The Codex Astartes is the foundational treatise that governs the organization, tactics, and doctrine of the Space Marines. Composed in the aftermath of the great civil war that nearly destroyed the Imperium, it was written to reform the superhuman warriors and prevent any single commander from ever again amassing the catastrophic power that had made betrayal so devastating.
Its most sweeping reform broke the enormous ancient Legions into smaller, self-contained Chapters of roughly a thousand warriors each. The Codex further prescribes how these Chapters should be structured, detailing companies, squads, ranks, and the roles each warrior fulfills, from scouts to veterans to specialist units. It also lays out a vast body of battlefield strategy, recording tried and tested formations and responses to nearly every conceivable situation.
Most Chapters treat the Codex with near-religious reverence, following its teachings as holy law and measuring their conduct against its ideals. Others adhere only loosely, preserving older traditions or unusual practices that predate its writing, and a few reject significant portions outright. Debate over how strictly it should be obeyed has divided the Space Marines for millennia. Whether embraced or resisted, the Codex remains the single most influential document shaping the Emperor's Angels of Death, a blueprint born of hard-won experience and the determination that past horrors must never be repeated.