Schism of Mars

The Schism of Mars was the deadly civil war that erupted on Mars between the Loyalist factions of the Adeptus Mechanicus and the Traitors of the Dark Mechanicus during the early days of the broader interstellar civil war in the 31st Millennnium between the Imperium of Man and the Forces of Chaos loyal to the Warmaster Horus. This great conflagration between opposing Mechanicus forces mirrored the larger conflict that consumed the galaxy during what became known as the Horus Heresy.

History
The Emperor of Mankind's Great Crusade continued to expand outwards into the galaxy in the early 31st Millennium, bringing untold numbers of human-settled worlds back under the control of a single human authority and ending the isolation that had consumed the human-settled galaxy during the 5,000 Terran years of the Age of Strife. This massive expansion of the Imperium continued unabated until the Emperor's realm 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 terrible interstellar 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 super-human, genetically-engineered sons, the Primarchs. The rebellion began 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 9 of the 18 active 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 earned immense personal loyalty amongst his subordinates. But even before the opening stages of his planned insurrection began, 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-priests 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 his own Sons of Horus Legion.

War Comes to Mars
The climate on Mars was full of discontent during this tumultuous time in the days just before Horus openly declared his rebellion against the Emperor. There were tense relations between the various Mechanicus Magi who governed Mars with sporadic outbreaks of espionage and violence being committed against the various forges that represented the primary sociopolitical units of Mars. There were even unconfirmed suspicions that the various Titan Legions of the Collegia Titanica, the most potent military forces available to the Mechanicus, had already secretly chosen sides in case of a potential conflict.

At the outset of the Horus Heresy, the Warmaster Horus sent Regulus, an Adeptus Mechanicus representative who had already thrown in his lot with the Warmaster, to Mars to secure the tentative support of the Fabricator-General of the Adeptus Mechanicus, Kelbor-Hal. Regulus convinced the Fabricator-General of the Warmaster's resolve to support increased autonomy for the Mechanicus against the autocratic rule of the Emperor. As a show of his appreciation for the Fabricator-General's support, Horus provided information to Kelbor-Hal that allowed the Tech-Priest to open a repository of forbidden knowledge known as the Vaults of Moravec, which the Emperor had ordered sealed two centuries earlier -- for they contained innumerable artefacts of technology that had been fashioned or corrupted by the malign power of Chaos. But their dark bargain was struck, and the Fabricator-General accepted Horus' proposal and joined forces with the Warmaster, assisting the Traitors with all of the technology of Mankind at his disposal.

When this repository was reopened, there was all manner of forbidden arcane knowledge and weaponry that had obviously been tainted by the corrupting influence of Chaos stored within. Soon the corruption spread throughout the forges and temples across the Red Planet as scrap code -- Chaos-contaminated digital source code that was infected with an arcane computer virus -- infested the logi-stacks and Cogitator (computer) archives of the Adeptus Mechanicus, causing literal Chaos to emerge in any Cogitator system that was networked to one of its infected counterparts. Later histories would record that the first blow of the Martian civil war was struck against Magos Mattias Kefra, whose forge in the Sinus Sabaeus region was housed within the Madler Crater. Titans of the Legio Magna marched from the southern Noachis region and within minutes had smashed down the gates of Kefra's forge. Howling engines daubed in red, orange, yellow and black, and decorated with flaming horned skull devices, ran amok within the high walls of the crater, crushing everything living beneath them and destroying thousands of years of accumulated wisdom in a fury of fire.

Vast libraries burned and weapon shops that served the Solar Guard were reduced to molten slag as the indiscriminate slaughter continued long into the night, the Magna Legion’s trumpeting warhorns sounding like the atavistic screams of primitive savages.

Further north in the Arabian region of Mars, the great engine fabrication yards of High Magos Ahotep in the Cassini Crater were struck by a hundred thermonuclear missiles launched from the atomic silos secreted within the isolated peaks and mesas of Nilo Syrtis. The explosions of the forbidden weapons filled the four hundred and fifteen kilometer diameter of the crater with seething nuclear fire, and sent conjoined magma-streaked mushroom clouds soaring nearly seventy kilometres into the sky.

Along the borders of the Lunae Palus and Arcadia regions, what previously been simply heated debate between the partisans of the Emperor and the Warmaster erupted into outright warfare as Princeps Ulriche of the Death Stalkers unleashed his engines upon the fortress of Maxen Vledig’s Legio Honorum.

Caught by surprise, the Legio Honorum lost nineteen Titans in the first hour of battle, before withdrawing into the frozen wastes of the Mare Boreum and seeking refuge in the dune fields of Olympia Undae. Their calls for reinforcement went unanswered, for all of Mars was tearing itself apart as the plague of war spread across the planet in a raging firestorm.

Amid the Athabasca Valles, the war machines of the Legio Ignatium and the Burning Stars Legion fought in bloody close quarters through the teardrop landforms caused by catastrophic flooding in an earlier, ancient age of the Red Planet. Neither force could gain the advantage, nor could either claim victory, so after a night’s undignified scrapping, both withdrew to lick their wounds.

At Fabricator Locum Kane's forge, the Mondus Occulum, the outlying forge Cogitators and logic engines had clogged with corrupt data, howling ghosts of sourceless machine-noise and dangerous code packets of infected algorithms that many of the most advanced protective Aegis Protocols were helpless to defeat. Only Kane’s swift action to shut down the Input/Output highways and the fact that the vast majority of his systems had recently been upgraded to take advantage of Koriel Zeth’s revolutionary system of noospheric data transference had spared them the worst of the attack, for an attack it surely had been. The opening acts of treachery had already occurred, yet the Fabricator-General could not march to war against the Emperor without the appropriate pretense, providing the excuse to silence his detractors and eliminate his rivals. The Fabricator-General and his Dark Mechanicus allies used the disruption unleashed by the scrap code attack to bide their time and marshal the strength of their forces. Kelbor-Hal and his allies used the tactics of sabotage and assassination in an attempt to eliminate those who were unwilling to join their cause. But the opportunity to go to war eventually presented itself when Techno-Magi Koriel Zeth, the mistress of Magma City, declared that she did not believe the Omnissiah actually existed. This open apostasy from the sacred doctrines of the Cult Mechanicus was the excuse the forces of the Dark Mechanicus needed to finally wage open warfare against their enemies, declaring them to be Heretics and apostates to the faith that had been sacred on Mars for millennia. Magma City would soon become a focal point in the struggle for those among the Mechanicus who remained Loyal to the Imperium. Open warfare eventually erupted across the Red Planet as Martian forces, both civilian and military, fought one another in a deadly and escalating civil war in which the destruction unleashed on Mars mirrored that unfolding in the wider galaxy between Loyalist and Traitor forces.

Forlorn Hope
For those still Loyal to the Emperor on the Red Planet, their salvation arrived in the form of a great Imperial expeditionary fleet. Malcador the Sigillite, the Regent of Terra, charged Rogal Dorn with a mission of the most vital importance -- to secure the forges of Mars for the Imperial war effort. Dorn informed the Sigillite that he would send his Legion's Champion and First Captain Sigismund, 4 companies of Imperial Fists and a large force of Imperial Army soldiers to secure the forges of Mars. Within the northern hemisphere of the Red Planet lay an arc of Loyalist control -- the important munitions manufactorums of Mondus Gamma and Mondus Occulum. Within these two vast industrial complexes the Mechanicum fabricated the weaponry and Power Armour for the Legions of the Space Marines. The forges of Mondus Occulum and Mondus Gamma alone produced the bulk of the armour and weapons of the Astartes. Dorn intended for his force to strike there to capture those forges and once they were occupied, the Imperial Fists and their allies would push outwards and secure the others.

Sigismund's companies landed at Mondus Occulum as the rest of the Imperial expeditionary force was fighting all across the surface of Mars. After a rapid deployment under fire in the shadow of Pavonis Mons, 13 companies of the Imperial Army's Saturnine Hoplites advanced on the Traitors' lines surrounding the forge of Ipluvien Maximal. Further south, 2 of the companies of Imperial Fists and 4 regiments of Jovian Grenadiers (nearly 15,000 Imperial Army soldiers in total) under the command of the Imperial Fists' Captain Camba-Diaz made planetfall in the Mondus Gamma forge complex.

Nothing of the Imperial Fists' mission to Mars panned out the way it had been expected. Camba-Diaz and the Jovian regiments soon became embroiled in a fight for their lives at Mondus Gamma forge, and the Saturnine Hoplites companies tasked with breaking the siege at Ipluvien Maximal’s forge had been repeatedly turned back by the horrifyingly altered weapon-creatures of the Dark Mechanicum. Though the fighting was desperate, Camba-Diaz eventually secured the armour forges and the ammunition silos, but his company was outnumbered a hundred to one. The Traitor Chrom’s troops pushed him back to the landing fields and his losses were grievous. Sigismund knew the Imperial expeditionary force would not be able to hold the forge, but a great deal of essential supplies had been secured for transit to Terra. Sigismund’s companies had descended upon Mondus Occulum not knowing whether they would have to fight to secure the forge, but it was a relief to find that Fabricator Locum Kane still held true to the Imperium. Sigismund secured vast quantities of munitions for transport back to Terra, including nearly 12,000 suits of Mark IV Power Armour and twice as many Astartes weapons. But the Loyalists had run out of time. Despite the locum's Servitors working at full capacity, it still was not rapid enough, for Sigismund's ship masters informed him of a sizable enemy force closing in, composed of infantry, armoured vehicles, Skitarii and at least two Traitor Titan Legions comprising nearly 60 war engines in all.

Sigismund's desire to wreak a bloody vengeance on the heads of those who had rebelled against the Emperor warred with the necessity of the mission his Primarch had given him to secure the vital armour and weapons of Kane’s forge that would be needed to defend the Imperial Palace at a later date. Reluctantly he knew he must stay true to his mission, for the forces arrayed against the Imperial strike force were simply too many and his orders did not allow for the pursuit of futile gestures of defiance. Locum Kane warned the Imperial Fists' First Captain that if both the Mondus Occulum and Mondus Gamma forges fell, the Imperium would have no way of replenishing the combat losses they would sustain in any meaningful way. After barely a few hours worth of fighting both Mondus Occulum and Mondus Gamma were burning, with vast swathes of their industrial machinery and manufacturing capacity destroyed. The loss of such irreplaceable technology and knowledge for all time would be felt by the Imperium for many millennia to come. Like comets launching from the surface of Mars, the Imperial starships fled for the heavens. Astartes and Imperial Army vessels jostled in the sky in their haste to depart the Red Planet. Barely a thousand Loyalist warriors were able to escape from the planet's surface before the manufactorums fell into the hands of the Traitors. The desperate rearguard action of the Astartes and other Loyalists on Mars and the sacrifice of many thousands of lives at least had secured tens of thousands of suits of newly-fabricated Mark V and Mark VI Space Marine Power Armour and other vital materiel that would ultimately prove to be the edge for the forces of the Imperium in the campaign against the Traitor Legions of Horus Lupercal.

Across the galaxy, open rebellion erupted as Space Marine Legion fought Space Marine Legion and Titans battled Titans. The members of what became known as the Dark Mechanicus supported Horus during the start of the Horus Heresy, participating in the horrendous attacks against the Loyalist Space Marine Legions during the Drop Site Massacre on Istvaan V. Throughout the period of the Heresy they would use dark and forbidden knowledge drawn from the inexhaustible well of Chaos to help destroy the Loyalist forces.

Source
[[Category:S]]
 * Horus Heresy: Collected Visions (Compilation), p. 337
 * Mechanicum (Novel) by Graham McNeill, pp. 273-274, 320-322