First Founding
- "They shall be my finest warriors, these men who give of themselves to me. Like clay I shall mould them, and in the furnace of war forge them. They will be of iron will and steely muscle. In great armour shall I clad them and with the mightiest guns will they be armed. They will be untouched by plague or disease, no sickness will blight them. They will have tactics, strategies and machines so that no foe can best them in battle. They are my bulwark against the Terror. They are the Defenders of Humanity. They are my Space Marines and they shall know no fear."
- --The Emperor of Mankind on the creation of the Adeptus Astartes
After he conquered Terra, the Emperor of Mankind set out on a Great Crusade to re-unite the lost human colonies into a mighty Imperium. He soon realized that bog-standard humans wouldn't be enough; some situations called for a small number of devastatingly powerful warriors, a surgical strike force to smash enemy linchpins. To fill this role, he created his Primarchs, and from their genes, he created the Legiones Astartes, the twenty Space Marine Legions:
| Legion Number | Primarch | Homeworld | Name of the Legion | Allegiance | |
| I | Lion El'Jonson | Caliban | Dark Angels | ||
| II | +++Records expunged+++ | ||||
|---|---|---|---|---|---|
| III | Fulgrim | Chemos | Emperor's Children | Traitor | |
| IV | Perturabo | Olympia | Iron Warriors | Traitor | |
| V | Jaghatai Khan | Chogoris/Mundus Planus | White Scars | Loyal | |
| VI | Leman Russ | Fenris | Space Wolves | Loyal | |
| VII | Rogal Dorn | Terra, Inwit | Imperial Fists | Loyal | |
| VIII | Konrad Curze | Nostramo | Night Lords | Traitor | |
| IX | Sanguinius | Baal | Blood Angels | Loyal | |
| X | Ferrus Manus | Medusa | Iron Hands | Loyal | |
| XI | +++Records expunged+++ | ||||
| XII | Angron | Nuceria, Bodt | World Eaters | Traitor | |
| XIII | Roboute Guilliman | Macragge | Ultramarines | Loyal | |
| XIV | Mortarion | Barbarus | Death Guard | Traitor | |
| XV | Magnus the Red | Prospero | Thousand Sons | Traitor | |
| XVI | Horus Lupercal | Cthonia | Luna Wolves/Sons of Horus | Traitor | |
| XVII | Lorgar Aurelian | Colchis | Word Bearers | Traitor | |
| XVIII | Vulkan | Nocturne | Salamanders | Loyal | |
| XIX | Corvus Corax | Deliverance | Raven Guard | Loyal | |
| XX | Alpharius Omegon | Unknown | Alpha Legion | It's complicated | |
Organization
The Legio Astartes, being far larger then current Space Marine Chapters, were organized along drastically different lines. At the head of the Legion was its Primarch Commander who oversaw the Legion's headquarters. Below this were the Chapter of the Legion, each of which was commanded by a Lord Commander. Each Chapter in turn was divided into a number of Battalions led by a Lieutenant Commander. Each battalion was then further divided into Companies led by a Captain. The Company, itself divided into a number of Squads, was the basic military division of the Legio Astartes. Due to the varying sizes of each Legion and the varying nature of supply and attrition, there was no fixed number of how many Chapters, Battalions, Companies, and Squads a Legion would contain. But even then, there was no fixed institution that a particular Company should be composed of certain units, with each Company specialising naturally as marines and officers became veterans of a certain form of warfare.
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 Legion of all was the Ultramarines. The Thousand Sons of Magnus 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.
The approximate sizes of a few of the Legions at the start of the Heresy have been given in various sources:
- Ultramarines - 250,000
- Sons of Horus - Between 130,000 and 170,000
- World Eaters - 150,000
- Word Bearers - 100,000 to 150,000
- Blood Angels - 120,000
- Night Lords - 90,000 to 120,000
- Iron Hands - 113,000
- Imperial Fists - 100,000
- Death Guard - 95,000
- Salamanders - 89,000
- Raven Guard - 80,000
- Emperor's Children - 110,000
- Thousand Sons - 10,000
Other Legions have not been so explicitly numbered, but their impression can still be gained by how they are described.
- Dark Angels - The literal "First" Legion had been on Crusade longer than any other Legion and two years into the Horus Heresy still had enough manpower to "crack Macragge in half".
- Space Wolves - Were considered to be smaller than most of the other Legions, but a pre-heresy skirmish with Angron showed that Leman Russ knew how to obtain his objectives extremely efficiently.
Horus Heresy
For all their power, the Space Marines and Primarchs were not perfect; half of them were sufficiently flawed to be tempted by (or driven to) Chaos, led by Warmaster Horus. The traitors were defeated, but at a terrible cost, including the near-death of the God-Emperor of Mankind and trillions of deaths.
Roboute Guilliman decided that the so-called Horus Heresy was proof that one man could not be trusted with power over one-twentieth of the Imperial Armed Forces, so he enacted several reforms to divide the Imperial Army into the Imperial Guard and Navy, and split the remaining loyalist Legions into Chapters, in accordance with his Codex Astartes. This event was later known as the Second Founding.
Two Unknown Legions
In the First Edition of Warhammer 40,000, the twenty First Founding Chapters were all known. It seems that Games Workshop had more love for some than others; when they made fluff revisions in the transition to Second Edition, the Valedictors and Rainbow Warriors were demoted to one of the "Later Foundings."
Nowadays, nothing concrete is known about the Legions II and XI, and likewise their unknown Primarchs. Officially, the Imperium deleted all records regarding the "Lost Legions"; the only reminder of the two legions were empty plinths in the Hegemon where statues of the Primarchs stood at the Imperial Palace. Throughout the Horus Heresy series, it is suggested that the Space Wolves destroyed them for some reason. The Horus Heresy: Massacre includes a timeline of the events at the end of the Great Crusade, and in 965 and 969.M30, the Space Wolves engaged in two missions from which all data was redacted. In the book The First Heretic, when a daemon takes the Word Bearers on a trip back in time to show them the creation of the Primarchs, the Word Bearers dialogue indicates that the XI Legion in particular did something bad enough for the Emperor to lead their purge himself, and that the left-over Marines were folded into the Ultramarines (their Chaplain dismisses it as a rumor, though it is public record that the Ultramarines received a major boost in troops at the time). In the book Fear To Tread, Sanguinius tells Horus that he hasn't revealed the existence of the Red Thirst to the Emperor because he fears that the Blood Angels would be purged as well, indicating that gene-seed flaws may have also been a factor. Deliverance Lost has a dialogue between taking place during Corax's first meeting with the Emperor where he asks why only sixteen of his brothers were waiting to meet him if he was the nineteenth Primarch to be found, only for the Emperor to deflect the question; consequently, we can assume that the Legions were purged sometime before Corax's discovery (and were never around for the Horus Heresy). He also forced the remaining Primarchs to swear an oath never to speak of their absent brothers, so whatever they did must have been extraordinarily bad. In Legion, a ship's captain notes that the naughtiness of the Alpha Legion isn't the first time that a Legion has "overstepped it's mark" and that the Imperial Army fleet should report the Alpha Legion "before they become too powerful," which might imply that one of the Legions got away with naughtiness for a long time and then used their entire Legion to fight the Emperor.
Or there may simply be no missing Legions. According to a source who allegedly worked for Games Workshop for five years (seen here), Rick Priestley (who helped write the first Rogue Trader) read about the Roman notion of Damnatio Memoriae, and simply threw in the idea of two "missing" Space Marine Legions as a "nod" to Imperial Rome (specifically, the three Roman legions whose numbers were never used after they were wiped out in the disastrous Battle of the Teutoburg Forest. So, nothing significant can ever be revealed about them, because they never really existed in the first place. Except they totally existed in the current canon, even if they started as a joke. Also, the real reason that they were kept around between Rogue Trader and recent revelations was so that people could make up their own pair of Primarchs for model painting and personal fanon (although this has lost its effect as doing so is a step in sending them down the path of the Mary Sue).