The Emperor discovered the Mechanicus on Mars, and chose to ally with the Fabricator General instead of attempting to conquer Mars. This milestone is an important marker in the creation of The Imperium. The Treaty of Mars is illustrative of this history, and no other Adeptus has this kind of autonomy.
Except the Adeptus Astartes.
"Because of their importance to the Imperium, the Adeptus Mechanicus was allowed, in quiet discord, to continue following its own mysterious strictures. The Cult Mechanicus does acknowledge the Emperor as the Master of Mankind, although they do not recognise the authority of the Adeptus Ministorum, despite its official sanction. While other religions were named as heretical for such acts, the Adeptus Mechanicus has been granted an unusual autonomy, a freedom of worship unparalleled save for that granted to the Space Marines for their unusual practices."
- 6E rulebook p.402
What the Treaty of Mars exactly is also depends much on interpretation - or the sources we may be reading. When White Dwarf #178 featured an article discussing the past of the Adeptus Mechanicus, it made it sound neither as if the Emperor conquered Mars nor the AdMech merely allying with him. They joined his empire:
"On Mars the Emperor was recognised as the long-awaited Omnissiah of cult legend. A frenzy of popular uprising swept through the entire Cult Mechanicus as word spread of his coming. When the Emperor arrived on Mars he was hailed as the Machine God Incarnate and the Tech-Priests and Techno-Magi alike came to acknowledge his leadership and marvel at the technical secrets at his command.
Not all amongst the Cult Mechanicus were happy with this turn of events. Many of the senior Magi resented the disturbance in the status quo threatening, as it did, their own power base. A few of these malcontents led a rebellion and seized the Temple of All Knowledge from where they called the faithful to war against the Emperor.
The conflict that followed was short and bloody, and ended in the defeat of the reactionaries and triumph of the Emperor's followers. Mars and Earth were reunited after millennia of separate development.
To establish their place amongst the new rulers of the Imperium the Cult Mechanicus became the Adeptus Mechanicus, and its cult leaders became High Lords of Terra. Over the following millennia the people of Earth and Mars would march forward together, welded into one mighty Imperium, yet distinctly different societies with their own governments and institutions.
It is now the forty-first millennium. The Emperor has endured for ten thousand years, his living spirit continues to guide the fate of mankind though his body is rank with decay. On the world of Mars the Techno-Magi celebrate the rites of the Machine God and his divine manifestation the undying Emperor. Throughout the Imperium there are countless Forge Worlds devoted to the pursuit of knowledge. The Titan Legions and the Tech-Guard armies of the Adeptus Mechanicus protect and expand the territories of the Machine God. In conjunction with the Space Marines and Imperial Guard, they form the fighting armies of the Imperium, the most potent force in the galaxy.
The Forge Worlds and Knight Worlds remain fiercely loyal to their Martian masters. Their industry and their tithes are to the Adeptus Mechanicus and the Techno-Magi of the Cult Mechanicus. Where other human worlds in the Imperium are part of the feudal empire controlled by the Administratum of the Adeptus Terra on Earth, the Adeptus Mechanicus retains direct ownership of its own territories. Thus the Forge Worlds and the Knight Worlds owe no obligations to the Adepts of Earth. They raise no regiments for the Imperial Guard and they pay no tithes to the treasury. They are not answerable to the Adeptus Terra but to the Adeptus Mechanicus. Only the Inquisition has jurisdiction in their territories, as there are no worlds in the Imperium where those warriors of righteousness may not walk freely."
The difference I am trying to illustrate is this: The Adeptus Mechanicus background makes a Tech Priest. The Forge World homeworld does not. There is a difference between a -/AdMech/Sage, and a Forge/-/Sage.
Fair enough, that's a good point. I'll have to read up again on the specifics - though it may be possible that it would still fit, if the Skitarii are similarly inducted into cult rites as the warriors of the Ecclesiarchy. From what I've read about the Forge Worlds, they function very similar to the Shrine Worlds of the Ministorum, just following a different religion - but one that still extends over every aspect of life.
... basically making the Tech-Guard the "Frateris Templar" of the Mechanicus, if you will.
I suppose it depends on how we see the Skitarii and life on a Forge World, again making it a matter of interpretation. But I'd have to re-check the exact mechanical effects of this background before I could be sure of my own opinion.
Edited by Lynata, 07 January 2014 - 01:51 PM.