Space Marines - The Codex Astartes: In the wake of the calamity that was the Horus Heresy, the foundations of the present-day Imperium were laid down during a period remembered as the "Reformation."The first High Lords of Terra established, under the direction of Guilliman, the structure by which the Adeptus Terra operated, and described the feudal responsibilities and duties of the Planetary Governors. One of the most important accomplishments was the reorganisation of the Imperium's military forces.This was undertaken almost single-handedly by Roboute Guilliman, who in his role as the post-Heresy Lord Commander of the Imperium moved with his characteristic speed and efficiency to codify the structure of the Astra Militarum, the Imperial Navy, and the Space Marines.With the threat of the Traitor Legions held at bay in the wake of the Horus Heresy and the Great Scouring, Roboute Guilliman turned to ensuring that such a catastrophe could never happen again.He distilled his formidable wisdom into a mighty tome known as the Codex Astartes. This text became a major part of his legacy and the cornerstone upon which the future of the Imperium would be based.Of all Guilliman's works, the most influential would prove to be the Codex Astartes, the great prescriptive tome that lays down the basic organisational and tactical rules for a Space Marine Chapter. The Codex Astartes decreed that Space Marines would be created and trained over a controlled period of time.Of special interest is the volume of the Codex that described in detail the tactical roles, equipment specifications, uniform markings, command protocols and countless other aspects of Space Marine doctrine. Though for all its multitudinous topics, the most lasting and contentious decree of the Codex Astartes was that the existing Space Marine Legions be broken up and reorganised into smaller organisations known as Chapters.Though many of his brother primarchs initially railed against Guilliman's decree, almost all eventually accepted the necessity of reorganisation for the security of the Imperium. Thus were the Chapters of the Adeptus Astartes born.Upon the Codex Astartes' implementation, in an event that would become known as the Second Founding, each of the old Legions became a single Chapter of 1,000 Astartes named for its forebear plus a number of other new Chapters.In addition to a name and heraldry of their own, each of these new Chapters would take for itself a homeworld or fortress-monastery, and use it as a bastion from which to defend the Imperium from all threats.The Codex Astartes stated that each Chapter would be one thousand battle-brothers strong and look to its own recruitment, training and equipment. Never again would one man be able to command the awesome, terrifying power of a Space Marine Legion.The Horus Heresy had also revealed the inherent weaknesses of the gene-seed of several Space Marine Legions. These defects had been exacerbated by the accelerated gene-seed cultivation techniques needed to keep the huge Space Marine Legions up to strength.Guilliman believed that the Chaos powers were able to exploit the resultant physical and mental corruption to turn Horus' troops against the Emperor.One of the key objectives of the Codex Astartes was to recognise and expunge these genetic weaknesses. As a result, the Codex decreed that Space Marines would forever more be created and trained slowly.The gene banks used to create Astartes implants would be carefully monitored and scrutinised for any defects. Cultivated organs would be subject to the most stringent tests of genetic purity.Young aspirants 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 Earth with setting up and maintaining gene 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 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. They alone had the power to destroy or create Space Marine armies at will.Over the millennia, there have been many subsequent Foundings of Space Marine Chapters. Many Chapters adhere rigidly to Guilliman's teachings. These Space Marines pride themselves on following the tenets within the hallowed pages of the Codex Astartes and applying its principles of warcraft and devotion to the Emperor.With the passage of 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 reanalysed, reinterpreted and modified countless times over the centuries.Indeed, the Codex Astartes of the late 41st Millennium is a highly developed treatise combining the experiences of hundreds of celebrated military thinkers throughout history.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.Most Chapters stick rigidly to the organisation laid down by the Codex Astartes for tactical roles and other processes. Others, such as the Blood Angels, Black Templars and Dark Angels, are organised according to general Codex doctrines but maintain troops, tactics and idiosyncratic traditions that set them apart from their brethren.A small number of Chapters are utterly different from the Codex, and owe nothing to it at all. The most famous of these is the Space Wolves. The sons of Leman Russ have never followed the Codex Astartes -- their strong-willed primarch moulded his Chapter very much in his own image, irrespective of other influences and dictates.