Mars, the crimson planet of the Sol System, serves as the second head of the Imperial Aquila, standing alongside Terra.
Under the rule of the Adeptus Mechanicus—the priesthood that worships the Omnissiah—Mars has entered the most glorious era of her ten-thousand-year history. Her will alone can decide half of the Imperium's affairs. However, Mars's glory did not begin with the Cult Mechanicus; it traces back to a far more ancient epoch.
When humanity first stepped into the stars, Mars became man's first colony beyond the homeworld, marking the start of her illustrious legacy. During the Dark Age of Technology, Mars stood as one of the most powerful realms among the human nations. Legend has it that Mars could compete on equal terms with Terra itself.
Yet, this prosperity did not last. The immense destruction of the Cybernetic Revolt ended the golden age that even the Master of Mankind remembers with longing. Subsequently, the Warp storms that engulfed the galaxy shattered the history of human civilization as a single entity.
Following the onset of the Warp storms, the vast human civilization that once spanned the stars was sliced into isolated fragments scattered across the void. The remnants of humanity struggled to survive upon the scorched corpses of their former civilizations; many regressed into feudal societies or even more primitive states.
The core worlds of Mars and Terra were no exception. While Terra descended into a meat-grinder of techno-barbarians and warlords, Mars fell into a hell of her own making.
When the humans on Mars collapsed into civil war, using the terrifying technologies they had once created to slaughter one another, the Red Planet became a nightmare mirror of her neighbor. Fortunately, Mars encountered her unifier much sooner.
Amidst the chaos of techno-barbarian skirmishes and warlord partitions, a sect worshipping a strange deity began to rise. In the madness of that era, no one paid much attention to this young cult; after all, factions rose and fell daily, and most people lived only for the next hour.
At the time, none could have imagined that this young religion, practicing such peculiar tenets and worshipping a deity called the "Omnissiah," would defeat all other powers to unify Mars. They viewed machinery as divine miracles and treated technology with religious awe.
In the fractured world of Mars, sects with bizarre beliefs were common, and this new cult seemed like just another anomaly. Consequently, no one went out of their way to monitor them.
This strange young sect would later be known by a simple, concise name: the Adeptus Mechanicus. It was during this era of darkness that the organization destined to co-rule the Imperium was born.
In its earliest years, the Cult Mechanicus was so obscure that later Imperial historians could not even identify the individual who founded the priesthood that would one day influence the entire galaxy.
Surviving ancient documents suggest that the early Mechanicus was a collective of scholars. However, even these individuals are represented only by a few names and fragments of dialogue buried within the mountain of Martian archives.
What is known is that, driven by their faith—the belief that "the pursuit of knowledge is the ultimate service to the Omnissiah"—these scholars began to gather the knowledge displaced by war.
Through the efforts of these strange priests, much of the data and technology that should have been lost to the fires of human conflict were recovered and rebuilt. To better practice their creed of restoration, the Mechanicus launched the Unification War of Mars.
To conquer the planet, the Cult Mechanicus evolved from a simple sect into a theocratic state. The Tech-Priests, originally responsible for spreading faith and gathering relics, transformed into warlords commanding vast armies.
To eliminate the enemies blocking their path, the Sages and Priests of the Mechanicus gathered followers and formed military orders. Aided by the orderly preservation and excavation of STCs (Standard Template Constructs), the Skitarii legions of the Mechanicus crushed their foes with superior weaponry.
Simultaneously, to protect their industrial zones and hive cities, the Tech-Priests assembled the Titan Legions. They took the massive bipedal machines once used for planetary terraforming and refitted them into "God-Machines" capable of bringing total annihilation to their enemies. In the future, these God-Machines would become the most indispensable element of the Imperial military.
Thus, led by an organization with a superior structure and advanced technology, the once-chaotic Mars was slowly unified through armed proselytization. A theocratic regime took hold, and the entire population of Mars became believers in the Omnissiah.
Following unification, the Adeptus Mechanicus transformed the Red Planet into a holy sanctuary for the Omnissiah. The world became a Forge World, covered in gargantuan foundries and hive cities, all designed to better please the Machine God.
Despite the ongoing Warp storms, the Mechanicus dispatched countless exploratory and colonial fleets into the galaxy to spread the faith. Many of these fleets were lost to the Warp, but many others successfully established colonies.
These successful colonies became the origins of the various Forge Worlds scattered across the galaxy. These Mechanicus settlers built their new worlds in the image of Mars.
With the unification of Terra complete, the Master of Mankind turned His ambitious gaze toward Mars, that crimson world overflowing with technological legacies. For the sake of the Great Crusade, He was determined to bring the Adeptus Mechanicus under His control.
When the Emperor descended upon Mars, it rained on the planet for the first time in millennia. According to an internal Mechanicus prophecy, when rain fell upon Mars again, it would signal the arrival of the Omnissiah's avatar, come to lead the Priesthood toward a truly great endeavor.
Witnessing the Emperor bring rain to a world that had been dry for thousands of years, the ruling Tech-Magi of Mars placed their absolute trust in this Terran warlord. Of course, the Emperor also offered generous terms: He guaranteed non-interference in their worship of the Omnissiah and allowed the Mechanicus to retain control over other Forge Worlds, making them an entity equal to the Terran government within the Imperium.
The shrewd Tech-Priests did not hesitate to sign the Treaty of Olympus. With that, the Adeptus Mechanicus and Mars joined the Imperium, becoming the primary armory for the Great Crusade.
This was the state of affairs when Perturabo arrived on Mars. The Adeptus Mechanicus was the next group he would have to face.