Schism of Mars

The Schism of Mars was a deadly civil war that erupted on Mars between loyalist Adeptus Mechanicus and Dark Mechanicum forces during the outset of the Horus Heresy. This great conflagration between opposing Mechanicum forces mirrored the larger conflict that consumed the Imperium.

History
As the Emperor's Great Crusade continued to expand outwards, bringing untold human-settled space back under the control after being cutoff from mankind during the Age of Strife. This large scale crusade continued unabated until the Imperium encompassed nearly the entire galaxy. At that time a new and unexpected threat emerged to challenge Mankind's dominance over the galaxy. This was the rebellion that would be known to later ages as the Horus Heresy. This revolt was instigated and led by the Warmaster Horus, the greatest and most beloved of the Emperor's sons, the Primarchs. The rebellion began on the farthest Eastern Fringes of the galaxy with the Warmaster's virus-bombing of those Space Marines whose loyalties to him were suspect on the world of Istvaan III, and soon took hold amongst half of the Space Marine Legions and many of the Titan Legions serving under the command of the Warmaster.

It is not entirely clear how Horus managed to turn such a significant percentage of the armies under his command against the Emperor, but he was known to be a very skilled and persuasive leader who commanded immense personal loyalty amongst his subordinates. But even before the opening stages of his planned insurrection occurred, he knew he would have to secure the support of the Adeptus Mechanicus and their superior technology and weapons if he was to defeat the Emperor and conquer the galaxy. Horus won over the loyalty of many of the Mechanicus' Tech-Adepts after promising them the lost secrets of ancient Standard Template Construct (STC) technology that had been recovered from the worlds of the recently subjugated Auretian Technocracy by the Sons of Horus Legion.