Space Marine Legions - Legiones Astartes: A Space Marine Legion was a frontline force of shock-infantry comprising tens of thousands of transhuman Astartes warriors armed and equipped with the finest wargear the Imperium could supply. A Space Marine Legion, unlike the 1,000-man Chapters of the present-day Adeptus Astartes created after the Second Founding at the end of the Horus Heresy, could number anywhere from 10,000 to more than 250,000 Space Marines, as well as the Legion's associated Imperial Army, logistical support forces and Armada Imperialis fleet elements.A force of a hundred of these genetically and biochemically-enhanced transhuman warriors could quell a rebellious city in solar hours. Thousands could conquer a world in only solar days, and tens of thousands or even hundreds of thousands wielded at once were the doom of entire species, capable of reducing alien civilisations to mere dust and memory in a span no greater than the single course of Terra's orbit around the Sun.Founded amid the bloodshed of the Unification Wars of Terra that swept the Emperor of Mankind to dominion over the cradle of humanity, the original military formation of the Legions was the division of twenty numbered units of enhanced warriors, organised very much along the lines of the Thunder Regiments that proceeded them in the Emperor's service and who they would eventually replace.Much of the discipline and organisation of the early Astartes Legions owed greatly to the ancient and proven Terran patterns of military strategy, hierarchy and functions as laid down in the revered texts of the Principia Belicosa of Roma and Krom's (Oliver Cromwell) fragmentary "New Model" that had survived in the hands of the tyrants of Old Earth down the blood-stained generations of the Age of Strife.To these venerable treatises the Emperor and His commanders had added their own genius and created a sturdy but adaptable strategic framework that spoke to the fundamental strengths and superhuman abilities of the Astartes themselves. At the outset of the Great Crusade in circa 798.M30, many of these early Legions were raised along the so-called "Terran Pattern" of organisation as formulated by the Imperium's Officio Militaris.Of the twenty original Space Marine Legions, eighteen would survive to grow into vast forces by the end of the Great Crusade in the early 31st Millennium; as for those that did not (the IInd and XIth Legions), nothing can be said of their fate in current Imperial records.Nine of these Legions would continue to stay utterly loyal and faithful to the Emperor of Mankind during the Horus Heresy and thus they serve as the genetic forbears of all current Loyalist Space Marine Chapters of the present era. The Space Marine Legions and their Primarchs were the Emperor's primary military force and most important advisers, respectively, and were intended to be the political and military speartip of the Great Crusade.However, the Emperor also wanted to create a civilian government for the Imperium of Man which would ultimately hold authority over even the Primarchs, a change embodied in the creation of the Council of Terra in the early 31st Millennium that angered many of the Emperor's sons and ultimately became one of the contributing factors to the outbreak of the Horus Heresy.The Legions were aided by the ground and naval forces of the Imperial Army, the predecessor of the present-day Astra Militarum and Imperial Navy, but these forces more often served in support and garrison duties while the heaviest combat and the most important and difficult missions were always carried out by the Astartes Legions.As the Great Crusade continued the expansion of the nascent Imperium into the galaxy, the discovery of the Primarchs and their newly adopted homeworlds helped to stem an impending crisis that was not widely known at the time outside of the exalted ranks of the Imperium's ruling War Council: namely, the diminishing stability of the Astartes gene-seed itself through over-use and the increasing need for more Space Marines in the field.This was a matter that only worsened as the Great Crusade pushed ever wider afield into the galaxy. Imperial forces could no longer be concentrated as easily as before, and attrition was taking its toll as standard years of near-constant battle became solar decades.To slow the relentless pace of the Great Crusade's progress was for the Emperor simply not an option. As such, the simple truth was that more Space Marines were needed and they needed to be created faster than before.A secret conclave of gene-wrights under the Emperor's direct supervision posited the solution that became known as Grabiya's Theorem, which demonstrated that a Primarch's genetic code could be used to stabilise and expand Astartes gene-seed stocks with what was hoped to be "minimal deviation."Alongside this accelerated gene-culturing technique, other previously unavailable genetic technologies were put into effect, reducing the processing time required to create a battle-worthy Space Marine to a single Terran year in some cases. Such accelerated gene-seed techniques, along with absent, inadequate or over-forceful psycho-doctrination techniques, were later found to have unseen fundamental flaws.Many Imperial savants since have come to believe that the drive to create larger Space Marine Legions at an accelerated speed played a prime role in the degradation of the sanity and psychological make-up of certain Legions and paved the way for the horror that was to come.Nine of the twenty original Space Marine Legions chose to follow the Warmaster Horus and pledge their lives and service to Chaos. They became the Traitor Legions of Chaos Space Marines that still threaten the Imperium today.In the end, the Horus Heresy made clear that no single commander could be trusted to wield the enormous power of a Space Marine Legion, never mind several at once. The Ultramarines' Primarch Roboute Guilliman led the post-Heresy Reformation of the Imperium that reshaped the Space Marine Legions into the Adeptus Astartes and broke up each of the remaining Loyalist Space Marine Legions into the myriad 1,000-man Space Marine Chapters to protect humanity from the rise of another Horus.The remaining two Legions and their Primarchs were lost to the Imperium some time before the Horus Heresy, with no trace of them remaining. It has been suggested that they were destroyed by, or under direct order of, the Emperor for some unknown, grave transgression, or flaw in their gene-stock, and all evidence of their existence was purged from Imperial records.