Horus Heresy - Disposition of the Traitor Legions: Much remains uncertain in regards to the strategic positions, deployments and configuration of the forces loyal to the Warmaster Horus in the early phases of the war, and even their full scope and extent cannot be ascertained with any genuine certainty by the historian. Perhaps a full half of the Titan Legions and the numberless hosts of the Excertus Imperialis -- hundreds of millions of soldiers, vessels and war machines -- had either through corruption, misguided loyalty to Horus or simple blind ignorance in compliance with their orders, sided with the Warmaster. But this alone would never be enough to overthrow the Emperor. A thousand Battleships might be wrecked against the defences of Terra, ten million Auxilia might spend their lives in besieging the Eternity Gate, and ten millions follow them, and again and again, but Horus knew this alone would never avail him. Not against the superhuman warriors who held it; not against the Legio Custodes and defences designed by the Emperor's own hand and garrisoned now by Rogal Dorn and his sons, the Imperial Fists. So it was that the greatest attention should be placed on the location and strengths of the Traitor Legions. The Legions at the Warmaster's side during this period at the start of the conflict comprised eight Legions. The shattered remnants of the Thousand Sons, who would later join the Traitor's cause in earnest, were not yet at this time fully active in the war, and were still largely of unknown disposition and even allegiance following the apocalyptic Battle of Prospero. Although the Drop Site Massacre had inflicted terrible wounds on the Loyalist forces involved, the Traitor Legions had not themselves come away from the cataclysm unscathed which, combined with losses suffered during the recent purging of the Traitors' ranks at Isstvan III and elsewhere, had weakened Horus' position from its notional strength when the die for war was cast.Hard facts regarding the operational Legiones Astartes numbers are impossible to ascertain, but credible estimates place Traitor losses during the battles of the Isstvan, Phall and Paramar Systems in the region of 100,000 Space Marine fatalities, compared to an unknown number, perhaps three or four times as high, as a death toll for those who remained loyal to the Emperor. This, by many estimates, left something in the region of 900,000 Legiones Astartes under arms in the Warmaster's cause, with perhaps two thirds or more of that figure in the Loyalist camp. This estimate, however, was still far from certain, one which is itself further distorted by the events that were to shortly unfold on Calth and Signus Prime. From this calculation -- as equivocal as it is -- it is possible to determine that even after the Drop Site Massacre, the military advantage Horus had yet gained was simply not large enough in scale to make an immediate and direct assault on Terra and the Imperial heartlands of the Segmentum Solar, roused as they were now against attack, a strategically viable option, let alone one that would guarantee victory. This was the reality that Horus faced in determining his next move.The Traitor Legions were the single greatest power and strength of Horus' armies, but they were far from alone. To achieve the ultimate aim of laying siege to Terra and casting down the Emperor, the Traitors would have to call upon and fight alongside a wide array of other forces, some simply obeying the will of masters of long association, while others were brought into the Warmaster's fold through bribery, pacts of alliance, coercion or the infamous progress of the Dark Compliance. As early as the Traitors' defence of the Urgall Depression during the Isstvan V Drop Site Massacre, mortal Auxilia troops were used with unspeakable callousness by their Space Marine masters, herded towards the enemy as bullet-soaks on which the foe's ammunition would be expended before battle was truly joined, and such disregard for the purely human component of the Warmaster's forces would be shown time and again throughout the bloody years that were to follow. This was not however universal, as at other times, Horus and his principal subordinates entrusted vital missions to the more reliable of his mortal allies, and split entire battlegroups of Traitor Auxilia and Armada units on independent operations either of conquest or destruction, although it was not uncommon for him to assign an officer or cadre of the Sons of Horus, Iron Warriors or Alpha Legion to oversee such forces in the field.