The Martian Mechanicus unites with the Imperium with the Treaty of Olympus. Not all tech-priests supported this, sowing the seeds for the Schism of Mars.
Via our good friends at Lexicanum
After the Emperor formed the Imperium, he engendered support with the Martian Mechanicum, an already existing empire. On Mars, he was commonly seen as the Omnissiah, the earthly representative of the Machine God, due to his ability to seemingly instantly repair machinery with a mere touch. After landing in a golden ship on Mars’ surface The Emperor came before the Mechanicum Parliament on Mars, pledging that in return for supply arms to his armies and building a mighty war fleet, the Emperor would protect and the respect the sovereignty of the Mechanicum’s Forge Worlds. In addition, he would donate six Navigator Houses to Mars, whose supply of Navigator’s had since dwindled. Given such incentives the Martian Parliament and Fabricator-General agreed to the terms, and the formal alliance between the Adeptus Mechanicus and Imperium was signed on Terra between the Martian Ambassador and the Emperor.
The Treaty of Olympus was a diplomatic agreement between the Imperium of Man and the Martian Mechanicum binding the former to the latter. The Treaty was signed shortly after the Emperor landed on Mars at the dawn of the Great Crusade, calling for unity and displaying seemingly supernatural powers over machinery, repairing a Knight instantly with a mere touch. Many on Mars began to consider the Emperor the Omnissiah incarnate. As part of the stipulations of the treaty, the Emperor agreed to not interfere in the internal structure of the Mechanicum and allowed them to continue their beliefs in the Omnissiah, the avatar of the Machine God. In exchange, the Cult of the Machine produced and manufactured the wide amount of weapon, ships and other tools for the Imperium for the Great Crusade.
Many members of the Martian Mechanicum, including Fabricator-General Kelbor-Hal, resented the Treaty of Olympus and saw it as a means for the Emperor to subordinate Martian forges. This tension eventually led to the Schism of Mars during the Horus Heresy.
The Treaty was not celebrated by all of the Tech-Priest elements on Mars. Many Tech-Priests saw the Emperor as a warlord seeking to subordinate the Mechanicum and limit their technological research. Among these was Kelbor-Hal.
During the Horus Heresy, the Mechanicum like most other branches of the Imperial military, found itself divided in loyalty. Many Mechanicum units declared for the Warmaster Horus, some retained their loyalty to the Imperium and some seceded altogether to remain neutral during the conflict. Horus was able to convince the Fabricator-General himself, Kelbor-Hal, to join forces with him on the condition that the autonomy of the Mechanicum be ensured, and that the STCs the Sons of Horus had captured from the recently subjugated but technologically advanced Auretian Technocracy be handed over to him. Kelbor-Hal’s deputy, the Fabricator Locum Kane, remained loyal to the Emperor however, which initiated a civil war on the Red Planet known as the Schism of Mars that mirrored the larger conflict raging across the galaxy.
The Mechanicum considered the division of the Mechanicum with Hal and Kane both claiming the title of Fabricator General an unresolved equation, known as the “binary succession”. In an attempt to resolve this, Ambassador Vethorel, Fabricator General Kane’s representative on the Council of Terra, proposed to resolve this by the creation of a new Adeptus, the Adeptus Mechanicus. Despite the Mechanicum’s concerns for their independence and the council’s fears of a Martian power-grab, the resolution passed after the Collegia Titanica threatened to abandon the Solar War and the Imperator Titan Magnificum Incendius of the Legio Ignatum marched on the Great Chamber of the Senatorum Imperialis. The Mechanicum units that fought against the Emperor would come to be known in later times as the Dark Mechanicum.
In the last 10,000 years since the Great Heresy, the Adeptus Mechanicus has become fanatically ingrained in the dogma of the Cult Mechanicus. As their Forge Worlds supply mankind with vital weaponry and technology, the Adeptus Mechanicus continues their Quest for Knowledge and vigorously hunt for remains of precious STCs. Major conflicts over the tenets of the Cult Mechanicus have plagued the Adeptus Mechanicus, most notable of which include the Moirae Schism, Martian Civil War, and Elucidan Schism. However over the millennia, the ability of the Adeptus Mechanicus to reproduce technology from the Great Crusade has gradually diminished. Even the Golden Throne, cornerstone of the Imperium, is proving beyond the ability of the Adeptus Mechanicus to repair and maintain.