So here is my first campaign, which was never ran due to players not being interessted in playing this sort of regiment and its been on the shelf for so long that I forget about it every time i have the chance of starting up a new game of Only War.
So lets get to it;
Arkan is a feudal world which have recently been conquered by the imperium, the population is a rebellious lot and there was quite a fight for the imperium to capture the planet and conquering its people.
It would have taken longer if it had not been for the Blood Angles chapter that decimated the majority of the military forces of Arkan, leaving the population with a built-in hatered for the space marine chapter.
The population is also a inventive bunch, which puts them at odds with the adeptus mechanicus for their almost heretical technology. (outwardly it looks very steampunk-ish but it not too far from "modern" tech, function over aestetics).
The Imperial Guard have recruited about 8 regiments of foot soldiers from this world before they finally were allowed to add the arkan walkers to its millitary assets, the Arkan walkers are a form for power armour that is too big and clunky to be a typical power armour, and it handles like a Sentinel Walker. It was mainly constructed to oppose the Blood Angles.
The population is split in two, its the common people, and the champions and nobility. In the regiments, the common people are outfitted with scouting gear and make exellent trail-blazers. While the nobility often make for the command roles, however, when the Arkan 1st. heavy recon was put togeather, it was only the nobility and the champions who were granted a Arkan Walker, while the rest supported these scouting vehicles.
It should be mentioned that champions are proven individuals, worthy of respect, these can be from the most primitive of the badlands raiders and still be viewed in awe by the noble court of the planetary govenor. This has a bit more to do with tradition than actual skills, but skill is still very important.
With the background out of the way, the regiment is sent to a ice world to fight the severus dominion, taking out a few outposts and flank in on supply-depots and installations while the majority of the enemy force is focused on the main IG advance (a few other regiments).
However, during one assault on an installation, they find a captures blood angle, or they assume so due to his lack of weapons, chains attached to him, no helmet and generally bad shape... but still beating up anyone in the installation.
This scene cause the majority of the NPCs in the regiment to feel the built in hatered of the blood angles and they open fire, unless the PCs can stopp them (they also have to do the test so that they are not completely free from their ancestral links, except certain support classes like commissars).
Depending on how this goes down, they can join the Severus dominion and fight back the imperial guard, then be stationed elsewhere in the Dominion, or disgruntedly continue to fight for the imperium while the regiments morale is becomming an ever increasing problem and shifts the focus of the game to internal struggles rather than external ones.
And that is what I had for this campaign, some loose notes is lying around somewhere and I can find it and refine the campaign if I were to ever run it.