Principia Belicosa - Legion Integration and Development: The Space Marine Legions of the Great Crusade were a fusion of dual natures. In one pan they were all the product of the Emperor's conquest of Terra and the Sol System. Their earliest initiates were all selected from Terran stock and moulded by the wars of the nascent Imperium of Man, and so 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 the warlike history of their homeworld, 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 techno-barbarians 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 Old 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 new intakes of initiates were drawn from Luna, Saturn's orbital stations, the Proximal worlds, and dozens of other worlds 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 rediscovery of the primarchs and in many cases the granting of new homeworlds as Legion fiefs (most commonly the worlds upon which their new masters had been found), this was to change the character and cultures 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, after their reunification with their primarchs. 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 standard 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 corrupting stain of the Warp would later taint them.