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Breton wrote:
You're forgetting the corruption and the snobbery. The unwashed masses are out of sight out of mind for the upper crust, the lower crust will use them and only ship off the ones that aren't protected by them or someone else.

I don't see any reason why elites would want useless mouths hanging around. That's a constant throughout history, by the way - the Optimates always want the Proles culled, whether we're in Athens, Rome, London or Moscow.

The Imperial system has to have some functionality to have survived that long. Empires require lots and lots of maintenance. The notion that the High Lords basically run the joint as a fortress perpetually preparing for siege and using outbound Black Ships to get rid of "surplus population" makes much more sense than it being terribly run and one month away from cannibalism and revolution.

Commissar von Toussaint wrote:

Breton wrote:
You're forgetting the corruption and the snobbery. The unwashed masses are out of sight out of mind for the upper crust, the lower crust will use them and only ship off the ones that aren't protected by them or someone else.

I don't see any reason why elites would want useless mouths hanging around. That's a constant throughout history, by the way - the Optimates always want the Proles culled, whether we're in Athens, Rome, London or Moscow.

The Imperial system has to have some functionality to have survived that long. Empires require lots and lots of maintenance. The notion that the High Lords basically run the joint as a fortress perpetually preparing for siege and using outbound Black Ships to get rid of "surplus population" makes much more sense than it being terribly run and one month away from cannibalism and revolution.

Because they're not the elites. Or if they are, they're the lowest rung. The people who would be putting the "proles" on the ships would be the same ones who would want to use them for more personal power because they have so little.

I like the idea of this thread, so here's my $0.02.

I think the vast majority of races we play in the game would find it easier to destroy Terra than to conquer it. Hundreds of dedicated warships and potentially thousand of other armed vessels would be around to defend it, including the Imperial Navy, Mars, any Space Marines, Grey Knights, Inquisitorial and Ecclesiarchal forces.

So let's go down the list!

Space Marines - If all the Space Marines suddenly turned traitor and decided to attack Terra, could they conquer it? Depends on the writer and how much bs plot armor Space Marines are given, but I want to say no. The whole point of breaking up the Imperial Army was to prevent something like that from occurring. If the Imperial Navy at its strongest point couldn't blunt the Space Marines, it becomes a very poor narrative point. Probably would come the closest to conquering Terra (though, funnily enough, out of the Imperial factions wouldn't do the most damage). Between the massive space forces in the area, Luna itself being essentially a planetary weapons platform, and that there would be forces around that could actually put up a fight, I don't see Space Marines conquering Terra without major plot assistance.

Adepta Sororitas - If all Sisters of Battle turned traitor and decided to attack Terra, could they conquer it? No. Not enough numbers or space faring vessels to overcome space and orbital defenses. On the ground, the Sisters would be drowned in bodies, other power armored forces, heavy metal, and Titans.

Custodes - Again, a lack of numbers. They'd be able to kill the Emperor easily, of course, which would do tremendous amounts of damage to the Imperium as a whole, but wouldn't actually be conquering Terra. If the Navy were so inclined, they could simply start lancing areas where the Custodes are operating to reduce their numbers until Space Marines or other specialist forces could eventually root them out.

Mechanicus - This is a little more iffy. Mars is one of the major deterrents to fighting over Terra. AM have their own fleets, ground forces, war machines, planets, and even Titans. I feel like in Mars vs. Terra, Terra would win. However, all of AM vs. only Terra, I think AM could eventually grind away at Terra, Luna, and all the other Imperial strongholds.

Astra Militarum - Not enough space based forces to transport them to Terra. Though if a trillion guardsmen were to materialize on the planet, I'd argue that they could eventually take it.

Imperial Knights - No, not enough numbers.

Chaos Daemons - Going to say no on the basis that they haven't done it yet. Probably one of the more dangerous scenarios if daemon portals were to simply open on the planet and unable to be closed.

Chaos Space Marines - This is going to be awkward. Narratively, they *should* be able to conquer Terra. After all, that's kind of their whole point--to be the big, bad boogey man to the Imperium, the dagger that could go to the heart and end everything. But...they couldn't even eliminate Cadia without cheating, they hide in the Eye, and a lot of their victories are often plot armored nonsense. Out of all the human forces, they should be the ones with the best shot. Just that the fluff doesn't really reflect it. But it should.

Eldar of Craftworlds, Dark, Clown, Death Cult, and Dinosaur riding flavors - Not without their end game scenario firing off where they eliminate Slaanesh and reunite, *and* finding a couple of caches of lost War in Heaven super weapons. If Terra were to become a problem, this race would find it easier to blow it up or direct another faction to kill it.

Genestealer Cults - Probably have the best, most realistic shot of conquering Terra, given that they had already infiltrated it and would've continued to grow if not for sheer dumb luck. Funnily enough, a Terra conquered by the GSC might not actually look all that different from your regular Terra...at least at first. I imagine the cults would start a slower, almost imperceptible decline of the Imperium--relief forces arriving too early or too late, supplies redirected, forces ordered to abandon planets to the hive fleets, starting wars with everyone to deplete resources, etc.

Necrons - ...Maybe? Another race that would find it easier to blow up or trap in a time loop or something like that. Given that Trazyn does have a spot for the Emperor/Golden Throne in his collection, it's either not something that can be done easily or it just simply can't be done. But all, or a significant chunk, of the Necrons? Sure, probably. Maybe.

Orks - Yes. They've proven to be dangerous before. Their unique approach to planetary invasions means they don't necessarily have to pacify local space before ramming their roks into Terra's surface.

T'au - Hard to judge. If the Sol system fell into a wormhole and popped into existence next to the T'au system? Realistically, I'd have to say that it would take hundreds of year for the T'au to eventually conquer Terra, but that would be more than enough time for the greater Imperium to respond. But T'au have some bs plot armor.

Tyranids - In theory, Tyranids could conquer Terra. I think we can all agree that an endless wave of self sufficient alien horrors that cannot be demoralized or dissuaded could conquer pretty much anything. Narratively, though, the Tyranids exist to lose--any wins they get are easily glossed over and they inevitably lose, usually in the dumbest of ways to forces too small to realistically stop a Hive Fleet. If a Hive Fleet were to materialize near Terra, the most likely action would be that the Imperium fought bravely, suffered massive but ultimately meaningless casualties (that would grow less and less significant with each edition), and were defeated by the flavor of the month.

I’m not sure I agree Tyranids never win. They’re not exactly in it to win or lose. Their objective is survive and eat. They manage that pretty regularly, and when driven off it’s invariably at staggering cost to the defenders.

 Gert wrote:
The Necrons aren't generally idiots though. Well apart from that one time

👍

I think overall Chaos, as a unified thing as portrayed in the background, numbers wise, could do it.

But? Do they have the overall cohesion to pull it off, or are those with the most power too obsessed with their own ego to pull it together?

 Mad Doc Grotsnik wrote:
I’m not sure I agree Tyranids never win. They’re not exactly in it to win or lose. Their objective is survive and eat. They manage that pretty regularly, and when driven off it’s invariably at staggering cost to the defenders.

When written about, the Tyranids eat never before mentioned worlds or defeat never before named Chapters (though also rarely are these utterly wiped out). These are basically disposable losses. No major characters or factions have "recently" in the time line seemed to suffer meaningful setbacks to the Tyranids and they always seem to pull through at the last moment through some form of "kill the Norn Queen" or "saved by the warp" deus ex machina. I am not saying a faction needs to be wiped out but when was the last time a campaign or story ended with the Tyranids having the lasting victory over a previously named major character or faction?

The action on Octarius is kind of a victory with the Tyranids killing the unnamed Overfiend of Octarius but the story seemed to be setting up for the Ork tech crown disrupting the Tyranids' cohesion and probably a new Overfiend will rise up. The Tyranids defeating the Adeptus Mechanicus and consuming Gryphonne IV and the majority of their Titan Legion could be a good story, if it were ever written up in detail, as Gryphonne IV was supposedly one of the most heavily defended worlds in the southern galaxy.

Breton wrote:

Commissar von Toussaint wrote:

 Irbis wrote:

First, Vault of Terra books tend to land on grimdumb side of plot. So, they aren't that good proof for that. Second, if there was a genuine siege/food crisis, the Terra PDF command wouldn't be sitting on their asses waiting for revolt but would actually start arresting most disposable stratas of society and start feeding them to corpse starch reactors to make sure there is enough food by increasing supply and decreasing demand. Hell, given how many supposedly surplus people in the form of pilgrims and such are on Terra it's probably the only world in the Imperium that can actually last a pretty long time operating on normal level even if the food supply is completely cut in an instant (and for that you'd need to not only cut Terran system from the rest of IoM, but also Terra from the other planets/space infrastructure in Sol system which would require even bigger force)...

One of the problems with GW fluff is that people are allowed to write authoritatively on things they know nothing about.

Given the totalitarian nature of the Imperium, I would expect everyone on Terra to have a very good reason to be on Terra or they would be deported on outbound Black Ships. Remember: no one likes empty ships. I suspect that's how the balance is maintained - bring in 100,000 psykers and take off 100,000 "useless mouths."

You're forgetting the corruption and the snobbery. The unwashed masses are out of sight out of mind for the upper crust, the lower crust will use them and only ship off the ones that aren't protected by them or someone else.

I think corruption is the key, and I suspect that the security systems on Terra are going to be as broken as everything else in the Imperium.

Someone playing the long game could easily infiltrate Terra with huge numbers of agents that could bring down more or less anything outside of the Mechanicum and Custodes, since we can assume they aren't as easy to bribe or corrupt.

As with any other Empire, the grunts are doing most of the actual work and yield a surprising amount of power.

Yeah, any attempt at portraying the Imperium as anything but an inefficient, bureaucratic hellhole is missing the entire point.

 A Town Called Malus wrote:
Yeah, any attempt at portraying the Imperium as anything but an inefficient, bureaucratic hellhole is missing the entire point.

I think there's two basic views on the question. The one I lean toward is that there has to be some residual level of competency in the Imperium for it to last as long as it has. Thus, there's a reason Terra hasn't fallen yet besides plot armor.

The other one is that it's a rotten apple waiting for the slightest breeze to knock it off the branch. Both positions are compatible with the fluff because the fluff is all over the place.

 A Town Called Malus wrote:
Yeah, any attempt at portraying the Imperium as anything but an inefficient, bureaucratic hellhole is missing the entire point.

There is a truly amazing line from an old film you may have heard of, called "Casablanca" with Humphrey Bogart and Ingrid Bergman.

In one scene Claude Rains character is addressing a throwaway comment by Conrad Velt's character, (A nazi major) about how American's are just a bunch of blundering fools.

"My Dear Major, you mustn't underestimate American Blundering. I was with them when they Blundered into Berlin in 1914."

Same with imperial inefficiency. They're inefficiently controlling a large portion of the entire kwown Universe. And their patron deity is more powerful than most of the Chaos "Gods"

Iracundus wrote:
I am not saying a faction needs to be wiped out but when was the last time a campaign or story ended with the Tyranids having the lasting victory over a previously named major character or faction?

The action on Octarius is kind of a victory with the Tyranids killing the unnamed Overfiend of Octarius but the story seemed to be setting up for the Ork tech crown disrupting the Tyranids' cohesion and probably a new Overfiend will rise up. The Tyranids defeating the Adeptus Mechanicus and consuming Gryphonne IV and the majority of their Titan Legion could be a good story, if it were ever written up in detail, as Gryphonne IV was supposedly one of the most heavily defended worlds in the southern galaxy.

When was the last time ANY previously mentioned character or faction was destroyed?

Squats? Was a nightmare, and they're technically back now too.

Tycho? Still in the codex.

Creed? Missing, not destroyed.

Even the Primarchs are coming back.

GW is all about the status quo. Even the fall of Cadia didn't disrupt the status quo, it just made the once a summer event of Every Faction And Their Sister invading the same planet at once marginally more believable.

FezzikDaBullgryn wrote:

 A Town Called Malus wrote:
Yeah, any attempt at portraying the Imperium as anything but an inefficient, bureaucratic hellhole is missing the entire point.

There is a truly amazing line from an old film you may have heard of, called "Casablanca" with Humphrey Bogart and Ingrid Bergman.

In one scene Claude Rains character is addressing a throwaway comment by Conrad Velt's character, (A nazi major) about how American's are just a bunch of blundering fools.

"My Dear Major, you mustn't underestimate American Blundering. I was with them when they Blundered into Berlin in 1914."

Same with imperial inefficiency. They're inefficiently controlling a large portion of the entire kwown Universe. And their patron deity is more powerful than most of the Chaos "Gods"

Except we’re specifically talking the defence of Terra.

Imperial inefficiency is largely down to its sheer size, it’s centralised beuracracy and depending on digging tunnels through Actual Hell to get from A to B.

Terra is the Throneworld. Terra is everything. Loose Terra, lose The Emperor and the Astronomicon.

The Sol System is such a vital, and tiny, part of the whole I’m not sure the grand scale inefficiencies would really be a factor when it comes to repulsing an attacker.

If anywhere has “Emperor Said No” Dark Age, Forbidden tech and weapons? It’s Terra. At least in concentration. If anywhere has a reliable in-system communication system, well maintained, with multiple redundancies? It’s Terra.

That’s not to argue “therefore always secret win button”. Just part of the fun of this topic is properly considering just what a hornets nest Terra could prove to be.

I’d need to check in-canon galactic charts etc, but the Sol System itself likely isn’t particularly isolated, so potentially significant reinforcements could be easily on hand.

Which all feeds into what’s going on in my head. Not that “loads of defences, therefore can’t be successfully attacked”. More that any successful strike would take truly vast forces. The closest anyone came was during The Heresy, which gives us some idea of what sort of forces might be required. Because sure, Terra has far fewer Marines to protect it*? But it’s had 10,000 years of relative peace in-system to sort its defences out.

*At least, so we’re told. Whilst I cannot prove so won’t produce as an argument, it wouldn’t at all surprise me if there are a load of Astartes nobody else knows about based on Terra. I mean, Cawl pulled off the Primaris Project in total secrecy, so someone taking tithed Gene Seed and creating secret Chapters, kept on ice doesn’t seem impossible. But that’s a flight of fancy rather than a strong argument.

Iracundus wrote:
The Tyranids defeating the Adeptus Mechanicus and consuming Gryphonne IV and the majority of their Titan Legion could be a good story, if it were ever written up in detail, as Gryphonne IV was supposedly one of the most heavily defended worlds in the southern galaxy.

I imagine the writers saw Matrix 3, and the walkers shooting at a hole filled with metal squid, and thought, we can do that!

Commissar von Toussaint wrote:

 A Town Called Malus wrote:
Yeah, any attempt at portraying the Imperium as anything but an inefficient, bureaucratic hellhole is missing the entire point.

I think there's two basic views on the question. The one I lean toward is that there has to be some residual level of competency in the Imperium for it to last as long as it has. Thus, there's a reason Terra hasn't fallen yet besides plot armor.

The other one is that it's a rotten apple waiting for the slightest breeze to knock it off the branch. Both positions are compatible with the fluff because the fluff is all over the place.

Given most real life organizations of any size are more akin to the rotten apple than residual competency, I can certainly see that applying after 10,000 years of decay. Then once you add in the tech that no-one understands and the reliance on methods no-one can remember the logic behind, it's not hard to imagine anyone being able to infiltrate most of Terra with some fake imperial seals and some dirty but pointless task to do. Would any governor deny access to an Imperial sewer surveying team? He probably wouldn't even want to meet them.

I think if anythings going to successfully bring down Terra, it'll be a fairly trivial plot like destroying the planets supply to toilet paper, or leaving all the doors locked, rather than a huge invasion force.

Herzlos wrote:
Given most real life organizations of any size are more akin to the rotten apple than residual competency, I can certainly see that applying after 10,000 years of decay. Then once you add in the tech that no-one understands and the reliance on methods no-one can remember the logic behind, it's not hard to imagine anyone being able to infiltrate most of Terra with some fake imperial seals and some dirty but pointless task to do. Would any governor deny access to an Imperial sewer surveying team? He probably wouldn't even want to meet them.

I think if anythings going to successfully bring down Terra, it'll be a fairly trivial plot like destroying the planets supply to toilet paper, or leaving all the doors locked, rather than a huge invasion force.

Most real-live organizations aren't designed to do anything other than make or spend money. You have to look at larger institutions and they both can last a long time and be very resilient. China's dynastic system lasted for 22 centuries and is arguably still in place after a brief flirtation with Marxism.

Japan's monarchy supposedly is 2,500 years old. All institutions have ups and downs, times of decline and times of renewal. Part of the issue is where in the cycle they are when you look at them.

Unlike all those other human institutions, the Imperium has the luxury of the same ruler - no risk of an inbred idiot son losing the kingdom.

I'm leaning towards the Doc's explanation - the greatest human intellect in history is likely paying attention to what goes on in his frontcourt.

Only beauty can tame the beast. The Johnson will be along shortly to handle his business.

Commissar von Toussaint wrote:


Unlike all those other human institutions, the Imperium has the luxury of the same ruler - no risk of an inbred idiot son losing the kingdom.

No, the Imperium has the same figurehead. The Emperor is a lighthouse, the High Lords rule the Imperium, and they are human like everyone, and there is just as much chance of an inbred idiot fething it up as can be seen in the many times that idiots fethed it up.

Whilst it’s indisputable that The Imperium, as a whole, is a nightmare of bureaucracy? Not only does the whole of thing somehow work, but on the much smaller scale we can look to documented planets and systems which work out about as well as can be expected.

Necromunda is one. A super densely populated, but for all intents and purposes, dead world. Yet….it still churns stuff out, endlessly recycling, remaking and making do.

It imports what it needs, and in the staggering quantities it’s needed in. It exports arms and soldiers, alongside luxury goods. We as modern humans might look at it and balk at the awful conditions all but the tiniest fraction endure - but it works.

It also shines a light on the Greater Imperium. Basically as long as Planetary Governor meets their tithe? Nobody particularly cares what goes on for the most part. Its territories are too numerous and far too wide spread for anything more to be done than occasional inquisition checks etc.


That in itself can prove problematic for blockading Terra. Yes there were will be favoured warp routes, due to varying stability. But even blockading those would require a pretty vast fleet - let alone all the other potential realspace relocation points.

And if needed, that’s a lot of traffic you need to blockade against, whilst seeing off the ships actively defending the Sol System.

As ever, incredibly difficult doesn’t mean impossible. But I just don’t think a blockade is a feasible approach. It would likely require a fleet of staggering size to begin with - and a way to reinforce as the inevitable battles take their toll. Now, The Imperium has reinforcements aplenty, albeit far flung and usually quite busy elsewhere. But defending Terra and breaking your blockade would, I fancy, become Priority One.

And let’s not forget Terra is not without her own shipyards. So unless you completely destroy a ship, chances are it can get back to port for repairs and come back at you. So again sheer attrition most likely isn’t going to be in your favour.

I just don’t think anyone has either the resources or temperament for that.

A direct assault seems a bit more possible. Yes you need to overcome fleet and planetary defences, but if you can gather sufficient ships, and wield them cleverly I think you’d have some chance of overwhelming the defences and defenders?

Altima wrote:
Space Marines - If all the Space Marines suddenly turned traitor and decided to attack Terra, could they conquer it? Depends on the writer and how much bs plot armor Space Marines are given, but I want to say no.

And yet, in canon a combined force of like 4 chapters did (during the reign of Goge Vandire, even though such a tiny fleet shouldn't make it past Pluto, never mind sieging the Palace somehow). Go figure.

Ditto with Vangorich, like 4 SM companies managed to capture the Palace then Assassin temples and eliminate him too despite Vangorich being in total control of the whole Imperium...

Adepta Sororitas - If all Sisters of Battle turned traitor and decided to attack Terra, could they conquer it? No. Not enough numbers or space faring vessels to overcome space and orbital defenses. On the ground, the Sisters would be drowned in bodies, other power armored forces, heavy metal, and Titans.

Sisters actually have bigger numbers than SM and while their fleets are individually weaker, there are a lot more of them, too. Not to mention the biggest Convent being on Terra itself which kinda makes the whole travel issue moot. Also funnily enough Vandire's proto-sisters kicked the butt of both SM force sieging Palace from outside and Custode one trying to do the same from inside, to the point Custodes actually tried parlay (and offered hostages) for the first and only time in their existence, as far as we know at least.

Custodes - Again, a lack of numbers. They'd be able to kill the Emperor easily, of course, which would do tremendous amounts of damage to the Imperium as a whole, but wouldn't actually be conquering Terra. If the Navy were so inclined, they could simply start lancing areas where the Custodes are operating to reduce their numbers until Space Marines or other specialist forces could eventually root them out.

Eh, no. Just no. Not only Palace has massive shields and defence guns, but Custodes have bombs planted on every single ship and command bunker in the whole Sol system. If they wanted to rebel they would just decapitate leaders they wouldn't be able to sway and order the rest around. After all, they the Emperor's companions, when they order you, not listening is both insubordination and religious heresy. If there is a force that can coup the Imperium in an instant, it's the Companions.

Also, dunno about killing the Emperor bit, seeing He would be able to obliterate anyone in the Throne room with a thought. If Emperor wanted it for some reason, maybe, but otherwise, no.

If a Hive Fleet were to materialize near Terra, the most likely action would be that the Imperium fought bravely, suffered massive but ultimately meaningless casualties (that would grow less and less significant with each edition), and were defeated by the flavor of the month.

Unless it was Imperial Armour book, then Imperium would lose in laughably stupid way to a force that shouldn't be able to do anything

 Irbis wrote:
And yet, in canon a combined force of like 4 chapters did (during the reign of Goge Vandire, even though such a tiny fleet shouldn't make it past Pluto, never mind sieging the Palace somehow). Go figure.

Yeah, that's not accurate. Vandire didn't have control over the Sol System when the end of his reign came around as the majority of his forces had been destroyed in a Warp Storm (the Storm of the Emperor's Wrath) and he was left only with the ten thousand Brides of the Emperor. He controlled the Ecclesiarchal Palace and portions of the Imperial Palace. The Mechanicus, led by the Fabricator General, were joined by units from four Chapters who were also then aided in secret by the Custodes. There was very little actual combat as the Mechanicus bombarded the Palace while the Custodes put their plan into action. The Palace was never stormed because Vandire was assassinated by the head of his own bodyguard.

Ditto with Vangorich, like 4 SM companies managed to capture the Palace then Assassin temples and eliminate him too despite Vangorich being in total control of the whole Imperium...

Also inaccurate. By the time of the Beheading, Vangorich had lost all control barring the Assassin Temples. When six hundred Astartes landed on Terra to depose him not a single other Imperial institution raised arms against the Space Marines. Half their number were killed assaulting the Imperial Palace and the remainder, barring Maximus Thane, were killed assaulting the Eversor Temple at the North Pole.

Commissar von Toussaint wrote:
Given the totalitarian nature of the Imperium, I would expect everyone on Terra to have a very good reason to be on Terra or they would be deported on outbound Black Ships. Remember: no one likes empty ships. I suspect that's how the balance is maintained - bring in 100,000 psykers and take off 100,000 "useless mouths."

Eh, it's literally the HOLY planet. People need no other reason to go there than pilgrimage. And your second point is actually what I meant - if Terra is cut off, the easiest solution to the problem of 'useless mouths' is to fed them to corpse starch factories instead of ships, and the High Lords are more than ruthless enough to order it...

Breton wrote:
When was the last time ANY previously mentioned character or faction was destroyed?

...

GW is all about the status quo. Even the fall of Cadia didn't disrupt the status quo, it just made the once a summer event of Every Faction And Their Sister invading the same planet at once marginally more believable.

Um, literally last two book campaigns had Belakor ruin big Forge World and obliterate major, named Knight house (both with subfaction rules)/Vashtorr crush major Imperial warp beacon with all the forces on massive Imperial fleet defending it (which should be at least dozens of chapters/Sister orders/IG regiments/etc, etc). Really now?

 Gert wrote:
Yeah, that's not accurate. Vandire didn't have control over the Sol System when the end of his reign came around as the majority of his forces had been destroyed in a Warp Storm (the Storm of the Emperor's Wrath) and he was left only with the ten thousand Brides of the Emperor. He controlled the Ecclesiarchal Palace and portions of the Imperial Palace. The Mechanicus, led by the Fabricator General, were joined by units from four Chapters who were also then aided in secret by the Custodes. There was very little actual combat as the Mechanicus bombarded the Palace while the Custodes put their plan into action. The Palace was never stormed because Vandire was assassinated by the head of his own bodyguard.

Um, wrong. The storm only destroyed one fleet out of hundreds he had. Yes, that was his most fanatic followers but that was specifically Church force, so the of remainder of the troops were untouched. Yes, a lot of remaining commanders started to disobey orders to attack SM and AM as it was suicide, but that didn't mean abandoning him completely and he needed to be dug out of Palace precisely because he still had control of most of the Imperium. If he really had nothing, there would be no need for the siege, they could just brick up the occupied part of Palace and wait for everyone inside to die of thirst.

And no combat? Seriously? Then who SM and Custodes repeatedly attacked to be thrown back again and again? Do you really think Custodes would beg for parley and give Brides hostages without a fight? They literally couldn't budge them so had to employ desperate gambit, drag Alicia before the Emperor at humiliating terms and hope He would convince her to eliminate Vandire. That doesn't sound to me like a tactic you'd employ against inferior force led by man who has nothing

Also inaccurate. By the time of the Beheading, Vangorich had lost all control barring the Assassin Temples. When six hundred Astartes landed on Terra to depose him not a single other Imperial institution raised arms against the Space Marines. Half their number were killed assaulting the Imperial Palace and the remainder, barring Maximus Thane, were killed assaulting the Eversor Temple at the North Pole.

Also wrong, population was scared to attack SM due to horrors of the Heresy being still remembered but Vangorich still remained the effective dictator of the Imperium and massive force of SM led by Ultramarines was needed even post his fall to purge his supporters and stabilize the government. This again doesn't sound like someone who has nothing, the fact local commander refused to obey suicidal orders does not mean the regime is out, doubly so if fight continues long after the leader is dead. If anything that's the mark of really strong, not weak institution.

 Irbis wrote:
Um, wrong. The storm only destroyed one fleet out of hundreds he had. Yes, that was his most fanatic followers but that was specifically Church force, so the of remainder of the troops were untouched. Yes, a lot of remaining commanders started to disobey orders to attack SM and AM as it was suicide, but that didn't mean abandoning him completely and he needed to be dug out of Palace precisely because he still had control of most of the Imperium. If he really had nothing, there would be no need for the siege, they could just brick up the occupied part of Palace and wait for everyone inside to die of thirst.

The troops that were disobeying his orders, defecting to Thor's side, or fighting against Thor's forces you mean? None of which were at Terra. Vandire had ten thousand Brides to defend him.

And no combat? Seriously? Then who SM and Custodes repeatedly attacked to be thrown back again and again? Do you really think Custodes would beg for parley and give Brides hostages without a fight? They literally couldn't budge them so had to employ desperate gambit, drag Alicia before the Emperor at humiliating terms and hope He would convince her to eliminate Vandire. That doesn't sound to me like a tactic you'd employ against inferior force led by man who has nothing

The Mechanicus and SM attacked the Palace and didn't breach the walls. The Custodes met with them and told the attackers to continue the siege while they solved the Vandire problem. They didn't beg with the Brides and the Captain-General left his men as hostages as a sign of good faith while he brought Alicia Dominica and her bodyguard to the Golden Throne.
It wasn't about the Custodes, Astartes, and Mechanicus struggling to win, it was about causing as little damage to the Palace as possible and ending the insanity of the Age of Apostasy, something the Custodes did at other points when things eventually went too far.

Also wrong, population was scared to attack SM due to horrors of the Heresy being still remembered but Vangorich still remained the effective dictator of the Imperium and massive force of SM led by Ultramarines was needed even post his fall to purge his supporters and stabilize the government. This again doesn't sound like someone who has nothing, the fact local commander refused to obey suicidal orders does not mean the regime is out, doubly so if fight continues long after the leader is dead. If anything that's the mark of really strong, not weak institution.

The regime was out. Vangorich had no allies left by the time of the Beheading and the only forces the Astartes fought were Assassins. The problem of a destabilised government came after Vangorich's reign of terror ended because when he ruled nobody moved against him. The beginning of his rule was effective but only because he removed every single problem he faced. He murdered all of the High Lords save one, had a puppet government loyal only to him, and in his later years began random purges and massacres.
If his government was so strong why did it fall apart immediately after he died? Why did it take a massive effort by the Astartes to stabilise the Senatorum if Vangorich's Senatorum was such a good one?
It's because it wasn't.