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{{SpaceMarinesPortal}}

The '''Codex Astartes''' is the doctrine of the [[Space Marine Chapter]]s, governing all aspects of Chapter organisation and battlefield tactics. For any given tactical situation, the Codex has hundreds of pages devoted to how it may be met and overcome. The wisdom of thousands of Imperial warriors has contributed to the Codex, and details on everything from unit markings to launching a full-scale planetary assault are contained within its pages.

The Codex was written and formalised after the [[Horus Heresy]] by the [[Primarch]] of the [[Ultramarines]], [[Roboute Guilliman]]. Some Chapters, such as the Ultramarines and their [[successor]]s, regard the Codex as a holy text, and follow it without question. Many other chapters also follow the Codex, though not always as strictly as the Ultramarines; these two types of chapters are collectively known as [[Codex Chapters]]. Among the Ultramarines, each battle brother must memorise whole sections of the codex, so that within a company there exists an entire record of the text.{{Fn|1}}

==History==

In the aftermath of the Horus Heresy, the [[Emperor]] had been incapacitated, and the Space Marines' numbers had been decimated by defections to [[Chaos]] and battle losses. With the Emperor confined to the [[Golden Throne]] the [[High Lords of Terra]] found themselves the new leaders of the Emperor's domain. Both the High Lords and surviving Primarchs became paranoid of the possibility of such events repeating themselves. The armies of the Emperor once again scoured the galaxy seeking out all forms of heresy and removing even the slightest chance of rebellion. The Ultramarines were left as the largest loyal legion, and Guilliman assumed the title of [[Lord Commander of the Imperium]].{{Cite This}} Concerned that once again their could be a threat from such a terrible force each Legion was to be divided into smaller Chapters and a new set of regulations would be created to govern them.{{Fn|9}}