Most of what I know comes from playing the Dawn of War PC games, the Horus Heresy novels (which strictly speaking, are more like Warhammer 30,000) and a few other novels set in "present day" mostly the Gaunt's Ghosts & Inquisitor novels by Dan Abnett, and the Caiaphas Cain books. Which, come to think of it, is actually quite a lot of books, but of very limited topic coverage, relatively speaking.

One of those issues concerns the Tyranids. In Dawn of War 2, when the Tyranids show up, none of the Space Marines know who they are. I know the Imperium is supposed to be this oppressive theocracy that has no problem lying to people for their own good, but I figured the Space Marines, at least, would be clued into stuff like this. Yet, only one of them, a crusty old veteran, who is noted to have been on detached duty with another chapter, is the only one who knows anything about them. In the Caiaphas Cain books, however, the ordinary soldiers of the Imperial Guard are not only fully cognizant of their existence and nature, but they are familiar with the nomenclature and various sub-species and have nicknames for them. The phrase they blithely toss about "shadow in the warp" or something like that to foretell the arrival of the Hive Fleets, is a trigger phrase only recognized by the crusty old veteran scout in the Blood Ravens in Dawn of War 2.

One thing that occurred to me was that DoW2 takes place in a much earlier time, but it is actually several campaigns after fighting the Necrons in DoW, with references made to that campaign in the game, including a couple of characters from the planet where the Blood Ravens encountered the Necrons. From the Cain books, I get the idea that the Necrons are a much newer arrival on the scene (or at least in the awareness of the Imperium) than the Tyranids, since they are a brand new thing to Cain when he encounters them, and later, IIRC, he has to brief others on how to handle them, because they are so new and rare. So, tyranids came first, but the Space Marines don't know about them?

Also, I recall someone, possibly Werthead, telling me on one of these boards, that the tyranids are one of the oldest enemies in W40K. Yet, despite the encounters with Space Orcs, Space Elves, and the origin of Space Mordor in the Horus Heresy (Horus is Space Sauron, down to the Eye symbol), there is no sign and no mention of the tyranids. Finally, in the Ravenor novels by Dan Abnett, (which take place before Gaunt's Ghosts, as Gaunt refers to Ravenor as an historical figure, and which in turn, predate the Caiaphas Cain novels, as they include a footnote about Gaunt as an historical figure), Ravenor and his buddies encounter tyranids in a time-traveling portal-doohickey, and when they don't recognize them, are told they have not yet shown up in Ravenor's time. The person telling this is kind of spaced-out, but rattling off the names of the Hive Fleets leaves no doubt that is what they are (also the description fits the velociraptor-looking things from DoW2).

So all this seems pretty cut and dried, and maybe I'm just remembering the account of the tyranids being an older foe wrong. But that doesn't explain the general ignorance of the Blood Ravens compared to the familiarity of the Imperial Guard (it would be like Delta Force, GI Joe & SEAL Team Six being ignorant of an enemy that the regular Army infantryman is thoroughly familiar with, only more so), AND in the very first Inquisitor book by Abnett, about Eisenhorn, who TRAINED Ravenor, who was supposedly the first inquisitor to meet the tyranids and live to tell about it, before he trained Ravenor (because in this first book, Ravenor has yet to suffer the injuries he is long past when he meets the tyranids), is discussing aliens with his minion, who makes a casual reference comparing the new breed to being even more strange and repulsive than tyranids! WTF?!

Also, what's the deal with Space Marine chapters? I thought they were all genetically determined, with each chapter all having the same genetic alterations that make them Astartes, and thus each one is genetically the "son" of their primarch and a literal brother to his fellows of the chapter. So how can they switch? If you deliberately play DoW2: Chaos Rising in the most evil way, your Blood Ravens characters end up joining the Black Legion at the end. In the Night Lords trilogy by Aaron Dembski-Bowden, a member of the Night Lords does the same thing, and a member of another evil chapter joins the Night Lords. I guess when the entire "nation" you fight for is called "Chaos" you can get away with things like that, but how does it explain how Cyrus joined the Deathwatch chapter, and then came back to the Blood Ravens, with no recriminations. In the same Eisenhorn book I referenced previously, Eisenhorn claims the Deathwatch is a special chapter founded just for the support of the Inquisition. IIRC, the Inquisitor of the Caiaphas Cain books supports this with occasional references to calling in the Deathwatch to deal with a problem too big to be solved by tossing a single, self-interested staff officer and his valet at it (which is her usual modus operandi, and might explain why no one references Amberly Vail as some sort of legendary inquisitor-hero).

Then there is another problem, related to that. I am planning to read another novel by Dembski-Bowden which is about the Grey Knights (who, in Dawn of War, are specialist units within the chapter, like Terminators or Librarians), who are implied to be an Inquisition chapter like the Deathwatch, and even more secret (but apparently not to the remarkably out-of-the-loop Blood Ravens? ). Their enemy is the Space Wolves.

Except here's the thing - in the Horus Heresy novels, A Thousand Sons and Prospero Burns, the Space Wolves' primarch is Leman Russ. The tanks of the Imperial Guard are called Leman Russ tanks, and in more than just Dawn of War. Why would they name their tanks after one of the arch-traitors? It can't be a tanker thing, because the Space Wolves don't seem to use tanks, and anyway, the Space Marines use a different model of tank. Russ is portrayed as a barbarian, who fights hand-to-hand and is highly suspicious of space magic, but is more sneaky and cunning than he lets on (btw, Abnett wrote the latter of those books, and the one more concerned with Russ & the Wolves). One thing that nags at me is the possibility that he might not be a bad guy (he's certainly more likable than his sympathetically-portrayed rival Magnus, and less arrogant or pretentious) and his legion betrays him as well as the Emperor when they turn to Chaos. However, this doesn't really track with how we have seen the legionaries react to their primarchs. While some do turn on Horus or Fulgrim or Mortarion it is because of a higher loyalty to the Emperor, and their decisions to turn are at great remove from their primarchs, many of whom are not particularly close to the loyalists who refuse to join them in their treason.

Also, the strong implication is that the primarchs shape the legions/chapters (another issue - I am given to understand that first there were the legions, and then after the heresy, the loyal marines were broken into chapters to keep the formations smaller and more under control, so the Blood Ravens are separate from Imperial Fists or Black Templars, despite having the same primarch - but then, at times they claim they don't know he is their primarch, so WTF; yet in Dembski-Bowden's HH novel, The First Heretic we see the Word Bearers are already broken into independent chapters - the Word Bearers go bad, and are even enemies in DoW, and their characterization in that game fits ADB's portrayal of them as religiously oriented), and the Space Wolves are by their own admission, ruthless and mostly ends-justify-the-means types, who outright state they will do anything they are told, and will stop at nothing. In one of the later HH novels (by Abnett, who thus characterizes the Wolves), the leader of the Ultramarines, whose name I am not remotely going to attempt to spell, and who is supposed to be some sort of brilliant military analyst, even agrees with that version, saying that they are one of four legions/primarchs who are so ruthless or unstoppable that he could win ANY war allied to one of them (interestingly two of them were good guys, though one was a sort of tragic-flaw guy who let his obsessive hatred of the traitors lure him into the fatal trap that kills him early in the heresy, so he might have been vulnerable to the Dark Side, as it were). This same genius primarch, however, also theorizes that the council were the Emperor outlawed space magic was a deliberately ploy to make the Imperium vulnerable to Chaos and its daemons. The architect of that action seems to Leman Russ, in "Prospero Burns". Because of that stricture, the Thousand Sons Legion are driven into outlawry and side with Horus even though Magnus, their primarch, first tried to use his forbidden space magic to stop Horus from going bad. Then Russ and the Space Wolves race off to try to whack Magnus before he can warn the Emperor, and they imprison the narrator of "Prospero Burns" presumably to silence the main outside witness before he realizes they are about to switch teams. The implication seems to be that once the Wolves made them think there was no going back to the Emperor for them, the Thousand Sons went to Horus because a. he was the only one left who would take them, and b. they were already halfway there with their abuse of space magic. All of this is supposed to be long-reaching, intricate and subtle plot by Chaos, in which Russ & the Space Wolves were a key component. Granted, the HH novels have not yet portrayed the official fall of the Space Wolves, but there were already signs of their bestial natures and mutant degeneration of their genetic line. We see similar things in the two novels about the Dark Angels and one about the Blood Angels, with their own corruption either overt (with the psychotic natures of the Blood Angels) or implicit (with the furtiveness of the Dark Angels and taint of their homeworld). Also, when the highest-ranking officers in a legion have names like Ahriman and Abbadon, why is anyone surprised when they go bad? So Russ was not only in cahoots with the powers that be to engineer the elimination of a witness to Horus' treachery and/or drive them out of the Imperium, he is also specifically noted by Magnus as one of the expressly sympathetic primarchs to Lorgar, the eponymous First Heretic of the same novel, and the religious, superstitious and primitive nature of the Space Wolves is highlighted in a way that (as with Lorgar's Word Bearers) puts them at odds with the secular & rationalistic ethos of the Imperium in the good old days before the daemons turned out to be real.

By all accounts, aside from his secret role as the Emperor's fratricidal assassin (and when you designate one of your kids to be the kill-anyone-even-my-other-kids specialist, again, can you be surprised when he someday goes off the reservation? I mean, did he have a Kill Russ guy prepped? Way back in M3, Captain Kirk knew the first rule of assassination was to kill the assassins), Leman Russ is absolutely the opposite of the sort of guy they'd name a tank after. It might be intended as ironic or a commentary on the fallen state of the Imperium, that so much is lost or forgotten, but it also strikes me as exactly the sort of thing the Inquisition would jump all over, even if the common people don't realize he's become an enemy (and 10,000 years ago too, so it's not like the habit of thinking of him as a good guy is so much newer).

So, um, that's my questions. I know I'm missing a ton of backstory and should probably pick either the Heresy books or stick in the "present day" stuff if I am not going to go totally into the setting/IP, but it seems almost like you have to take a class to really get all this, and I just like the games (yes, I have come around on DoW2, thank you) and like five of the 800 million authors involved (Abnett, Dembski-Bowden, Graham McNeill, Sandy Mitchell & James Swallow, basically). As anyone from the WoT board can tell you, overthinking the world-building stuff is what I do. So if anyone can clarify some of this mess for me (or refer me to some definitive yet concise resource, emphasis on the latter modifier, since they really get way too in-universe with explanations on most sources), I'd really appreciate it.