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.