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"The wise commander utilises his enemy's weakness even more than he utilises his own strength."

—Introit to the Principia Belicosa

The Palatine Aquila, ancient symbol of the Imperium of Man

The Principia Belicosa, also spelled Principia Bellicosa, was the principal tome of Imperial military organisation, strategy and tactics used during the Great Crusade era. It was based on those used by ancient Roma and other military guides of the human past. It was utilised by the Legiones Astartes as a source that dictated their disciplined structure and organisation when they were created during the latter years of the Unification Wars in the late-700's.M30.

This sage wisdom was taken from the ancient and proven Terran patterns of strategy, hierarchy and function as laid down in this revered text, as well as Krom (Oliver Cromwell)'s fragmentary New Model, that had survived in the hands of the tyrants of Old Earth down the blood-stained generations of the Age of Strife. To these venerable treatises the Emperor of Mankind and His commanders had added their own genius and created a sturdy but adaptable strategic framework that spoke to the fundamental strengths and transhuman abilities of the Astartes themselves.

The chain of command was simple and direct, and the Legions' officers, themselves mighty warriors, would lead them into battle personally as had long been the wont of the techno-barbarian tribes of Ancient Earth, and the battle would be taken always to their enemy. This was because to defeat a foe was never enough for the Legiones Astartes, only the utter destruction of an enemy was victory.

This cold logic, coupled with their inhuman strength and bearing, and the sheer dread they inflicted on friend and enemy alike, was to give them one of their earliest appellations, and perhaps their most appropriate --"the Angels of Death." It would be a name they would earn time and time again.

At the outset of the Great Crusade, ca. 798.M30, many of these early Legions were organised along the so-called "Terran Pattern" of organisation as formulated by the Imperial Officio Militaris.

Legion Integration and Development

The Space Marine Legions of the Great Crusade were a fusion of dual natures. In one pan they were the product of the Emperor's conquest of Terra and the Sol System. Selected from Terran stock and moulded by the wars of the nascent Imperium of Man, these Legions held a certain commonality of character.

Given these Astartes were all of the first generation, born only of Terra, they shared the imprint of their genetic forging and warlike history, and it is no surprise that members of the Legions at this time regarded each other as siblings, as brothers. Training, indoctrination and the shared experience of battle reinforced this belief within the Legiones Astartes -- that they were a family born in the cradle of war. This bond of brotherhood would survive as the Imperium grew, but perhaps it was never as strong as it was when the Legions first conquered under the Terran sun.

The second face of each Legion was that of people and cultures which had flourished under different stars. As the war to unify Earth became a crusade to conquer the galaxy the Legions grew. Casualties had to be replaced, and as the wars increased in scale so too did the number of losses and the number of recruits that were needed to take the places of the fallen.

Initially intakes were drawn from Luna, Saturn's orbital stations, the Proximal worlds, and dozens of others of the near Segmentum Solar fed the Legions' need for warrior stock. As this occurred, the Terran foundation of each Legion became diluted but was never overwhelmed, for the warrior tribes and cultures of Old Earth were many and their commitment to the Great Crusade resolute.

With the discovery of the Primarchs and in many cases the granting of new homeworlds as Legion fiefs (most commonly the worlds upon which their new master had been found), this was to change the character of the Space Marine Legions profoundly. Some alterations were superficial: a habit of speech, a change in close-quarter tactics, martial traditions and warranted additions to Legion iconography and even language.

But for others the change would prove dramatic, with entire paradigms of culture, tradition and even ideology overwriting what had come before, such as in what came to be known as the Space Wolves and Dark Angels Legions, for example. In many cases the stamp of the Legions and the will of the Primarchs on their recruits came to largely outweigh differences of birth or blood.

By the middle years of the second century of the Great Crusade (ca. 950.M30) the on-going effects of these shifts within the Legions had resulted in wide disparity between them. The outward sign of this was the development of a distinct character which meant the original Terran military patterns they had adhered to at the outset of the Great Crusade had been largely abandoned or become so modified and diluted as to be in some cases unrecognisable.

There were, of course, exceptions in whole or in part, such as could be found within the Ultramarines and Iron Warriors Legions who built upon rather than abandoned what had gone before, as well as those such as the White Scars to who the strictures of the Principia Belicosa could no longer be even notionally applied. There are those that have cited this increasing idiosyncrasy in hindsight as the seeds of division -- of a sense of insularity and "otherness" growing between one Legion and another.

This itself only fuelled rivalries and feuds that had begun to simmer beneath the surface of the Great Crusade, both between the Primarchs and their Legions, that would later be exploited by Horus and his heretical conspiracy, and lay them open to the introduction of the Davinite Warrior Lodge cult structures into those he favoured -- the avenue by which the stain of the Warp would taint them.

Legion Structure

Throughout the early years of the Great Crusade, many Legions kept much of their outward structure as it had existed since the wars for Terran and Solar Unity, which had adhered closely to the Terran Pattern laid down by the Officio Militaris before the Legions' unification with their Primarchs. However, by the time of the outbreak of inter-Legionary hostilities during the Horus Heresy in the early 31st Millennium, the command and organisational structure of a Legion was often more a mirror of the character and preferences of its Primarch and its abiding culture than formal writ.

While certain formations and features were common, as an outgrowth of practical matters such as deployment and logistics their organisation and use was far from standardised. The various terms of convenient external classifications for internal Legion formations (Great Companies, Regiments, Chapters, Battalions, Cohorts, Demi-Chapters, etc.) were realities during the Great Crusade.

More often the practical reality of the Legions' disposition would vary still more as terminology used within different Legions for equivalent ranks and specialisations bore the mark of each Legion's character and culture rather than the desires of central administrators for a common nomenclature. In some cases this discrepancy increased as local planetary languages came to replace Imperial Terran -- the language known to later generations as High Gothic.

Principia Belicosa Excerpts

"Battles are won by the application of superior force; wars are won by the application of superior resources." - The Principia Belicosa of Terra, Imperial Commentaries

"Superiority of force is key and the key to superiority is above all structure. With structure comes order, with order the ability to carry out purpose, and only through clear purpose can victory be perceived." - Principia Belicosa, Ch. XXXIV, Vs. I-IV


  • The Horus Heresy - Book One: Betrayal (Forge World Series) by Alan Bligh, pp. 29, 32
  • The Horus Heresy - Book Two: Massacre (Forge World Series) by Alan Bligh, pg. 119
  • The Horus Heresy - Book Three: Extermination (Forge World Series) by Alan Bligh, pp. 18, 87, 112, 258
  • The Horus Heresy - Book Four: Conquest (Forge World Series) by Alan Bligh, pg. 82
  • The Horus Heresy - Book Five: Tempest (Forge World Series) by Alan Bligh, pg. 231