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, 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 End of the Age of Strife
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 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.
The Emperor's first obstacle to the reunification of Mankind was the existence of the Mechanicum empire of the Tech-priests of the Cult Mechanicus on Mars, who had historically been fiercely independent of Earth. Instead of an outright war, the Emperor used the threat of an assault by his Space Marines on Mars and the offer of new technologies to form an alliance in the 30th Millennium with the Tech-priests of what would later be called the Adeptus Mechanicus. This alliance formed the heart of the newborn Imperium of Man and was symbolised by the adoption of the two headed-eagle or Aquila that is still the primary icon of the Imperium. Despite his disdain for their reliance on religion and cybernetic augmentation of their bodies, the Emperor knew that He would need Mars' great manufactoria (factories), mechanical output and ancient technological knowledge to build the military force required to reunify humanity across the galaxy, and He incorporated them into the Imperium. The Tech-priests of Mars, likewise, saw the Emperor's extraordinary scientific knowledge and technological skill as proof that He was the Omnissiah, the living avatar of their Machine God in the material realm, who the creed of the Cult Mechanicus had promised would return one day to the Red Planet and offer the true way forward for humanity.
The Conquest of the Void
The Great Crusade the largest and most ambitious military endeavour ever undertaken by Mankind. As mighty and valiant as the hosts of the Emperor were, this epic undertaking would have been entirely impossible without the countless thousands of Warp-capable vessels that transported hundreds of thousands of the superhuman warriors of the Space Marine Legions and many millions of Imperial Army soldiers from one star's light to the next. The Great Crusade saw a staggering array of vessels constructed, reclaimed or pressed into service. Some were used for a matter of solar months before being declared obsolete or wearing out and degrading to destruction, quite apart from losses incurred in battle, while others gained a permanent place in the canon of war, with successful designs endlessly copied and modified as the decades progressed. The first vessels to enter the service of the Imperium were constructed in the orbital foundries of Terra, and later Mars' Ring of Iron and the orbital shipyards of Saturn, under the scrutiny of the Emperor and the Forge-wrights of the Mechanicum, and indeed it was only that in alliance with Mars that the trans-solar expansion was possible in any meaningful way. This was further aided when at last the Saturnyne Dominion, with its accomplished ship-masters, joined the Imperium after their alien overlords were overthrown, and as the Imperium expanded, many more great orbital shipyards were added at Voss, Grulgarod, Lorin and Cypra Mundi. All these grew to near rival Mars itself in voidship production. Driven by the will of the Emperor, the first Expeditionary Fleets pushed outwards into the galaxy. Preceding each great Expeditionary Fleet of hundreds, sometimes thousands, of vessels often ranged smaller contingents of independent flotillas led by a class of martial leader that would become known as the Rogue Traders Militant. Many of these individuals were former rulers of the numerous realms the Emperor had cast down first during the Unification Wars and later as the Great Crusade spread, formerly independent human worlds. They were offered a stark choice--bend the knee before the Emperor and swear service to the Great Crusade, or die by His hand. Though many set pride before what they regarded as slavery, others chose service and took up the Emperor's Warrant of Trade. There was a price, however. The Rogue Traders Militant were expected to scout ahead of the leading edge of the Great Crusade, accompanied by their own armies as well as whatever assets had been ceded them by the Emperor. Operating so far ahead of the Emperor's crusading armies, the Rogue Traders Militant could expect little or no aid should they encounter foes too powerful for them to overcome. After several solar decades penetrating the inky black of the void, Rogue Trader Militant fleets often appeared as ramshackle vagabonds, many of their starships taken from defeated enemies, sometimes including xenos vessels of entirely novel or esoteric form. They were forbidden to return to Terra, for in His wisdom the Emperor sought not to just rid Himself of powerful rivals, but to ensure that even in their deaths they might serve Mankind. Many vanished alone and unheralded; slain, consumed or enslaved by nameless xenos abominations far from the light of Terra.
As the Imperium expanded, so too did its fleets. Countless long-lost wonders of technology were recovered, some wrested from the dead hands of unwilling custodians, and others surrendered willingly as fitting tribute to the Master of Mankind. Some vessels were unique, constructed by methods even the most accomplished Adepts of Mars could not hope to replicate: the Terminus Est, the Nicor, the Mirabilis and the Phalanx foremost among them. Other patterns and classes proved possible to reproduce and replicate, and before long the various arms of the Imperium's military acquired their own distinctive panoply of warships. Those of the Legiones Astartes were often blunt of prow and slab-armoured, built to endure the withering storm of fire that accompanies a planetary invasion, their plasma furnace-hearts powering some of the most destructive weapons known to Mankind. But beyond these practical needs, each fleet favoured the nature of its Legion, from the sable black marauders of the Raven Guard to the baroque crimson and gold battlecruisers of the Blood Angels to the brute functionality and unadorned steel of the Iron Warriors' siege-barques. The ships of the Emperor's wider naval armadas, the Armada Imperialis, were more diverse affairs, built for void supremacy. They ranged from stately Battleships, multi-kilometre long engines of doom, their armour concentrated to the fore and their flanks repleted with rank upon rank of broadside batteries, to lithe and deadly Destroyers and stripped-bare Warp Runners, to watchful piquet Frigates and lumbering star fortresses. Beyond these were innumerable classes of transports, arks, conveyers and supply ships, the forge vessels of the Mechanicum and their own strange space-going engines of war.
Rediscovery of the Primarchs
Even with access to the vast fleets of starships and military materiel that the forges of Mars could churn out, the expansion outwards into the galaxy from Terra was slow, primarily due to a lack of the necessary manpower and skilled leadership. Without access to the Primarchs' unique genomes, the time required to create a Space Marine was far longer than ideal. The Emperor was also adamant that all the genetic material for new Astartes gene-seed must come directly from Him, to keep the gene-seed organs pure and 100% effective, as mutation had a habit of setting in even at this early date in the Astartes' existence. Only a short time after he had launched the great expedition to reunite Mankind that he called the "Great Crusade," the Emperor was reunited with one of His Primarchs after His Great Crusade Expeditionary Fleet, the Pricipia Imperialis, reached the nearby Mining World of Cthonia during one of its first journeys through the Warp; this was Horus, the future Warmaster of the Imperial armies. Being the first Primarch rediscovered by the Emperor, the first of His beloved sons who had returned to Him, Horus and the Emperor soon formed a truly unique bond of friendship and paternal love, saving one anothers' lives on many occasions. Horus was given command of the XVI Legion of Space Marines, the Luna Wolves, and he soon set the precedent for raising all future Astartes for his Legion from among the men of his former homeworld, in this case Cthonia. With the increased Space Marine production made possible by direct access to a Primarch's genome, the Great Crusade began picking up steam, with the Emperor's "Imperium of Man" bringing many new worlds into "Imperial Compliance" either through negotiation and diplomacy or outright conquest when all other methods failed. Each world was also introduced to the Imperial Truth, the Emperor's chosen philosophy and the foundation for Imperial culture, which was stridently atheistic and rejected all forms of human religion or spirituality as mere superstition to be replaced by the cold rationalism of scientific progress. Though the Emperor loathed war, He understood that if Mankind was to survive the rigours of a hostile universe it must stand united, and that unity must be brought quickly, even if it had to be forced in certain instances. After 30 Terran years of successful crusading, the Emperor discovered another of the Primarchs, the Wolf-King, Leman Russ on the frozen world of Fenris. Although Horus was pleased at the discovery of one of his brothers, he secretly hoped to always remain the Emperor's favourite son, no matter how many of his brothers were rediscovered. Over time, all of the Primarchs were rediscovered by the various Expeditionary Fleets of the Great Crusade and given command of the Space Marine Legion for which their genetic code had been used as the foundation of that Legion's gene-seed. The order in which the Primarchs were discovered is not entirely clear in Imperial records. Horus was clearly the first to be rediscovered, and Alpharius of the Alpha Legion was the last. Rogal Dorn of the Imperial Fists was the seventh of the Primarchs to be found, while Lion El'Jonson was discovered on the Death World of Caliban after Leman Russ of the Space Wolves had already been recovered by the Emperor on Fenris. Magnus the Red of the Thousand Sons was found on Prospero before his brother Lorgar of the Word Bearers welcomed the Emperor to Colchis. Angron of the World Eaters was rescued against his will from his homeworld of Nuceria after the Great Crusade recovered Perturabo of the Iron Warriors from his homeworld of Olympia. The Crusade also grew larger as the solar decades of conquest passed. With each new world brought into Imperial Compliance and made accepting of the Imperial Truth, its resources could be harnessed for the construction of ever more starships and the massive amounts of materiel needed to keep the entire enterprise moving forward. With the rediscovery of all of the Primarchs, the time required for Space Marine production was dramatically decreased; a new Astartes ready for combat could be created in as little as a standard year at the height of the Great Crusade. This rapid creation process, however, would prove disastrous: mental defects and unique eccentricities began to creep into the ranks of the Astartes as the manpower needs of the Great Crusade called for larger and larger armies. Yet, by the early 31st Millennium, some 200 standard years after it had begun, the Great Crusade had reunited vast swathes of the galaxy under the leadership of the Emperor of Mankind, seemingly initiating a new Golden Age of progress and reason for the human race.
Blood and Illumination: The Pax Imperialis
The Great Crusade brought peace to Mankind, ending wars between peoples and nations, binding all together in unity. It broke the chains of superstition and freed billions from the whims of tyrants and murderous xenos. Where before there had been strife and the cloud of ignorance, now there would be peace and truth. These were high ideals indeed, but ideals that came at a price. Mankind had to be dragged to illumination, and many tried to pull it back into the shadows, into the old ways of ignorance and discord. The Emperor had unified Terra not only through words and alliances, but also through force of arms. Illumination and peace had to be won by blood. This was the basic truth of the Pax Imperialis, and when the Great Crusade took to the stars, this truth went with it. But it was not only in open war that the blood price for universal peace had to be paid. The enemies of the Imperium were manifold, and even before Horus turned from the Emperor, there were those who nurtured treachery in their hearts, who mouthed words of loyalty whilst seeding discord. While the ideals of the Great Crusade saw warriors marching in open war, the truth was that there were other methods and other less noble battles fought to keep the peace. The Assassin Temples, the networks of informers and the Silent Agents of Malcador the Sigilite, the Annihilation Protocols, and the ancient and all but forbidden weapons wielded by a chosen few in the Legions, all these were the tools and warriors by which the Imperium enforced its ideals and, for a time, brought peace to the galaxy. For the Emperor, the end justified the means in the pursuit of His great dream, for He knew better than most, that His time was limited. Mankind must be united and ready to face the future before the Chaos Gods could unite to stop Him.
Fractures in Loyalty
As the Imperium grew and the Great Crusade fractured into hundreds of different Expeditionary Fleets that could be separate from one another for solar decades of relative time, the Primarchs and their Emperor began to grow distant from each other. The Night Lords and World Eaters Legions grew infamous for their ever-greater atrocities during the conquest of new worlds, the Thousand Sons' pursuit of knowledge transformed into the use of sorcery through the temptations secretly offered by Tzeentch, and the Word Bearers' fanatical religious beliefs in the Emperor's divinity led to a rift between them and the Emperor when He dramatically rejected their attempts at worship in light of the atheistic nature of the Imperial Truth he sought to impress on all the worlds of the Imperium. The Imperial Army, composed almost entirely of normal human soldiers (some regiments such as the Geno Two-Five Chilliad were the products of selective breeding and genetic-engineering dating back to the Unification Wars to produce more capable soldiers or officers), provided much-needed manpower for garrison and support duties for the planets brought into Imperial Compliance by the Crusade. Freed from garrison duties, the Space Marines who comprised the core of each Expeditionary Fleet could conquer yet more worlds in a shorter space of time. Many Astartes and Imperial Army soldiers began speaking only of their loyalty to the Primarch that they served, and not to their Emperor. Owing to the isolation between the Legions and the Emperor after the Ullanor Crusade when the Emperor returned to Terra and left the Great Crusade in the hands of His favoured son Horus as the new Imperial Warmaster, this growing breach between the military and the people of the Imperium as represented by the Emperor essentially went unnoticed back on Terra.
Twilight of the Great Crusade
After Horus' successful campaign to destroy the largest Ork WAAAGH! known in the history of the Imperium to that point at the world of Ullanor during what became known to later Imperial historians as the semi-legendary Ullanor Crusade, the Emperor declared it the greatest victory of the Imperium to date. Horus' reward was the title and position of Imperial Warmaster and with it the supreme command of all of the Imperium's armed forces, Space Marines and Imperial Army starships and troops alike. After doing so, the Emperor made it known that He was needed back on Terra, where he would undertake a secret project intended to open the next phase in human cultural evolution. What the Emperor intended was to make use of the ancient artefact known as the Golden Throne, an instrument of technology created before the Age of Strife that had been unearthed on Terra that acted as a powerful psionic augmentation device, to open the Aeldari Webway to the use of humanity. This would allow all the worlds of the Imperium to be instantaneously interconnected through the Webway in a way that would make travel by starship obsolescent, and also end humanity's need to rely on psykers for interstellar navigation and communication with all the risks this entailed. The creation of a human Webway was intended to be the Emperor's greatest gift to Mankind and usher in millennia of progress and peace once the Great Crusade was complete. Horus was not informed of the Emperor's plans on Terra, and felt deeply troubled by his father's sudden unwillingness to confide in him, once his closest confidante, and by his decision to retreat to Terra with the Great Crusade yet unfinished. To make matters worse, while some of his fellow Primarchs accepted his promotion, others, chiefly Angron, Konrad Curze and Perturabo, openly begrudged Horus his new authority. There was also the widespread erroneous belief among the Astartes that the Emperor would disband or reduce the Space Marine Legions to the level of peacekeeping forces once the Great Crusade was completed. Horus resented his brothers' feelings and thought that while he was winning new worlds for the Imperium the Emperor was handing the Imperium he, his brothers and the Astartes had won to corrupt mortal bureaucrats and Terran nobles who knew nothing of honour or sacrifice. These existing seeds of bitterness, jealousy and pain were all the Ruinous Powers of Chaos needed to sow the seeds of dissension in the Warmaster's mind.
Events came to a head following Horus' wounding at the end of the Great Crusade during a battle on the moon of Davin against Nurgleite undead by the Chaos-tainted weapon known as the Kinebrach Anathame that had been wielded by the Nurgle-corrupted former Planetary Governor of Davin, Eugen Temba. Even Horus' superhuman immune system could not defeat the terrible Nurgle-created toxin that had laced the blade of the Anathame, and in their desperation to save his life, the Captains of the Luna Wolves gave in to the machinations of the Word Bearers Legion's First Chaplain Erebus and gave Horus into the keeping of the Chaos Sorcerers of the Temple of the Serpent Lodge on Davin. As part of their "healing" ritual, the sorcerers sent Horus' mind into the Warp, where Erebus, the First Chaplain of the Word Bearers Legion who had long served Chaos, acting as the agent of the Dark Gods, sought to corrupt him. Erebus used a deceptive vision of the future of the Imperium where the Emperor was worshiped as a god to convince Horus, who was filled with jealousy at the sight of thousands of pilgrims worshiping in a grand cathedral dedicated to the Emperor, to replace his father as the master of the galaxy. When Horus awakened fully healed, he awoke already corrupted, ready in his terrible ambition to turn on his father and attempt to wrest control of the Imperium from the Emperor no matter the cost, unleashing the terrible interstellar civil war known as the Horus Heresy. In return for his allegiance to their cause, the Chaos Gods offered Horus the power of the Warp, cementing the corruption of his soul. Horus would never realise that the dark future of the Imperium that the Chaos Gods had shown him, where Mankind was oppressed by superstition, ignorance and constant war, would be caused by his actions and his betrayal of his father. The Heresy would end with the death of Horus, the exile of his 9 Traitor Legions into the Eye of Terror, and the quasi-death of the Emperor of Man following His internment within the Golden Throne. This was ironically the very instrument through which He had hoped to open the Webway and create vast new vistas of progress for Mankind.
Expeditionary Fleets
During the Great Crusade, the vast forces of the Emperor were organised into a number of different formations which became known as the Expeditionary Fleets. At first there was but one fleet, the Principia Imperialis -- commanded by the Emperor Himself -- but as the Crusade spread across the galaxy more and more were launched. Trusted and powerful generals were appointed to lead these new fleets. Later, after the discovery of several of the Primarchs in the early stages of the Crusade, the Imperial fleets were divided into three main thrusts across the galaxy, one commanded by Horus, the other by Ferrus Manus, and the last by the Emperor Himself. In order that they could be effectively supported and their movements across the galaxy tracked with some degree of accuracy, each of the Expeditionary Fleets was designated a number by the War Council. As the Great Crusade drew to a close 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. The composition of each of the fleets was not fixed. The various fleet groups, Imperial Army regiments, auxiliary troop formations and even Space Marine Legions that made up a fleet could come and go over time, as prevailing strategy dictated. But the designation numbers remained and, while most never held any particular significance, some became famously associated with specific Primarchs or other prestigious leaders of the Imperium. The numerical designation of an Expeditionary Fleet was also applied to worlds conquered by its forces -- for example the Imperial world initially known as 63-19 was the nineteenth world discovered by the Warmaster Horus' 63rd Expedition. It was therefore possible for the Imperium's administrators to plot the movements of a specific fleet by the worlds it encountered, in chronological order. While all of the original Expeditionary Fleets had been assembled in Terran space by the Mechanicum, by the height of the Great Crusade they had been dispersed throughout the Milky Way Galaxy to reunite all the human-settled colony worlds under the Emperor's rule to bring to fruition His dream of a united human race dominating the stars. The Expeditionary Fleets were given a broad remit to bring human-settled worlds into the Imperium by diplomacy if possible but by force if necessary, and to cleanse any xenos species from the galaxy who represented a threat to humanity. As almost all intelligent alien species encountered by Mankind had proved hostile, outright genocide was the usual expected remedy. The fleets were also tasked with the secondary mission of exploring the galactic wilderness following the terrible anarchy of the Age of Strife so that new frontiers could be opened for human settlement and resource extraction in formerly virgin regions of the galaxy.
While the Emperor hoped that most human civilisations would willingly accept reunification under the Imperial aegis because of all the benefits interstellar unity offered, He knew that many human worlds would forcefully reject Imperial Compliance. As such, each Expeditionary Fleet was a potent military armada usually under the overall command of a high-ranking officer of the Imperial Army who bore the rank of Lord Commander and the expedition's fleet elements were commanded by an officer with the rank of Master of the Fleet. Additionally, each of the Emperor's Primarchs was given command of an unusually large Expeditionary Fleet, to which was attached the main body of their Space Marine Legion as well as substantial starship and Imperial Army assets. These Expeditionary Fleets, which were quite rare since the majority of the Great Crusade's fleets possessed no Astartes contingent at all, usually undertook the major thrusts into completely new areas of the galaxy. Smaller fleets with less combat capacity would then follow in a major fleet's wake, branching off to ensure Imperial Compliance on worlds that presented less human or xenos resistance than those faced by the fleets carrying Astartes complements. However, the Primarchs of several of the Legions were known at times to split off smaller units of Space Marines from their main fleets to accompany Expeditionary Fleets that were otherwise lacking in Space Marine support. In such situations, the ranking Space Marine officer served as the fleet's Lord Commander since Astartes always assumed command over normal troops in any military situation where they were present during the Great Crusade.
Sources
-Horus Heresy Collected Visions (Art Book), pg. 143
-The Horus Heresy - Book One: Betrayal (Forge World Series) by Alan Bligh, pp. 18-22, 80
-The Horus Heresy - Book One: Betrayal (Forge World Series) by Alan Bligh, pp. 18-22, 80
-Warhammer 40,000: Compilation (1st Edition), pg. 20
-The Horus Heresy - Book Three: Extermination (Forge World Series) by Alan Bligh, pp. 22, 76-77, 106, 110, 150, 155, 255
-The Horus Heresy - Book Three: Extermination (Forge World Series) by Alan Bligh, pp. 22, 76-77, 106, 110, 150, 155, 255
-Warhammer 40,000: Rulebook (5th Edition), pg. 102
-White Dwarf 321 (US), "Warhammer 40,000 Tactica: Walkers"
-The First Heretic (Novel) by Aaron Dembski-Bowden
-Legion (Novel) by Dan Abnett
Galaxy In Flames (Novel) by Ben Counter
The Horus Heresy - Book Seven: Inferno (Forge World Series) by Alan Bligh, pg. 10