Space Marine Legions

A Space Marine Legion or Legion Astartes was the original unit formation of the Space Marines created during the First Founding by the Emperor of Mankind on Terra before the start of the Great Crusade that reunited the scattered worlds of humanity underneath the banner of the Imperium of Man. A Space Marine Legion, unlike the 1000-man Chapters of the present-day Adeptus Astartes created after the Second Founding following the Horus Heresy, could number anywhere from 10,000 to more than 100,000 Space Marines ( the Ultramarines and Word Bearers were closer to the latter number), as well as the Legion's associated support forces and fleet elements. There were originally 20 Space Marine Legions, of which nine sided with the Emperor of Mankind during the Horus Heresy and are thus the ancestors of all the current Space Marine Chapters of the present. The Space Marine Legions and their Primarchs were the Emperor's primary military forces 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 even over the Primarchs, a change that angered many of his 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 Imperial Guard and Imperial Navy, but these forces more often served in support and garrison duties while the worst combat and the most important and difficult missions were always carried out by the Astartes Legions.

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 the 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 Legion Astartes 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 sometime 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 purged from all Imperial Records.

Number of Legionaries
The Legions were massive armies, and the size of each could vary tremendously. A precise number was never truly achieved and maintained. Even during the Great Crusade some Legions were very numerous, while others were not. The numbers would always vary with new recruits and inevitable battle-losses, and also important was the availability of potential recruits and the administrative skills of the Primarch and his officers.

The most numerous Space Marine Legion of all was the Ultramarines, followed by the Word Bearers (according to references in the Horus Heresy novel "The First Heretic"). As the Word Bearers were mentioned to number at least one hundred thousand Astartes, this may be used as a baseline for the Ultramarines' own numbers.

The Thousand Sons of Magnus the Red were of a small number as many of them had developed mutations or uncontrollable levels of psychic powers. Fulgrim's samples had been largely lost, and this left the legion of the Emperor's Children also with a very small number. Both of these Legions would increase their numbers to acceptable standards only after their Primarchs were found.

Original Legion Names
The Astartes Legions were for a very long time after their initial creation under the direct command of the Emperor and originally all Space Marine recruits were drawn from Terra itself rather than the later Legion homeworlds of the Primarchs. The Terran Space Marines would always remember these times and the Legions initially had overwhelmingly Terran traditions and origins. Later, as the Primarchs were found, the new recruits would have very strong loyalties towards them, and some of the primarchs would re-name their legions. The Emperor saw no problems with this development; he believed the loyalty of the Primarchs to him was unshakable. This reasoning would be proven to be sadly mistaken as the Emperor misjudged his sons; for all their seeming physical and mental perfection they were still human and prone to all the sins of humanity, including greed, anger and the lust for power, recognition and advancement--all emotions that left them susceptible to the temptations offered by the Ruinous Powers of Chaos.

The first Primarch to be found on the world of Cthon, Horus, was the Emperor's favorite and he and his Legion, the Luna Wolves, were even granted the honour of being re-named as the Sons of Horus following Horus' elevation to the rank of Warmaster and his survival of the wound that nearly killed him on the moon of Davin. The First Legion was given the new name Dark Angels by Lion El'Jonson, and the Dusk Raiders received the name Death Guard by Mortarion, and while its original name is unknown, the highly honorific name Emperor's Children was awarded by the Emperor only after a speech made by their Primarch Fulgrim. It is unknown if other legions were also re-named.