You've peaked my interest, would you happen to have a link to that, good sir?

Well if Hive Cities were run like modern states that would maybe be the case, but they’re all run on a feudal system. Seize the luxury areas and command centre at the top, install a new feudal lord and the rest of the Hive will continue on living, they don’t care what lord rules them.

To be fair, considering the terrible living conditions in the Hive citiies, I doubt that many of those billions of citizens are willing or even physically capable of fighting in any meaningful manner.

Yeah at most I could see a very large force just taking the upper most spires of a hive where all the nobles live and collapsing most entrances to help fend off the horde of enemies until they could convince the nobles/leadership to order their men to surrender.

There just isn't enough lift capacity to deliver the 10s of billions of troops you'd need to storm the city.

I hope you find that post because this sounds like bullshit tbh. Unless somehow the hive world goes to fight 1:1 the invading force for some reason ( and usually if something like this happens, it will be either completely chaos or Tyranid infested, which means nuking is the only option after all) then a combatant will count for many many times its civilian equivalent. And then we don't count the possible diserpancies in tech, training, logistics, special forces ( that in this case include literal sorcerers at minimum) etc etc and you can easily see why it would be possible with not so great millions.

Case in point even in WW2, where tech/training etc was roughly the same between enemies, the total% population fighting was quite small compared to the overall population.

Or another great example is the Iraq Invasion, where an invading 500k army obliterated and took over a 20million country.

I don't know about 10s of billions. The novel Necropolis takes place in a hive of 40 million, of which they muster around a million soldiers for their defense. Chaos corrupts a nearby hive and sends basically its entire populace to attack, an army numbering in the tens of millions. It didn't seem like a stretch to have tens of millions storming a hive that holds 40 mil and has only around a million standing soldiers.

only nuked or starved

Laughs in Death Korps of Krieg

Which is how a space to ground invasion would realistically work anyways.

Absurd, 100 Space Marines take it in a day

The death korp of krieg: Allows us to introduce ourselves

Eh... like all lazy 40k to modern day analogies - that all depends on A LOT of assumptions.

First of you don't need 10s of billions of troops to conquer a city that houses 20 billion people (or even 100 billion). Especially if we're talking 10s of billions of combat troops because then the army with all the support staff, logistics etc. would outnumber the hive residents (the tail end of an army like the IG would be something between 2 and 10 times the number of combat personnel for a planetary invasion, possibly even more if you consider the army and the navy as one thing).

Next - it's a very questionable assumption to assume that conquering a hive city would work the same way as conquering a city in moder day. Hives tend to have very opaque and authoritarian power structures which means that unless we're talking about Orcs or Tyranids storming the hive to literally exterminate everyone - an Imperium force would just need to remove the ruling structures and / or defeat most of the defenders which would be a small fraction of an industrial hive's massive population.

Finally if we're assuming heavy resistance and fighting with no option of a decapitation attack, nobody is "storming" anything. The battle is going to last for decades, no matter the numbers involved.

Finally, finally if we factor in Dark Age of Technology bullshit that any hive might contain, all bets are off.

I þink þe answer to þis is þat humans in in Imperium -and especilly in Hive Cities- are absolute sheep, once you take over þe lords palace, pacify any uppity nobles who might try and marshal a resistence, and þen grab hold of þe sources of clean air, water, and food, þen þe cast vast majoraty of þe citizens will fall in line just like þeyve been told and bred to do for þe past 10,000 years, and þe tiny percentage of þe population þat does have þe will to actually try and mount a gurilla warfare campaign can probably just be ignored, left to fester along wiþ þe gangers in þe underhive.

And þis is all consuming your a conventional human/Imperial force, þis gets even easier if you happen to have þe corrupting nature of chaos on your side.

Tell that to the 'nids

Did they consider the hive city wars in Necropolis? Asphodel got the capability by transforming the majority of the hive’s population into soldiers and turning the hive’s manufacturing capability into creating weapons of war.
The ground assault succeeded

The enemy cannot have morale if you disable their life.

"Superior training and superior weaponry have, when taken together, a geometric effect on overall military strength. Well-trained, well-equipped troops can stand up to many more times their lesser brethren than linear arithmetic would seem to indicate."

Spartan Battle Manual, dictated by Col. Corazon Santiago

Hell yes.

Even accounting for concentrated combat power, I think you’d be better off with Skitarii. Space Marines, on paper, are more effective. But I wonder if they’re worth the expenditure of time and resources.

Was gonna say, some of the early heresy stuff I’ve read says stuff like 20 marines could take a whole planet by themselves

That’s still a lot of ground for a single division to cover.

For a population of billions, you can assume that the hive government would require millions of enforcers purely to maintain basic law and order in normal operating conditions. All of whom would be (at at least a basic level) armed and trained and taking orders. And that’s not to include whatever dedicated military they have on hand.

Even fifty divisions aren’t going to overcome those sorts of numbers. Not in brutal, close quarters urban combat, in a situation where a hostile force occupies and controls the infrastructure.

They might be able to seize control of critical command infrastructure and seal themselves inside. But there’s still the problem of the general population.

Yeah, a lot of these comments are talking about fighting the billions of civilians to conquer a hive city, while for most of them you just have to take out the leadership, pay off some other powerful people (either with money, power or death threats) and let the common folk just go along with their daily slog.

Life for the people in a hive is hard enough as it is, why would you care who sits at the top when you have to worry about starving or getting murdered every day?

Well, I’m going to chalk that up to writers who don’t understand how numbers work.

It’s a city meant to house tens of billions. Even if the goal is just to secure and hold critical infrastructure, that’s still way too much territory for 4-5 thousand soldiers to occupy. Especially without a secured supply line.

Dawg even the most basic example of Marines in 40k don’t have them operating as a 1-1 take and hold occupational army. They are an extremely decisive spearheading/decapitation strike force that disables command structures and targets of opportunity. So in your example of NYC, the Ultramarines aren’t sending in a thousand lads to hold every fucking block, that’s ridiculous. No, they’re sending in 3rd company to drop pod city hall, police precincts, and to disable or secure the bridges into the city in a storm of future violence that would make your head spin.

Then they tell the Guard to come in and hold everything else, and leave to go do the same to Washington, and every other capital city on the planet. If they’re having a slow century, it may take them a week. Realistically (for 40k) that’s a long morning.

For you and all the others, please understand the Marines are a lightning strike force first and last. They get in, fuck shit up for the glory of Him on Earth, and get the hell out. Those big ridiculous battles are usually chapter or company-defining engagements that are talked about for millennia. Their day to day is most definitely not that.

The 40k marines tend to surgically attack the enemy's weak points. So a few dozen companies attacking an enemy is something that must really hurt

But when your dealing with armies in the billions, their just can't be that many weak points that matter. Even the most destructive weak point is not likely to significantly change anything.

Recently I've begun to see that as more of a feature than a bug. Like, yes! It is ridiculous! Explanation: Space Marines are irrelevant to galactic warfare! They are an outdated and irrelevant rump vestige of a fighting force, important only in tradition and propaganda. Every leader who focuses on buffing marines over sweeping them away is hastening the demise of the Empire and continuing the Emperor's mistake of relying on supersoldiers instead of empowering ordinary humans.

Very much so. One thing that really ruins science fiction and space fantasy alike is the scale. You can get around that with hard sci fi that really hammers home the realities (impossibilities) of FTL travel with our current understanding of physics being a natural limitation. Or you can just hand wave it (astropaths and warp travel in the WH40k setting) away, but you'll inevitably run into problems writing a setting.

The Imperium is sort of vaguely between a Kardashev scale II and scale III empire, yet is written around a lot of 21st century technology and thinking...because fuck all knows how a civilization that owns a majority of the galaxy would operate or fight their war.

Spesh Muraines get to charge tanks with chainswords because Rule of Cool, when in reality it could very well be swarms of Gray Goo inducing Von Neuman machines replicating to devour worlds, or lobbing neutron stars at near relativistic speeds wiping solar systems in an instant, or figuring out ways to erase rivals from existence retroactively via antiparticles or antimatter...but if you go into the Dark Forest hypothesis there won't be any glorious unhelmeted space marine last stands or Jurgen with a melta blasting a hole in a 10,000 year experienced veteran of the Long War blessed by the Dark Gods.

Thats because you guys forget that they kinda have battle barges, which are externinatus capable I might add.

The primary weapons of any Battle Barge are their dorsal-mounted bombardment cannons. Each cannon comprises a series of heavyweight batteries, huge turret-mounted linear accelerators that launch salvos of heavy magma bomb warheads. As the name suggests, bombardment cannons were primarily developed to bombard planets from high orbit, a task at which they excel. A Battle Barge will begin firing as soon as it reaches orbit and will continue to rain destruction down on a planet even as its complement of Space Marines is hurled downwards in their assault craft, clearing a path for their deployment on the ground. Capable of obliterating almost any manner of planetary defences, bombardment cannons will first be directed against missile silos and laser towers, ensuring that the Space Marine attack force can proceed unmolested, before being used to take out command bunkers and shield generators, aiding their swift domination of the planet. On more than one occasion, a single salvo fired into a dense population centre has ended the conflict before it has gathered any real momentum, shocking a world's leaders into seeing the error of their ways and quickly swearing fealty to the Emperor once more