The Warhammer 40,000 universe (hereafter referred to as just 40k), is absolutely ridiculous, which is why it has so many fans. It's the single largest tabletop miniatures game in the world, and has been around now for over thirty years and ten different rules editions at time of writing. Its popularity is through the roof, to the point that even people who've never painted a single miniature or fielded an army on a card table consume the copious quantity of official literature released by the Black Library. And if you're a newcomer to the setting, especially if you're a fan of other mega-popular science fiction universes that involve galaxy-scale wars like Star Wars, Halo, or Star Trek, you're likely nerdy enough to have wondered what would happen if some other sci-fi franchise went toe to toe with 40k. Who would win?
Before I dig into this question, I want to repeat a point that someone else made years ago on the internet. Most fans of a particular sci-fi universe are fans because they'd actually enjoy being a part of that universe. Star Trek enthusiasts love the idea of the United Federation of Planets. Star Wars fans dream about being taken in their sleep and awakening in a Jedi temple. Harry Potter readers absolutely want to go to Hogwarts. You will be hard-pressed to find a 40k fan who wants to spend even a few hours in that version of the Milky Way, much less live there.
I'm not saying people like that don't exist, but I am saying if you find one, steer clear of them, because they are masturbating-in-their-own-feces levels of insane. The 40k universe is mental in a way most other science fiction universes cannot even fathom. And it's why the discussion of "Who would win if...?" is almost always pointless once the Warhammer team enters the chat. The main reason is that the 40k universe is bellicose and militaristic to a degree most other sci-fi universes can barely comprehend.
Take the Galactic Empire of Star Wars. We all know what that looks like: the Death Star, fleets of Imperial Star Destroyers, thousands upon thousands of planets under the control of their own regional governors, hundreds of thousands of ground vehicles, armored assault units, heavy artillery, and space-based fighters and bombers. At the height of their power, headed by Emperor Palpatine, they virtually wiped out the Jedi, shut down the Senate, and pretty much took control of every world that they viewed was worth controlling. Laws are enforced by Imperial code, with arrests, detentions, extractions, and rebellions handled courtesy of the poster boy for the Galactic Empire, the Imperial Stormtrooper.
Jokes aside about their lack of accuracy when targeting heroes with plot armor, the average Stormtrooper is nothing to scoff at. Whether vat-grown clone or a human inducted into the armed forces, Stormtroopers are highly-trained, highly-disciplined fighters wielding rapid-fire blasters, carrying thermal detonators, and wearing full body armor. They fight in squads and can be geared for virtually any environment, from arctic wastes to desert hellscapes, allowing for survival even in near-total vacuum (if only for a short time). Going by in-universe lore, the Stormtroopers are terrifying to most denizens of the Star Wars universe, and you don't want to mess with them if you value your life. While no one knows exactly how many Stormtroopers the Emperor commanded at the height of the Galactic Empire's power, a rough estimate indicates that there are roughly one billion assigned to personal defense of the Empire's Star Destroyer fleet alone, and it's safe to assume several hundred million beyond that were deployed to garrison all the worlds under the Empire's control. That's not even counting anyone else like ship pilots, officers, flight crews, and all the other people you'd need to handle the logistics of a military that size, we're just talking basic infantry.
That's a lot of freaking Stormtroopers. I mean, you'd have to be some kind of sociopathic, death-wish-having asshole to believe you could stand up to a military of that size.
Switching over to 40k then, we'll take a look at their own poster boy for the franchise: the Space Marine.
On the surface, this guy looks pretty much interchangeable with a Stormtrooper, right? I mean, it's all there: full body armor, some kind of rifle, that unblinking helmet meant to strike fear into the heart of an adversary. Heck, they even pledge undying allegiance to their own Emperor. To an outsider looking in, you'd be forgiven for making this assumption. But anyone who knows anything about 40k is already laughing, because a single Space Marine makes even a legion of Imperial Stormtroopers look like a joke.
Stormtroopers are inducted much like most countries handle their modern-day armed forces: they are recruited from the standard populations of their worlds, they undergo basic training, and then they are deployed. Pretty much anybody able-bodied and willing to pledge support to the Emperor can join the military, and at the end of the day, they're perfectly ordinary people given access to high-capacity blaster rifles and armor that provides protection from most physical projectiles and lighter classes of laser and blaster weapons.
By contrast, to become a Space Marine, you are put through a battery of genetic tests and physical conditioning at age 9 or 10 meant to weed out all but the hardiest and most-determined of individuals. For most groups, that would be enough: pass that and you get to call yourself a Space Marine. For the Imperium, you're just getting started. Round two is even harsher and nastier than the first, all but guaranteeing only those possessing apex-levels of physical fitness, stamina, fortitude, killer instinct, raw aggression, and mental toughness complete the trials. But you do that, and you're a Space Marine, right?
Nope. You've only won the right to undergo the genetic enhancement which, if you survive it, will allow you to become a Space Marine. It's during this phase of things where you get cut open and meddled with to an absurd degree, fitted with extra organs like, you know, a second heart in case something happens to your first, an ultra-powerful immune system which renders you impervious to external infection, an extra gland in your mouth which allows you to spit poison if you like, upgraded blood which clots instantly, and some extra genetic programming which bulks your skeletal and muscular structure so that by the end of it, you stand somewhere between 8 and 10 feet tall, carry the physique of a 'roided out bodybuilder, but move like an Olympic gymnast and sprinter. But the most important upgrade you receive is the Black Carapace, which essentially turns every Space Marine into a man with a Wolverine-style skeleton, subdermal Kevlar armor, and the ability to interface directly with his new suit of power armor which, despite its bulk, allows him to move like he's completely unencumbered. And those are just some of the upgrades you get: I'm not even getting into things like the ability to chew on a part of your enemy and absorb its memories; a brain implant that allows you to sleep and be awake at the same time by deactivating different parts of the brain in turn so everything gets a rest without the Marine needing to actually stop; a stomach addition that allows you eat virtually anything and derive sustenance; or a third lung that allows you to breathe virtually anything, even water, for extended periods of time.
At this point, you barely qualify as 'human', even outside of your armor. You're also about 18 or 19 years old, where the only thing you have known during your most formative years is the conditioning and surgery no one else except your brothers-in-arms have gone through, making most humans you meet seem like an entirely different species.
But let's get back to your gear. Space Marine power armor is manufactured from one of the hardest substances in the known galaxy, meant to stand up against everything from small-arms fire to direct impacts from heavy artillery. It is a sealed shell, allowing the Marine to operate in zero-g and full vacuum environments and under any atmospheric conditions as though it was completely normal, and the strength of the Marine combined with the added power afforded by his armor means doorways are merely suggestions: a Space Marine can tear through virtually any material with his bare gloves, and they will happily smash through stone walls, reinforced bulkheads of ships, and even the ground itself if that's what it takes to achieve their objectives.
Then there's that gun. The standard weapon of the Space Marine is a bolter, a .75-caliber semi-automatic assault weapon that fires a self-propelled armor-piercing explosive shell with every pull of the trigger. If you're like, "What does all that even mean?", then allow me to help you.
You remember that scene from Saving Private Ryan where the Germans wheel out a 20mm anti-aircraft gun to use on the Allied infantry?
Now imagine that, only every one of the shells it fires is a miniature nuclear warhead. And it can be fired thirty times before it needs reloading. And you can carry it in your hands without an inconvenient fire team following you around to re-position it every time the enemy moves out of its arc. That's the standard armament for a Space Marine. And they operate in squads, so there's never just one.
We aren't even getting into the more ridiculous things, like Flamers, Plasma Cannons, or Heavy Bolters (which are just like normal Bolters, except they fire an even larger-caliber round and can still be lugged around along with their ammo packs by the average Space Marine). How, pray tell, is nearly anything the Galactic Empire can field supposed to stand up to that? A single ordinary run-of-the-mill Space Marine in the Star Wars setting is so over-powered as to be absurd, and we aren't even talking about Primaris Space Marines, or Space Marine Terminators, or Librarians (Space Marines with added psychic powers and abilities), or even the Dreadnoughts, tanks, APCs, artillery, artifacts, and other vehicles a Chapter can muster against enemies where the humble Space Marine squad of five isn't enough.
But here's where it gets absolutely absurd. At this point, if you don't know 40k, you're probably asking yourself why a Space Marine needs to be so ridiculously over-powered as to be a nearly-indestructible walking weapons platform. And the reason a Space Marine needs to be that grossly OP is because the first fight he gets into, there's a good chance he's going to die, as he'll be facing things that are just as ridiculously strong as he is, using weapons strong enough to pierce his armor. In the world of 40k, as strong and durable and powerful as they are, Space Marines are actually pretty low on the power scale when you account for things like Imperial Titans the size of entire city blocks and ships fitted with cannons that can create literal black holes.
Warhammer 40k is absolute lunacy, because just when you think you've found the most comically OP unit or tech in the entire game, some other faction comes along with something capable of curb-stomping that. And it has to be that way, because everything in the Warhammer 40k universe that isn't on your side wants to kill you. Some of them then want to eat you, or strip your soul from your corpse and eat that, or convert your body and every other organic thing around it into biomass to fuel its hive fleet and create more of itself:
But surely the Galactic Empire's space fleets should have some say in this matter, right? I mean, your average Imperial Star Destroyer is 1.6 kilometers in size, and the Empire's got roughly 25,000 or so of those ships in service, not to mention the hundreds of thousands or even millions of their smaller ships, like the TIE-class fighters and interceptors, they could bring to bear on someone in a straight-up fight. And you'd be absolutely right.
The problem is, 1.6 kilometers is the size of the average Frigate in the Warhammer 40k universe, the ships that comprise the bulk of the Imperium's space force. A Cruiser, the next class up, ranges from 5-12 kilometers in size. Battleships muster in at 8 to 17 kilometers, with the larger Gloriana-class Battleships falling into the 19-20km size. And from there, they just get bigger. Imperial Dreadnought-class ships clock in at over 25km. What I'm getting at is that the Imperium fields light cruisers that dwarf the size of the Empire's average battleship. Are there larger vehicles in the Star Wars universe? Sure. But how many Eclipse-class Dreadnoughts are out there? The answer is two, and both of them were destroyed, so depending on when this fictional conflict takes place, the Empire might not even have them to field.
But what makes the Imperium's navy so terrifying to engage is that their ships are built to be used as weapons themselves. Ship-to-ship combat in the Star Wars universe is generally predicated on the vessels staying far apart from each other, flinging proton torpedoes and blaster fire at one another until somebody's shields go down, at which point the ship will be obliterated in the next barrage or, like we saw at the beginning of A New Hope, disabled and boarded. If one ship runs into another of roughly equal size or mass, that's generally a bad thing, with one or both vessels getting destroyed, or at least no longer being in fighting shape. Ramming one ship with a second in the Star Wars universe is either the result of a suicide run, or ineptness on the part of the pilot.
In the 40k universe, "Ramming speed!" isn't a last-ditch effort to bring down an enemy vessel, it's the order given when an Imperium ship gets within range of an enemy it hasn't completely obliterated with its guns. Driving your ship into an enemy is a standard tactic of combat. Assuming the Imperium ship doesn't just cut its target in half or pulverize it from the shock of the impact, they will deploy boarding torpedoes, inserting Space Marines into key locations to destroy critical infrastructure like engines, shield generators, and weaponry. That, or they'll just reverse course and ram it again. I reiterate: 40k is utterly mental.
But really, one can argue and nitpick individual scenarios all day long ("How many Battleships could the Death Star wipe out before it got destroyed?", "How many Stormtroopers would it take to bring down a Space Marine?"), but there's one reason above all else why 40k so often wins in these scenarios, and that has to do with psychology.
See, even under the worst and most brutal regime of the Galactic Empire, most citizens are afforded at least the basic pretense of their own humanity. Unless you're enslaved, your life is pretty much yours to do with as you please as long as what you're doing doesn't piss off the Empire, and even then, if you're far enough away in the Outer Rim, you're unlikely to garner their attention. It's why Luke doesn't really worry too much about things being on Tatooine. As he explains to Obi-Wan, he doesn't have any particular fondness for the Empire, they're just such a long way from his home that their existence may as well be meaningless. Him growing up on a moisture farm with his aunt and uncle isn't the glamorous life he dreamed about, but it's at least his life. Only the most callous and uncaring of Imperial captains would order the deaths of every enlisted person and officer under his command in a suicide charge towards a superior foe. Retreat may not always be praised, and might be punished, but it's still seen as a valid and understandable strategy when deployed under the right circumstances.
The Imperium of Man in 40k recognizes no such thing. To be human in the 40k universe is to live and toil under the crushing, all-pervasive gaze of the Emperor of Mankind and the never-ending bureaucracy of the Imperium. You could be growing up, doing your own thing on a farm world, when suddenly the Imperium ships descend from the clouds and out comes a force of Imperial Guardsmen who have a quota to fulfill. Your world needs to provide seven million able-bodied men and women for the Imperial war machine to chew up and spit out as the Imperium's needs dictate, so congratulations, conscript, you're now a soldier in the Emperor's army. Average lifespan of a newly-minted Imperial Guardsman dropped into a combat scenario is fifteen hours. Not months, or days. Hours. Best of luck.
Maybe you think you can get out of it by turning to a life of crime. You know, smuggle stuff like Han Solo does. Worst case scenario, they catch you and chuck you in prison, right? LOL nope! Most crime in the Imperium is punishable by death or hard labor (usually until you die from exhaustion), but even if you luck out and only get thrown behind bars, that doesn't mean the Guard can't conscript you into a Penal Legion so you might redeem yourself by giving your life for the Emperor in a suicide charge. You survived the suicide mission? Great, there's another one coming up in an hour or so. Repeat until you're either dead or so battle-hardened they advance you in rank and put you in command of other greenhorn Penal Legion peons, all of whom will be thinking about fragging you and running away instead of getting themselves killed. Good luck.
The 40k tagline is, "In the grim darkness of the far future, there is only war." And that is a truth everyone who grows up in the Imperium understands and acknowledges. It also makes the Imperium a truly fearsome adversary, because as far as the Empire is concerned, it's more important for a Guardsman to safeguard their lasgun than it is to safeguard their own life. A lasgun, after all, has to be manufactured using precious resources which, while vast, are still limited in the grand scheme of things. But in a galaxy with a million worlds under its control, human life is an abundant, renewable, and incredibly cheap resource. Imperium commanders think nothing of ordering tens or even hundreds of thousands of people to their deaths to hold a mile or two of terrain on their side of the lines, and Commissars stand ready to personally execute anyone who disobeys orders or displays even the barest hint of cowardice or corruption by outside forces.
Even in the Galactic Empire's most brutal times, the loss of a hundred thousand Stormtroopers would be noticed, whatever caused it would be noted, and strategies would be adopted to prevent that from happening again. In the Imperium of 40k, the loss of a hundred thousand Imperial Guard troops would probably result in a commendation for the officer overseeing the battle, because that's a drop in the bucket and casualty estimates were ten to twenty-five times that.
When faced with superior firepower, overwhelming enemy forces, and a lack of supplies and reinforcements, the Galactic Empire's troops will cut and run. The Imperium, by contrast, fixes bayonets and charges because the concept of 'innate human value' is a joke. "Live to fight another day," is the credo most societies take to heart. The Imperium inverts this to, "Fight to live another day," and every member of their armed forces, from the lowliest Guardsman recruit, to the most powerful Space Marine Captain, is born into a system that indoctrinates them to it. There are few, if any, science fiction universes set up to handle a foe who believes in this way, and it's for this reason, above any technology or weaponry, that Warhammer 40k (almost) always comes out on top with regard to these scenarios.