survived the scattering of the [[Primarch]]s by the [[Gods of Chaos|Chaos gods]]. These forces would play the major role in the upcoming Great Crusade alongside the [[Imperial Army]], [[Titan Legion]]s, [[Legio Cybernetica]], [[Sisters of Silence]], and [[Custodian Guard]].{{cite this}}

In the early stages of the Crusade, the Imperial forces were divided into three main thrusts, one commanded by [[Horus]], the other by [[Ferrus Manus]], and the last by the [[Emperor]] himself.{{Fn|8}} As each Primarch was discovered, he was given control of his own Legion of Space Marines. As more of the Empire was carved, the Emperor's forces were spread more and more thinly. Eventually the need for Space Marines was greater than the supply. As such, Accelerated Gene Culturing was necessary to provide sufficient legions. Unfortunately, this process was unstable and led to flaws in the gene seed of the legions. This would lead to such afflictions as the [[Red Thirst]] and various mutations in the [[Space Wolf]] gene seed.{{cite this}}

Also, without the Emperor's guidance the Primarchs that had previously friendly rivalries were beginning to develop into dangerous feuds. Some of the Primarchs had reached the point of intolerance with one another, others like [[Magnus the Red]] had simply become isolated in the confusion of the Great Crusade. Already the [[Word Bearers]] Primarch [[Lorgar]] had secretly turned against the [[Emperor]] decades before the Crusades end after the destruction of his "perfect city" of [[Monarchia]] by the [[Ultramarines]]. Meanwhile [[Leman Russ (Primarch)|Leman Russ]] clashed with both [[Angron]] and [[Lion El'Jonson]]. [[Perturabo]] was shunned by most due to the nature of his hand in the Crusade and was in turn envious of [[Rogal Dorn]]. [[Konrad Curze]] also had a disagreement with Dorn that escalated into a brutal surprise attack that left Dorn badly injured and unconscious. This mistrust and hatred would form the linchpin of the [[Horus Heresy]].{{cite this}}

The age of the Great Crusade ended with the virus bombing of [[Isstvan III]] by [[Warmaster]] [[Horus]], which announced the beginning of his [[Horus Heresy|Rebellion]]. The loyalist legions were recalled to Terra to defend against the traitor Warmaster, but not all of the Legions were able to return. The [[Ultramarines]] were mired in the Ultramar system fighting [[Kor Phaeron]] and his retinue of [[Word Bearers]] that had come to destroy them. The [[Space Wolves]] were finishing their battle at Prospero with the now openly traitorous [[Thousand Sons]], the [[White Scars]] were mired in the [[Chondax Campaign]], and the [[Dark Angels]] were in the midst of a civil war on Caliban and a slow campaign against the [[Gordian League]].{{cite this}}

In many ways the Emperor was now gone, The Imperium suddenly finding itself without the great leadership of the Guardian of Mankind. The brief age of recovery which the Great Crusade represented was over, and mankind began to decline into the state it is now, with fear and chaos rife, [[Inquisitor]]s seeking out heresy at every corner and billions of people living in forced, and willing, slavery to their God-Emperor.{{cite this}}

*{{Endn|7}}: [[The Horus Heresy Book One - Betrayal]]