The Great Crusade was the period in the early history of the Imperium of Man, lasting from approximately 798.M30 into the first decades of the 31st Millennium (approximately a little over two standard centuries), when the Imperium was expanding as it sought to bring all of human-settled space back under the control of a single ruler, the Emperor of Mankind.
The Great Crusade marked a brief era in human history of rebuilding, reunification and rapid technological advancement following the complete regression of Mankind into techno-barbarism during the 5,000 Terran years of the Age of Strife. It was a time when the Emperor of Mankind still lived in the conventional sense and led His people in person. The Great Crusade era is considered the Golden Age of the Imperium before the long, slow slide into cultural stagnation, political repression and increasing warfare that marked the ten millennia after the Great Crusade and the Horus Heresy had ended.
The Emperor of Mankind sought to unite all of humanity under one banner following the Long Night of the Age of Strife, and end inter-human conflict. Once united, the Emperor intended to begin the next stage of His great plan to ensure human domination of the Milky Way Galaxy, which He judged to be necessary if humanity was to survive the never-ceasing threats to its existence embodied by Chaos, myriad xenos races and its own fragile human nature.
To carry out the Great Crusade and reunite all the scattered colony worlds of Mankind beneath the single banner of the Imperium of Man, the Emperor created the Space Marine Legions that would form the core of the later Adeptus Astartesand the Imperial Army to serve as a support and garrison force for the Astartes. The use of accelerated gene-culturing of gene-seed organs to fill the need for vast numbers of Space Marines later led to the development of genetic flaws in many of the Legions, which would be one factor in bringing on the Great Betrayal of the Horus Heresy.
These forces, with the technological and military aid of the Mechanicum as required by the Treaty of Mars, would bring the light of the Imperial Truthand enforce Imperial Compliance with the new regime on every human world encountered. At its peak in the early 31st Millennium, some 203 standard years after it began, there were 4,287 primary Expedition Fleets engaged upon the business of enforcing Imperial Compliance and extending the Imperial aegis across the galaxy as well as 60,000+ secondary deployment groups involved in regulating Compliance or Imperial occupations.
A further 372 Expedition Fleets were regrouping or refitting in the Sol System or other Imperial hub star systems or resupplying as they awaited new orders. Some 4.3 million Remembrancers, official Imperial artists and journalists, were sent out with these fleets in the final days of the Great Crusade to record for posterity in pictures, words and music all that the Imperial warriors of the Crusade were accomplishing in the name of Mankind.
Finally, every Imperial Expedition Fleet carried with it a corps of Iterators, Imperial philosophers, rhetoricians and propagandists who sought to spread the Imperial Truth to every human world and convince their populations of the benefits to be had from Imperial rule.
The Great Crusade was under the direct, personal command of the Emperor of Mankind until the great victory won against a massive Ork WAAAGH!during the Ullanor Crusade. After the defeat of the Orks achieved a new high point for the Great Crusade, the Emperor retired to Terra to begin work on a secret project deep within the Imperial Palace to open the labyrinthine interdimensional Aeldari Webway to humanity's use so that the Imperium could be tied together as never before and forever shielded from the depredations of the Warp.
The Emperor named His favorite son Horus, the Primarch of the Luna Wolves Space Marine Legion, as the Imperial Warmaster and gave him full authority to direct the Great Crusade's progression as he saw fit, including the right to command all of his fellow brother Primarchs and their Legions of Astartes. The Emperor also established the Council of Terra and the Imperial bureaucracy, the forerunners of the later Senatorum Imperialis of the High Lords of Terra and the Adeptus Administratum, to begin shifting the Imperium from His personal rule to a government that more closely served the needs of the Imperium's common people.
During the Age of Strife the Emperor, who had spent all the years of His immortal life since His birth in the 8th Millennium B.C. seeking to guide Mankind to a better destiny, was trapped on the techno-barbarian-ravaged Earth. This was due to the massive Warp Storms prevalent throughout the galaxy in the 5,000 standard years before the birth of the Chaos God Slaanesh once more made travel and astropathic communication through the Warp possible.
Not content to simply sit back while humanity suffered through the long years of Old Night, the Emperor of Mankind spent this time performing intensive biological research on the human genome in gene-laboratories hidden deep below the Himalazian (Himalayan) Mountains, seeking a way that he could ultimately reunite all of humanity so that it could once more pursue its destiny of taking control over the Milky Way Galaxyas other intelligent species like the Eldar had already done.
The Emperor, who did not bear that title at this early date, had always worked to advance His agenda from behind the scenes and a long list of different identities over the millennia, some of them very well-known. But to save Mankind from the disaster of the Age of Strife, it had become apparent that He would have to take a far more open political role than He had previously preferred if the human race was to be saved from ultimate extinction.
The result of His scientific work was the creation of the first prototype supersoldiers who would serve as a template for the later development of the Space Marines. Using these first genetically-engineered supersoldiers called the Thunder Warriors, the Emperor unleashed the Unification Wars upon the techno-barbarian tribes and nation-states of Terra, forcing them to unite under a single government represented by a banner called the Raptor Imperialis with Himself taking the title of the "Emperor of Mankind" after years of savage warfare. This event was later known in Imperial records as the Unification.
After this, the Emperor realised that to reunite all of humanity, He would need generals and proconsuls in addition to Himself whom He could trust absolutely and who possessed capabilities similar to His own. So he ordered the creation of 20 highly advanced, genetically-engineered transhumans known as the Primarchs. These men, created using the Emperor's own genome as the foundations of their genetic code, were intended to lead the reunification of Mankind.
The Ruinous Powers of Chaos, however, fearful of the affect the Emperor's plans would have upon their own power within the Immaterium since they were dependent upon mortals for their very existence, managed to bypass the arcane wards protecting the Emperor's secret gene-laboratory under the Himalazian mountains, and teleport the unborn Primarchs, still in their gestation capsules, away from Terra. They were scattered across the galaxy to different human-settled worlds and several were tainted with the touch of Chaos that would bloom in time.
Facing this setback, the Emperor decided to change His tactics if not His ultimate goal of reunification. He gathered the remaining genetic material that had been used to create the Primarchs and from it cultured thousands of sets of gene-seed. These could be gestated into individual sets of 19 separate organs that when implanted within the body of an adolescent human male would transform him into one of the transhuman warriors known as the Astartes in High Gothic, the first true Space Marines.
The Emperor raised 20 Legions of Astartes, each making use of gene-seed organs cultured from the DNA of one of the Primarchs, from among the population of a newly unified Terra. After five millennia and the birth of Slaanesh following the events that led to the Fall of the Eldar, the massive Warp Storms finally dissipated across the galaxy. With their absence allowing interstellar travel once again, the Emperor quickly moved forward with his plans to begin the conquest of the galaxy.