"Alone, a Legionary is a formidable foe as far beyond a man as the wolf is beyond the sheep. Together, bound by ties of unshakeable loyalty, a Legion is a force that can extinguish the stars and shake the very heavens." — Attributed to Lorgar Aurelian, Primarch of the Word Bearers
The Space Marine Legions (also known as the Legiones Astartes) are the unit formations of the Space Marines created during the First Founding by the Emperor of Mankind on Terra in the late 30th Millennium. This initial Founding happened before the start of the Great Crusade that would reunited the scattered worlds of humanity beneath the banner of the Imperium of Man, while the Unification Wars were still raging on Terra. At the start of the Crusade, as the Legions were placed at the forefront of the Imperial Expeditionary Fleets that left Terra to reunite the human-settled galaxy, the Astartes were renamed Space Marines and their formations the Space Marine Legions. The Space Marine Legions are the primary frontline military forces of the Imperium of Man.

Legiones Astartes Edit

A Space Marine Legion was a frontline force of shock-infantry comprising tens of thousands of Astartes warriors armed and equipped with the finest wargear the Imperium could supply. A Space Marine Legion, 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 fleet elements. A force of a hundred of these genetically and biochemically-enhanced super warriors could quell a rebellious city in hours. Thousands could conquer worlds in days, and tens of thousands and even hundreds of thousands wielded at once were the doom of entire species, capable of reducing entire 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 entirely replace. Much of the discipline and organisation of the early Astartes Legions owed greatly to the ancient and proven Terran patterns of strategy, hierarchy and functions as laid down in the revered texts of the Principia Belicosa of Roma and Krom's 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 Legionaries themselves. At the outset of the Great Crusade in circa 800.M30, many of these early Legions were organised along the so-called "Terran Pattern" of organisation as formulated by the Imperial Officio Militaris.

The Space Marine Legions and their Primarchs are the Emperor's primary military forces and most important advisers, respectively, and were intended to be the political and military spear-tip 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 civil war. The Legions were aided by the ground and naval forces of the Imperial Army, the predecessor of the present-day Imperial Guard 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 of at the time outside of the exalted ranks of the Imperium's ruling War Council: namely, the diminishing stability of the gene-seed itself through over-use and the increasing need for ever greater numbers of 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 years of near-constant battle became decades. To slow the relentless pace of the Great Crusade's progress was for the Emperor simply not an option, and so, 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 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.

Legion Size Edit

The Legions are massive armies, and the size of each could vary tremendously. A precise, pre-set formation was never truly achieved or maintained. Even during the start of The Great Crusade some Legions were very large, while others were not. The size of each Legion was fluid, with the numbers of combat-ready Astartes dependent on the number of new recruits, the inevitable battle-losses, the availability of potential recruits, and the administrative skills of the Primarch and his officers.

The largest of all the Space Marine Legions were the Ultramarines, followed by the Word Bearers. The Ultramarines numbered approximately 250,000 Astartes and the Word Bearers numbered approximately 100,000. It was the Ultramarines' sheer size, as well as the purity of theirgene-seed.

The Thousand Sons Legion of Magnus the Red was most likely the smallest of the Astartes Legions as many of the Space Marines within its ranks had developed rampant mutations or uncontrollable levels of psychic powers and had to be granted the Emperor's Peace. The Primarch Fulgrim's gene-samples had been largely lost on Terra making it difficult to develop new gene-seed for his Legion, and this meant that the Emperor's Children found it difficult to recruit new Astartes, also leaving it with a relatively small number of Space Marines. Both of these Legions would increase their numbers to acceptable standards only after their Primarchs were found by the Emperor during the course of the Great Crusade.

The approximate manpower available to several of the Space Marine Legions at the start of the War in the early 31st Millennium is recorded as follows:

Legion Number Legion Loyalty Legion Strength
XIII Ultramarines Loyalist 250,000 Astartes; the Ultramarines were the largest Legion at the Emperor's service
XX Alpha Legion Traitor Exact figures for the martial strength of the Alpha Legion at the outset of the wars of the Imperium are impossible to obtain. Sources and estimates vary wildly, some making extravagant claims which may both overestimate or undervalue the Alpha Legion's manpower. 120,000 Marines at best
IV Iron Warriors Traitor At the time of the outbreak of the War, the Iron Warriors Legion is estimated to have had an active strength of between 150,000 and 180,000 Legiones Astartes, along with a very substantial war fleet of over a hundred Capital class vessels. More accurate assessment than this is just an assumption.
XVI Luna Wolves Traitor 130,000-170,000 Astartes during the latter days of the Great Crusade; The Luna Wolves were renamed Sons of Horus for their deeds at being the forefront of the Great Crusade
XII World Eaters Loyalist 150,000 Astartes estimated at the XII Legion's peak. It is commonly estimated that of all the Legions that had fought for the Emperor, The World eaters have suffered the worst. Aside from the many wounded, it is recorded that a number had succumbed entirely during the protracted fighting to an insane bloodlust and had to be forcibly restrained and removed back to the World Eaters fleet for containment.
XVII Word Bearers Loyalist Exact numbers unknown; approximately 140,000 at the time of the Council of Nikia though following their rebukement at Khur, the XVII Legion had grown to far greater strength then was originally reported.
IX Blood Angels Loyalist Approximately 120,000 Astartes estimated at the Legion's peak.
X Iron Hands Loyalist In excess of 113,000 Astartes at the start of the War, it is unknown about the current strength of the Iorn Hands
III Emperor's Children Traitor 110,000 Astartes during the latter days of the Great Crusade;
VII Imperial Fists Loyalist The strength of the Imperial Fists was never large in numbers, though not small, their tallies of warriors during the Great Crusade never rose above 100,000. The vagaries of Warp travel, Astropathic communication and the volume of space across which the Great Crusade was fought means that all that is certain is that this tally is nothing but plausible estimation.
XIV Death Guard Loyalist 95,000 Astartes at its peak during the Great Crusade
XVIII Salamanders Loyalist Total number unknown; approximately 89,000 Astartes at the end of the Great Crusade.
XIX Raven Guard Loyalist Total Number just under 80,000 Astartes, with all assets, both surface and space borne, in attendance.
I Dark Angels Loyalist 20,000 Astartes comprised the I Legion during the latter days of the Great Crusade.
XV Thousand Sons Loyalist 10,000 Astartes during the latter days of the Great Crusade
VIII Sons of Konrad Traitor The Night Lords were estimated to comprise slightly over 90,000 Astartes, though others put their number closer to 120,000.
V White Scars Loyalist The V Legion is believed to have comprised only 7,000 Astartes at the end of the Great Crusade, making it one of the smallest of the Space Marine Legions, if not the smallest.
VI Space Wolves Loyalist Estimates of the Legionary strength of the VI Legion at the time of the Great Crusade and the Horus Heresy are not currently available 
II Damocles Hammer Loyalist (I have no fucking idea)
XI Azure Knights Loyalist (I have no fucking idea)