Andrzejef: So, bearing the authority of the Emperor himself they told Lord General Alexander to withdraw his forces, which clearly conflicted with the orders from Astra Militarum, which bore the authority of - surprise - the Emperor himself :)
muttly13: Incorrect Sir! The easy answer is this... The Space marines are answerable to only the Emperor while the Imperial Guard answer to the Adeptus Administratum (in other words, regular old human government). While this seems one and the same, the nuance is always what made 40K cool for me anyway... The Original Emperor tried (and succeeded) to squash "religion". I quote this as i dont mean it as in todays version, but in the 40k universe it is perilously associated with Chaos, the Great Enemy. The Space Marines were made in his image and VERY distinctly did not worship him as a God, but rather a Brother/Father figure.
Queue Horus Heresy, mass hysteria, dogs and cats living together... The Emperor is out of action, the Space Marines operate as normal, The Adeptus Administratum rises (and a lot of other organizations, forgive the brevity 40k purists) and begins to worship the Emperor as a god. They create the Imperial Guard and a host of other military, para military organizations that report to them, not direct to the Emperor, seeing as he is essentially comatose.
Aaaaaaaanyway, flash forward a few millennium and the two organizations ideas sometimes conflict. For example, a number of Space Marines Chapters have been exterminated (by both other Space Marines and the Adeptus Administratum) not so much for heresy but mutation, misconduct, not worshiping the Emperor, blah blah. For example, The Space Wolves have been subject to this a number of times but have worked their way around it one way or another. Space Marines are not supposed to have fangs and go berserk in battle, yadda yadda.
Andrzejef is totally correct in why they fight in Dark Crusade. Essentially the Blood Ravens are worried about being caught out as a freak show chapter and exterminated. I am just being nit picky that the IG do not report, but rather worship the Emperor, hence a different command structure entirely than the SM. That, and clearly I have a lot of extra time on my hands.
Side note: All of this story line used to be a 1000 times cooler prior to the Great Rift (otherwise known as GW"Lets forgo 3 decades of story line to make it all millennial-ly!") were everything has been solved by bigger, better, even more supery human Primus Space Marines! Like straight outta "oooh my wand core is the same bird as yours sukka!"
Yes, I could have been a bit more precise. Also than you for your corrections. :)
While SM are a bit more exact extension of Emperor's will (the original one), and treat him more like the honored father, the post-heresy Empire was build around Emperor's cult (based off - mind you - the Book of Lorgar, for which Word Bearers were reprimanded and disciplined when "big E" was still actively around) that is treating him as most holy figure, from which the authority of "governmental institutions" is derived. This way (the way I understand it at least) Administratum operates - theoretically - by his authority (at least that's what's being told to the citizens, and pretty much everyone that thinks otherwise gets a brief face to face quality time with their local Ordo Hereticus representative). So, from IG point of view - brainwashed as it may be - they were in the right, and space marines decided to defy his "most holy" will, thus going all heretical.
I guess also having the Ravens descend - allegedly - from Thousand Sons legion doesn't really help their cause either :)
About that Great Rift, that is apparently causing by itself another Great Rift of sorts (couldn't help it, sorry, Laughing God influece I guess), it seems that Astartes decided to learn from their foes, assimilate and then deploy an otherwise familiar tactic - "By the Emperor, there cannot be enough Dakka", known also as "Astartes is da biggest and da strongest", or "Waaaaaagh!" for short. :P
You think Great Rift is 40k counterpart of Fantasy's Age of Sigmar?