Codex Astartes - History: In the early 31st Millennium, in the wake of the calamity that was the Horus Heresy, the foundations of the Imperium of Man were reformed to make such a tragedy less likely in the future. The first High Lords of Terra, under the direction of the Lord Commander of the Imperium and Imperial Regent Roboute Guilliman, the primarch of the Ultramarines Legion, established the structure by which the Adeptus Terra operated, and described the feudal responsibilities and duties of the planetary lords.One of the most important accomplishments was the reorganisation of the Imperium's fighting forces. This was undertaken almost single-handedly by Guilliman, who with characteristic speed and efficiency codified the structures of the newly-created Astra Militarum, the Navis Imperialis, and the Adeptus Astartes.Of all of Guilliman's works from this time, the most influential is the Codex Astartes, the great prescriptive tome that lays down the basic organisational and tactical rules for the Space Marine Chapters.The initial inspiration for Guilliman's Codex Astartes came from Sergeant Aeonid Thiel, an Ultramarines veteran of the Underworld War of the Battle of Calth during the Heresy, who was present on Calth during that conflict against the treacherous Word Bearers.In the aftermath of the initial battle on the surface of Calth, when the fight devolved into a series of grinding assaults between small Astartes units surviving in the subterranean holds of that world after the poisoning of its sun, Thiel founded small, independent task-force squads called the Red Marked, which were made up of surviving Ultramarines. The Red Marked were responsible for rooting out and eliminating the forces of the Traitor Marine warbands that were hiding on Calth and the other ruined worlds of the Realm of Ultramar.During the meeting between the Ultramarines primarch and Thiel on one of Macragge's space stations in which Thiel reported his experiences, after listening to Thiel's report Guilliman tore apart the documents upon which had been writing his old tactical doctrines and made his message clear: the Ultramarines would no longer be a "horde under a warlord" that followed their primarch but "hundreds of thousands of individual Legionaries each in support of the other."Thus was the vision for a series of smaller Space Marine units that could operate as rapid-strike planetary assault forces conceived, an idea that would eventually be fully realised later in the doctrines of the Codex Astartes.Following the end of the Horus Heresy and the Great Scouring and the retreat of most of the Traitor Legions into the Eye of Terror, the Codex decreed that the nine remaining Loyalist Space Marine Legions would be divided into 1,000-warrior Chapters, the Chapter Masters of which would be directly beholden to the Emperor Himself and no other, not even the primarchs of their original founding Legions (save in the case of the single Chapter that would remain under each primarch's control and retain its Legion's original name). No one person in the Imperium could ever again control the transhuman might of an entire Legion of 100,000 or more Space Marines.The Horus Heresy had revealed previously unknown genetic weaknesses in the gene-seed of the primarchs and the Space Marines among the original 20 First Founding Space Marine Legions, weaknesses that left the Legions in question greatly exposed to corruption by the Ruinous Powers of Chaos.This risk was exacerbated by the rapid nature of Space Marine recruitment during the centuries between the start of the Great Crusade in ca. 798.M30 and the outbreak of the Horus Heresy itself in the early 31st Millennium. With the Imperium of Man expanding so quickly across the galaxy during the Great Crusade, the need for fresh recruits in the Space Marine Legions was great. So much so that some Legions had not been as particular in their gene-seed screening practices and recruit selection processes as they should have been.The first objective of Roboute Guilliman in writing the Codex Astartes was to both recognise and purge these weaknesses. As a result, the Codex Astartes decreed that Space Marines would forever more be created and trained slowly. The genetic banks used to create Astartes organ implants would be carefully monitored and scrutinised for any defects. Cultivated organs would be subject to the most stringent tests of purity. Young initiates would undergo trials of suitability before they were accepted, and only those of the very sternest character would be chosen.As a final safeguard, Guilliman tasked the Adeptus Terra on Terra with setting up and maintaining genetic banks to produce and store tithes of Space Marine gene-seed. These banks were to provide all new gene-seed for subsequent Foundings of Space Marine Chapters.To prevent cross-contamination, the genetic stock of each First Founding Legion was isolated whilst that of the Traitor Legions was placed under a time-locked stasis seal, though at the time many believed they had been destroyed.By taking direct control of these genetic tithes, the Adeptus Terra could ultimately control the Space Marines, thus ensuring that normal Humans and not the transhuman Astartes remained the true rulers of the Imperium. They alone had and have the power to destroy or create Space Marine armies at will.The Codex outlined a new, more measured process for Space Marine selection and recruitment and insisted that each newly-created Successor Chapter would tithe 5% of its genetic material to the Adeptus Terra and the Adeptus Mechanicus for testing and monitoring. It also decreed that only the Emperor Himself, through the auspices of the High Lords of Terra, would ever again be able to order the creation of a new Space Marine Chapter.All gene-seed would be subjected to the greatest genetic scrutiny before being used in the creation of new Space Marine implants. To maintain firm genetic control over the Space Marine gene-seed lines and prevent the emergence of potentially dangerous mutations or hybrids, the Codex rebuked the practice of sharing gene-seed between different Chapters. From then on, each Chapter would have to rely solely on the gene-seed produced in the bodies of its own Space Marines.The Codex also further defined the accepted tactical doctrine, Chapter organisation, order of battle, and recruitment practices for a Space Marine Chapter. It explained the different battlefield roles assigned to each squad of Space Marines in a Chapter, originally defining them as Tactical, Assault or Devastator Squads and assigning different equipment and purpose to each (see the excerpts below). These roles were later revised after Guilliman's resurrection in the Era Indomitus and broadened, to include both Firstborn and Primaris Marine units categorised as battleline, close support, fire support, Veteran and command.There were many other topics covered in the original Codex and all of them displayed Guilliman's formidable intelligence and hard-won wisdom. Most of the old Legions were divided into fewer than five Chapters following the horrific losses they had suffered during the Horus Heresy, but the Ultramarines, being by far the largest of the Legions, were divided many times.The exact number of new Chapters created from the Ultramarines is uncertain: the number listed in the oldest known copy of the Codex Astartes (the so-called Apocrypha of Skaros) gives the total as 24, but does not name them all.These Chapters would consist of ten companies of 100 Space Marines each. The breaking of the remaining nine Loyalist Legions into multiple Space Marine Chapters is known to Imperial historitors as the Second Founding, which occurred in ca. 021.M31, seven standard years after the death of Horus.As a result of the Second Founding, the Ultramarines' gene-seed became the favoured genetic stock of most subsequent Foundings. The new Chapters created from the Ultramarines are often referred to as the "Primogenitors," or "first-born." All of the Primogenitor Chapters venerate Roboute Guilliman as their founding father and patron.Only one of the original Space Marine Legions, the Space Wolves, has never been broken down into the ten companies decreed in the Codex Astartes. The Space Wolves continue to organise their forces into thirteen "Great Companies," different in organisation to those specified by the Codex.Note that the oldest copy of the Codex, the Apocrypha of Skaros, refutes the commonly-held belief that the Space Wolves have never been broken down into separate Chapters, stating that two Second Founding Chapters were created from the Space Wolves (Index Astartes I, pg. 13). However, the Space Marine Codex (5th Edition) lists only one Second Founding successor to the Space Wolves: the Wolf Brothers, who were disbanded at an unknown time because of extreme gene-seed corruption and mutation. Only in the Ultima Founding would new Chapters of Primaris Space Marines finally be raised using the gene-seed of Leman Russ.The Codex Astartes further defines the tactical roles, equipment specifications, and uniform identification markings of the Space Marines. With the passage of many Terran centuries, some Chapters have strayed from the strict letter of the Codex, introducing unique variations on its teachings but remaining broadly faithful to Guilliman's basic principles.Furthermore, the Codex has been reanalyzed, reinterpreted and modified countless times over the centuries, including most recently by Guilliman himself. Regardless, the Codex Astartes remains, as it has always been, the Space Marines' authoritative guide to waging war. As such, it is revered by every battle-brother as a holy text; the wisdom of the ancients serving as both scripture and the unbending rod by which they are measured.These guidelines have evolved in practice from Chapter to Chapter over the millennia, and the treatise and wisdom of hundreds of military thinkers have been absorbed in their own battle philosophies. The Codex Astartes is revered as a sacred text, and many Chapters regard its recommendations as sanctified by the Emperor Himself.