Astra Militarum - Foundation of the Imperial Army: The force that would evolve into the Astra Militarum first came into being during the Horus Heresy. Since those days, in battles beyond number, countless Imperial soldiers have forged a roll of honour so mighty that an entire continent of Terra is given over to its immortalisation. Yet every victory is but a drop in an unending ocean of warfare, for new threats assail the Imperium every single solar day.The Imperium of Man was carved from the stars during the glorious centuries of the Great Crusade. The Emperor Himself fought at the forefront of Imperial expansion, supported by His mighty gene-sons the primarchs, and their gene-progeny the Space Marines. Yet even such illustrious and all-conquering heroes could only be in so many places at once. As the bounds of Mankind's conquests spread ever further outward, so the original Space Marine Legions were spread thin.Rather than risk losing control of conquered worlds, the Emperor commanded that the Imperial Army be formed. Drawn from worlds already in Imperial Compliance, these formations appear to have originally comprised a collection of volunteers, mercenaries and the survivors of indigenous armed forces, and were little more than garrisons or peacekeepers. Each newly conquered or liberated world brought into Imperial Compliance during the course of an Imperial campaign of the Great Crusade was assessed and a census taken of its population. From this, the Emperor's administrators of the Corps Logisticae, the forerunner of the later Departmento Munitorum, calculated the tithe that each world was to pay in the form of regiments of soldiers and war materiel.The numbers of regiments raised from each world varied enormously in accordance with the size of each individual planet's population. Sparsely populated worlds would be tithed to supply only a handful of regiments annually, whilst the overcrowded Hive Worlds near to the galactic core would have to supply hundreds of regiments every standard year. However, the demands of the Great Crusade continually increased; by the time the burgeoning new realm of Humanity reached the Eastern Fringe of the galaxy, the Imperial Army was a fully-fledged arm of its war machine fighting on the frontlines. Imperial Army forces were self-contained and autonomous, possessing control of their own Warp-capable warships and transports. Vast and sprawling, these potent fighting forces were commonly assigned to Space Marine commanders and could conquer whole worlds in their name. All this changed after the horrors of the Horus Heresy.When the Warmaster Horus turned upon the Emperor, fully half of the Space Marine Legions turned with him. Compelled by misplaced loyalty, fear or personal ambition, so too did the majority of those Imperial Army forces under the Traitors' direct control. Millions upon millions of trained fighting men and women, possessed of their own dedicated armour units and interstellar transportation, were suddenly set at the nascent Imperium's throat.Worlds burned in the fires of orbital bombardment or were ground beneath the churning treads of countless tanks. Old scores were settled as formerly Compliant worlds turned upon those who had brought them enlightenment beneath the muzzle of a gun. One-time comrades fought tooth and nail amid the blasted rubble of Loyalist worlds as the Imperial Truth was torn down, replaced by heretical idols to ancient and malicious gods. The Traitor Legions of the Heretic Astartes were a terrible threat to the Emperor's domain, yet on every front of the interstellar civil war the situation was made worse by the countless warships, tanks and troops of the Imperial Army's rebels.