Great Crusade

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 Astartes and 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 Truth and 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.
The Emperor's secrecy and willingness to hand power over to others produced enormous tensions with some of the Primarchs and ultimately became one of the weaknesses that the Ruinous Powers exploited to corrupt 9 Primarchs and their Space Marine Legions into the service of Chaos, unleashing the terrible civil war known as the Horus Heresy. This conflict ended the Great Crusade and the Golden Age of the Imperium. From that time to the present in the late 41st Millennium, the Imperium has known only stagnation, oppression, religious strife and endless war.