Lore Questions Answered
Image is The Detective.
Drawn by Grace Bruxner.
date: 04/09/2025
Over on Bluesky Starshine asked a series of 40k lore questions and I said I would answer! So this is me doing that. Questions in bold followed by my answers. I will try and link to sources where I can but otherwise I am just vibing.
do tyranids hold ground: so the short answer is "no" and the longer answer is "maybe sorta technically, and even then not for long, and also there is one exception". The overwhelming consistent depiction is they are modelled on a host of locusts directed by a gestalt consciousness Hive Mind - they arrive at a planet, consume all the biomas, and move on, all as directed by the General Nid Will. They only "hold" territory for as long as it takes them to strip it bare. However, there are two maybe sorta kinda exceptions to this.
The first exception concerns their strategy for taking planets wherein the Hive Mind anticipates resistance. They send out what are known as "vanguard organisms" ahead of their main fleet (sometimes many many years, even centuries, ahead) who can, through various sneaky means, infiltrate a society. They then over the coarse of generations execute an incredibly elaborate plan to put those they have either brainwashed, bred, or just convinced through (invariably deceptive) rhetoric to serve their agenda, in charge of the planet. Or at least, put themselves in a position whereby they are confident they can significantly disrupt its defences when war breaks out. When this is achieved they send out a psychic beacon to attract the Hive Mind, basically letting it know that dinner is served. (For a fun book depecting part of this general procedure see here.) So if they do go the route of actively taking over the planet through these means there is a sense in which agents of the Tyranids are "holding it" in the period between them taking over and sending out their beacon and the tyranids-proper actually turning up and eating everyone.
The other exception is the planet Ziaphoria. For some reason they didn't eat that one. It is, in universe, a mystery as to what exactly is going on with that. I personally think the Doylean explanation is a lot of the lore is written for there to always be an exception to any general rule, since that allows people to style Their Guys in the way they see fit. 40k is an extremely soft sci-fi where the overarching objective is to encourage people to buy figurines, everything is bent to that end. So, yeah, if you did for whatever reason want to build and paint a Territory Defender Tyranid Swarm now you have an excuse!
do chaos space marines enjoy a chaos astra militarium or just vague non space marine Heretics: some do, some don't! The Chaos Space Marines are splilt up into incredibly diverse groups of incredibly diverse sizes, levels of organisation, and agendas. Some are running inter-stellar empires basically exactly modelled on the Imperium, including in how they combine relatively small elite Astartes shock-troop forces with large levies of professional mortal soldiers. The Exilarchy and the Pure in the (generally excellent, like straight up just a good book,) Spear of the Emperor work like that. Likewise the Iron Warriors are consistently depicted in various media to work this way. Others are small bands of sadists barely able to hold it together and certainly not capable of levying anything like that sort of logistical force. And others are somewhere in between, for instance the Alpha Legion are often depicted as employing (and genuinely having a respectful working relationship with, oddly) small groups of elite mortal operatives. Finally, a common alternate mode of doing things (most often associated with the Word Bearers and Death Guard) is to have masses of brainwashed slave-cultists, or literal actual zombies, whose job is to die quickly expending the enemies' ammunition in so doing. The point is, again, to allow a lot of creative freedom, and in this case to do so by stressing an incredibly wide range of Antagonist Faction vibes
It's worth noting, because it's been what I based things on for this very website, that we also see things depicted the other way round. There are disciplined(ish) armies of mortals that are actually led by generals which have no particular relationship with any Astartes. The Moebian 6th from Darktide are an instance of this. There are also instances of Astartes actually working for mortal generals, as in the Blood Pact whom I based my little guys on. So it's not always mortal armies in service to chaos space marines!

as seen in this lore-explainer video by dystopnianchimp.
does the imperium have an imperial core?: yes! The Segmentum Solar is the administrative and religious hub of the Imperium.
Okay so whats the purpose of having infantry in 40k: technology of the day allows for shielding which for-reasons-I-couldn't-explain-and-in-universe-are-literally-partly-magic-plus-in-any-case-obviously-just-exists-to-make-the-game-make-sense cannot be penetrated from space but can be shut down planetside. Such shields are also not generally sufficient to cover the whole planet, but can cover key points of interest. So if you want to control a planet and extract resources thereon is a big premium in being able to control (or eliminate) the sites from which these shields are generated. Infantry are often useful in so doing.
does anyone in the imperium have a not bleak life?: the short answer is "yes, lots!" the longer answer is "in absolute numbers many do, by proportion of people it is overwhelmingly grim and horrible". So I have a longer essay on the general economics of the Imperium here, which I hope yinz enjoy! But the pertinent thing there is that a) due to how population density works the median person lives in the Imperium incredibly overpopulated incredibly sprawling continent-or-even-planet-sized-city known as a "hive" (no relation to the tyranid sort). And b) because of how the political-economy of this setting work most of those people are incredibly poor and oppressed. Humans are among the most populous sentient life forms in the 40k galaxy and the Imperium is where the vast vast majority of them live, so it really is just bad times for most people. There are some very rich humans in the Imperium who live decadent lives, and humans who live on planets that are not hiveworlds (which is the most by number of planets! just nowhere near the most by population) that are often depicted as poor-but-not-nightmarish. It's just the hiveworlds and all their misery basically swamp out everything else.
Of the non-human groups we get: the Eldar, space Elves, are very few. Some of them actually live rather idyllic lives of spiritually fulfilled abundant space communism but they have a vanishingly-small-in-galactic-terms population so it doesn't really make a dent in things. The Space Dwarves are also few in number; generally their lives are not depicted as nightmarish per se, but also not really lovely either. Just slightly grimy space traders. The tyranids are vast in number but more or less just fleshy robots who aren't really sentient. The Hive Mind is always either angry or hungry (relatable). The vast majority of the Necrons are barely sentient slaves, and the more sentient upper classes are canonically depicited as either losing themselves to madness or prone to a kind of debilitating body dysmorphia which is incredibly nightmarish. (This book is also one of the genuinely-actually-just-great books of 40k.) The Tau are... actually not so bad. If you have to be born into the 40k universe you probably want to be born as Tau. They are populous enough to be a small-but-noticeable interstellar and multi-species Empire, and they actually have a government that considers raising its population's living standards to be a worthwhile policy goal. It's still an expansionist anti-democratic caste society, but, like... not the worst?
The genuinely difficult case is the Orks. The Orks of 40k are more or less biological robots built for war. The galaxy of a grimdark wargame setting is, for them, their absolute best life. They are also incredibly numerous. So there is a sense in which they sort of balance out the cosmic scales of the 40k verse by just having so much fun. However, their idea of fun is constant violence and brutality.
space marines: mechanized infantry or armored division?: yes.
(Ok ok, proper answer is: sort of both, can be skewed more heavily in a particular direction to serve different needs. They are generally depicted as a kind of shock troop, very much on the model of medieval knights, designed as much to over-awe and discourage further resistance as actually be an effective fighting force. But, again, the overriding concern of Games Workshop is to make it clear that there are myriad different variations on how to use Astartes and you, yes you, should feel free to customise them to be more or less like any real world or fantastical fighting force you can imagine. So they get all sorts of depictions in lore, and the clear intent is to encourage you to be creative; or, more carefully, to never ever discourage you from thinking that your headcanon would be a great modelling project to start buying plastic for!)
did horus have something like chaos imperial guard? also like,,,why are chaos space marines still a problem if theres only a 1000ish of them? like if space marines are like spec op organic tanks how are chaos marines performing that same function with just vague heretics as their meatshields: this one there is an answer to! So when Horus was about Space Marines worked differently; they were the main fighting force of the Imperium in the 31st millenium, and each legion of Astartes fielded huge armies across multiple planetary systems. The limitation to only having 1000 marines came after the resolution of that conflict, and was specifically to prevent any one Astartes commander being able to wield armies that might seriously threaten the entire Imperium ever again. The traitors, of course, were subject to no such limits; so the only thing impeding the size of a traitor warband is just how many people they are able to hold together and supply.
This also means, and this relates to some of the questions above, there is not any one role the chaos space marine is meant to play on the battlefield. They are not subject to any standardisation at all, and each is free to work out how their own skills are best deployed. Beyond their mainstay as either soldiers or spec-ops we see them try their hand at government, or tacticians for smaller elite strike forces as their day job but with active spiritual lives and hobbies they pursue as their main passion, or even as not especially interested in being on the battlefield and playing more of a policeman role. They adpat themselves to what their circumstances permit.