So if i remember reading correctly, in the book it was mentioned that sometimes Rogue Traders are put in charge of Crusades. I'm wanting to plan an endeavor where the Rogue Traders are tasked by the Inquisition with rooting out some heretics on a planet, only to find that its not just 'some heretics' but close to a full blown heresy. When they report back to the Inquisitor they will somehow start a crusade. My questions, being as how i'm sort of new to the 40k universe, is how would they go about recruiting space marines for said crusade? Does anyone have any ideas for endeavors to use for recruitting or otherwise gaining the assistance of the space marines to join them on this crusade and purge the chaos taint from the world. My understanding is the Inquisition itself could mandate the space marines help them, but i'd like to make something more of it, having the explorers either gain the support and trust of the marines or otherwise make it very obvious that their help is warranted. Thanks in advance.

hmm. Getting Marines involved seems unlikely, unless the heretics are ones that the Space Marines you want to get involved have a personal interest - Space Wolves vs. Thousand Sons, for example. But the chances are remote that a Rogue Trader would even know about that back history, and the Marines and the Inquisition would very much like to know how they found out such secrets.

Other Imperial forces are much easier to involve - indeed, some Warrants state that the bearer may demand ships and troops from any governor he likes, as long as he pays for their upkeep and is acting towards endevours listed in said Warrant..

Well it could be that the heresy has to be something of high note to the inquisition as they do hold some command over the Astratas. What could happen is that they may not have a ship close by that can get them their quickly and in turn could request transport and orbital support from the Rouge Trader.

The problem is you'd need something that was worthy of the Astrases attention given their numbers are not that large (considering the size of the imperium) and most other problems are solved by the imperial navy instead.