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 OldMate wrote:
I think personally it goes well with Games Workshop's complete inability to grasp numbers in actual warfare, let alone global warfare or galactic warfare. (which to be fair is pretty common in sci-fi, look no further than starwars).

Same with a spacemarine chapter consisting of a thousand marines. Even ignoring innitiates and vehicle crews this force is so tiny that even, as genetically engineered super soldiers in the finest armour(which with the advent of the grey knights and custodes they clearly are not) with the best and most effective deployment options, the force strength would be negligible in a planetary war. They'd have to seriously be doing covert-ops just to minimise losses(which they can't afford because of the risk of loss of geneseed).

But marines are awesome and I'm not trying to bum them out.

During the Great Patriotic War, the Red Army conscripted 29,574,900 men in addition to the 4,826,907 in service at the beginning of the war.

Just pulled this off Wikipedia as I was reading this thread. And that was one country not a planet.


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I agree that marines should be used for surgical strikes, except for when they're not. Look at the war on Ultramar. They attempted to hold ground against nids. Even with a couple thousand extra marines from sucsessor chapters I do not see this going well.

Besides Imperial fists and ultramarines basically function as frontline units (or garrison) on a regular basis.

Well, sometimes a Marine force has no choice- if you look at the attack of Behemoth, the fleet was fully engaged, with no ability to support Space Marine insertions. Tyranids also use a lot of methods to reduce the mobility of enemy forces, such as seeding tge atmosphere with spore mines- these can and will down Thunderhawks.

With that in mind, the only option left to Marine forces on the planet is to act as pockets of elite troops supporting the Macragge PDF, and targeting the most high-value enemy units. They were also defending enormous fortresses, so there is a huge force multiplier there.

The same is true of garrison squads in keeps- they have limited mobility, so they use the force multiplier of the fortress to make up for lack of numbers. It is not for nothing that Marines make very heavy use of Tarantulas and other automated sentries to defend installations.

Also, I don't think the force strength of a thousand Marines is neglibible in a planetary war with the degree of strategic mobility they usually possess. A Marine strike force can generally hit wherever it likes, and will usually be able to outnumber the enemy force defending the target they are attacking. More importantly, Marines can maintain a combat intensity that will kill normal humans. We don't see this on the tabletop, but as soon as an objective is achieved, the Marines are moving against the next one. They can do literally constant operations, day and night. Add to this that most Marine combat casualties will actually recover and be back on the frontlines in a short period due to their physiology (a "dead" model in game can actually just be unable to fight for now), and you can see how those 1000 Marines can be worth much more than just their number and equipment would suggest.

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Cadia only had 850 million people.765 of those were combatants of varying utility? Jesus, no wonder why it fell.

Maybe they should have stopped exporting their prime breeding population to other parts of the imperium for at tleast a generation or two. I mean it is a key fortess stopping the horrors of the warp from invading the imperuim.

I imagined it having something like at least 50billion. Maybe 3-400 billion. You have ultra-dense stratified housing. Squeeze'em in, ship'em out.

I think the big problem for Cadia was that the proximity of the Eye of Terror caused near constant Chaos cults to be popping up. With a significantly larger population, this would become unmaneagable and topple the planet from the inside. Cadia had its own Ordo of the Inquisition just to keep an eye on the constant cults.

PDFs though are restricted to a single world, have less capable equipment, less training etc. remember the guard regiments are the cream of the PDFs crop

Yeah, Haighus I know about the marine self-endued coma thing. This really becomes a weakness if you lose the ground.

Think about it if you are fighting spacemarines you make sure they're dead. You use superior numbers and excessive force to bog them down, use localized superiority of material to bring them down, and when they go into coma use a sharp stick to keep them down because its not like they can defend themselves.

(hilariously enough space marine are apparently easy to indoctrinate, this self induced coma is a suggested means for chaos space marine recruitment. and I wouldn't want to have my marines going into comas near any blood ravens... rather they die than be stolen)


850 million is less than the current population of China. Eleven billion should not be a big number to enforce for a galactic empire (as the Cadian gate literally defends the Imperium from hell itself, this should be a centre of anti-heresy forces not a centre with a anti-heresy force attached to it)]

If you're worried about cults amoung the troops why not just have a fortress world and cycle random regiments through it, then process each force as it leaves.
Or execute them if you're not willing to do that much paperwork.

To hold anything(especially if your key tactic is (apparently a guard tactic?) to drown your enemies in bodies) Cadia must be very small.

Then again as chaos is a reliably self handicapping force with literally no logistical power, maybe it is a suitable garrison and the eye of terror is a backwater talked up for propaganda. in order to draw attention away from an empire faltering from within...

That's my point. The PDF have a workable organizational structure with brigades, division said etc. Based on the fluff that post-Heresy the Imperium wants to break things up to prevent sedition the PDFs should have fractured command stuctures. Instead it's the polyglot expeditionary forces that have unworkable span of control.

The imperium still wants PDFs to 'work'. The point is that even if a system rebels, the rebels are stuck in a single system until the imperium gets around to bombing/invading/whatever them. The same reason guard don't get transports, the Imperial Navy doesn't get ground troops, and the astartes get warships but (in theory) not ones which can go pound-for-pound in a fleet battle against 'proper' ships of the line.

It's far more significant that the SDF doesn't get warp-capable ships. Hobbling the PDF is more likely to cause trouble when they're called on to actually protect the world from an outside threat.

Also, I don't think the force strength of a thousand Marines is neglibible in a planetary war with the degree of strategic mobility they usually possess. A Marine strike force can generally hit wherever it likes, and will usually be able to outnumber the enemy force defending the target they are attacking. More importantly, Marines can maintain a combat intensity that will kill normal humans. We don't see this on the tabletop, but as soon as an objective is achieved, the Marines are moving against the next one. They can do literally constant operations, day and night. Add to this that most Marine combat casualties will actually recover and be back on the frontlines in a short period due to their physiology (a "dead" model in game can actually just be unable to fight for now), and you can see how those 1000 Marines can be worth much more than just their number and equipment would suggest.

Agreed. Plus every marine is as capable a commander as most guard officers and their armour has C&C support not far short of a command chimera with staff.
Their main weakness is logistics - they can go without sleep and food for quite impressive durations, but bolt weapons do eat ammunition in a way lasguns don't. A clip holds 24 angelus-calibre mass-reactives, and - even assuming marine awesomeness allows one-shot-one-kill - they can only be carrying so many.

I think the big problem for Cadia was that the proximity of the Eye of Terror caused near constant Chaos cults to be popping up. With a significantly larger population, this would become unmaneagable and topple the planet from the inside. Cadia had its own Ordo of the Inquisition just to keep an eye on the constant cults.

Not to mention that, yes, shock troops, and yes, Kasr fortresses, but the core of the Cadian Gate is the Segmentum Obscurus Bastion Fleet. It's primarily a Naval strongpoint, with Cadia just being a secure anchorage, otherwise a.n.other warmaster could just sail right on by and say "you know, I think I'll go invade somewhere less well defended....."

GW has no real idea of numbers. Just rule of cool it, imagine a regiment as millions of soldiers or part of an invasion force of millions of regiments. Or that the planets are really quite tiny(although I do not endorse this view as it is against the point of galaxy wide warfare).

But everyone has nasty weapons and a marines can be killed by common weapons employed by all factions

Marines die pretty fast in the writing and there is a lot of nasty stuff out there.

So a thousand is a weirdly small amount(even if you are only counting power armoured battle brothers( which you are not))especially when they then are parcelled into smaller groups. Coma or no coma.


Having forces so small is sorta contrary to the optic GWs promote of battlefields crowded with warriors and monsters and mountains of skulls and bodies.

 OldMate wrote:
GW has no real idea of numbers. Just rule of cool it, imagine a regiment as millions of soldiers or part of an invasion force of millions of regiments. Or that the planets are really quite tiny(although I do not endorse this view as it is against the point of galaxy wide warfare).

But everyone has nasty weapons and a marines can be killed by common weapons employed by all factions

Marines die pretty fast in the writing and there is a lot of nasty stuff out there.

So a thousand is a weirdly small amount(even if you are only counting power armoured battle brothers( which you are not))especially when they then are parcelled into smaller groups. Coma or no coma.


Having forces so small is sorta contrary to the optic GWs promote of battlefields crowded with warriors and monsters and mountains of skulls and bodies.

You actually don't need that many soldiers to conquer and control a planet. The Wehrmacht numbered roughly 10 million soldiers at it's largest, and arguably came close to conquering a world of 2.5 billion people.

An army with modern technology and continuous reinforcements could probably conquer modern day earth with 60 million men.

and like 80% of the Wehrmacht

(and their allied forces the Hungarians, Italians and Romanians(I'm not including the Finns here because they buggered off after taking their land back. -quite a smart move as unlike Germany they were not annhilated-)(yeah it wasn't all just the Germans on their own side in WW2)

came up against the Soviet Union which crushed them with an army of 29 million soldiers. It didn't come close to subduing the Soviet Union, its single largest adversary, against which they never had a chance.

60 million men couldn't hold earth against its current population let alone its emphasis here: *Earth's combined forces*. I admit as things stand you may be able to carve out a foot hold through use of alliances and coercion to avoid conflict with some powers. But against a hostile population (thought yanks knew this stuff...I mean its how you got your independence) 60 million is not enough. Continuous reinforcements is not 60 million though, and a standing force of 60 million would have to have staggering mobility (most likely air mobility, which an over reliance on can be devastating if your enemy can cripple its effectiveness)

Even nuclear annihilation aside, I doubt 60 million could beat a mobilised Europe (Russia included here).



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That being my logical argument I'd also like to add my emotive argument.

Sure you don't need big numbers to hold a planet if your defences are adequate an you (for some reason) are not concerned about them getting a foothold.

But what the hell is the point of grim sci-fi and an empire of such scale (it literally spans the milkyway) if you're going to have battles that could, when viewed in comparison of scale and history be seen as tiny?

Imagine a titan legion breaking an enemy front line followed by a mass spearhead of armoured and mechanised units that stretch to the horizon and beyond. Imagine this happening on several continents simultaneously. Imagine this is proceeding a bombardment of such apocalyptic proportions that fills the sky with dust and radiation and puts the planet into a nuclear winter. And is onto itself an extinction level event.

Wouldn't it be epic if you had wars of billions of soldiers?.
Wouldn't it be grim if this force was expected to take horrific losses to achieve its objectives.
Wouldn't it not be dark?


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Now imagine like 60 million guys on a rock shabbing it out with a force of similar size. Its going to be a whole n'ather conflict. Look at North Africa vs Eastern front. Judge numbers by what conflict you want to portray.

w1zard wrote:
The Wehrmacht numbered roughly 10 million soldiers at it's largest, and arguably came close to conquering a world of 2.5 billion people.

This is completely false. Germany came nowhere near conquering the entire world and defeat was inevitable the moment they declared war on Russia and the US. Even at the height of their temporary victories they controlled part of Europe, leaving entire continents completely untouched. And that's the absolute best case scenario for Germany, winning early gains against enemies that were not prepared (French incompetence, Russia being slower to mobilize) and getting into a defensive position where they could draw the war out a little longer. It wouldn't take a whole lot for Germany to utterly fail and lose the war in 1940.

Okay, going away from the exploits of the Wehrmacht and back towards the OP:

The Regiment being the main organizational unit for the IG makes a whole lot of sense.
The battalion is a monotypic unit formation, i.e. a tank btn has only tanks, a mech inf btn has only mechinf and so forth.
A regiment by contrast is the first unit that has different assets. It contains infantry, armor, engineering support, medical staff, recon and artillery, sometimes augmented by more specialized assets. It is the first tactical element, that can be autonomous on the battlefield and this is the smallest unit you can send of to any warzone with the reasonable expectation of doing something valuable on their own.

Now, today's armies were planned and equipped for WW3 that never came. In that scenario, a single regiment wouldn't be a meaningful tactical element. The armies were supposed to fight large-scale conflict, this the monotypic batalions were bundled together into Brigades - larger units with a full range of autonomous fighting capabilities.

The Imperial Guard however, is called upon to do a plethora of missions. Sure, there are black crusades. And then there are backworld pacification campaigns for which you just need a couple well-supported companies, thus making the regiment as opposed to the brigade preferable.

 Haighus wrote:

Spoiler:

locarno24 wrote:

 Haighus wrote:

 OldMate wrote:
This maybe off topic, but how many soldiers are meant to make up a standard 40k guard regiment? To hold/ invade a planet the size of earth you'd need close to a billion soldiers. And it seems common for single regiments to be fighting on a planet at one time(maybe really small planets?). It seems the stuff I have been reading (Redemption Corps and Ultramarines Omnibus + killing fields) seems to be a grossly undersized affair manpower wise.

40k armies are generally undersized- the only way to square that circle is to assume the use of terror tactics and indiscriminate firepower is widespread. Insurgencies don't often survive well if the invaders will readily commit genocide.

Therefore, I think the Imperial Navy is the more important branch by far- they can destroy whole armies from orbit whilst the Guard defend key points that cannot be simply razed. Space Marines definitely operate in this way- most of the work is done by the fleet with the actual Marines performing surgical strikes to assist the fleet and knock out key targets.

This. A guard army attacking a planet doesn't need to fight open field engagements - only fighting in close to either industrial or civilian infrastructure you don't want to accidentally hit from orbit, or fortresses with anti-orbit guns that prevent a warship coming in to bombard it. In the case of a major hive city, of course, these are often one and the same thing...

For a war like the Third war for Armageddon, billions of troops should be mobilised from the Hives. However, unless the Hive militia regiments are stupidly big (a million or more strong), the Imperial forces on Armageddon will only number a few million in a total war scenario.

I don't think the hive defence forces have ever been referred to by scale. The only worlds whose PDF we have a meaningful size for are Cadia (pre being blown up...) which had 850 million inhabitants but 90% of them were at least 1st line reservists, and Vraks (which was able to put about 5 million soldiers in the field with full mobilization).

We have lists for the number of regiments deployed on Armageddon at points in the war, and it is something like 200 regiments total of Armageddon regiments (mostly Hive militia regiments, with Shock troop and Ash waste militia regiments adding to this). Unless the militia regiments are colossal, this frankly isn't very many troops.

I think the tricky bit is working out how such small forces defend worlds when the Imperial Navy has been driven off! I suppose Imperial settlements could be built in such a way that they are massive force multipliers for defenders (above and beyond the usual 3:1 advantage in modern times, something more akin to the near-invulnerability of good medieval castles in their day whilst provisioned), so they can hold out against Ork waaaghs and the like at least long enough for a larger fleet to arrive.

I agree that the number of Imperial soldiers suggested to be on Armageddon seems way too low. On the other hand these low numbers could plausibly defend the planet considering that according to the back page of Codex Armageddon there are 1307 Ork Warbands and a warband numbers between 600 and 3,000 orks*. So according to this page there would be at maximum there would be 3,921,000** orks on Armageddon, and probably considerably less than that.

Of course I'm not sure if these numbers are supposed to include the feral ork population or grots, They're also a single source from an in-universe Imperial report, so not necessarily reliable.

I prefer to believe that there are at least hundreds of millions, possibly even billions of orks on Armageddon. I do like it when GW puts out lists and charts, but I just end up headcannoning most of their numbers into something that makes more sense.

*Interestingly this is way larger than most references to the size of ork warbands, which are commonly described as being a hundred or so orks. This size is more in line with a tribe, which isn't very well defined but somtimes said to be a few hundred to a few thousand orks. It could be that warbands have merged and grown in average size on Armageddon. It could also be another instance of the person compiling the Imperial report being wrong. I like researching the various groupings in the ork hierarchy, but I figure the orks themselves aren't overly concerned with such things.

**I had to manually count the listed warbands, so I might be off by a few.



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 Kosake wrote:
The Regiment being the main organizational unit for the IG makes a whole lot of sense.
The battalion is a monotypic unit formation, i.e. a tank btn has only tanks, a mech inf btn has only mechinf and so forth.
A regiment by contrast is the first unit that has different assets. It contains infantry, armor, engineering support, medical staff, recon and artillery, sometimes augmented by more specialized assets. It is the first tactical element, that can be autonomous on the battlefield and this is the smallest unit you can send of to any warzone with the reasonable expectation of doing something valuable on their own.

Imperial Guard regiments are typically portrayed as being specialized as an anti-heresy measure. So if, for instance, the commander of an armored regiment turns traitor they wouldn't have infantry or artillery support unless commanders of other regiments also turned traitor.

There are probably examples of combined arms regiments in the fluff though, so if a person wanted to do something like that for their own IG army I wouldn't yell at them for doing it wrong. The first I remember reading about regiments being specialized was in the Gaunt's Ghosts novels, so I'm not sure if this anti-combined arms policy existed prior to then or if it is a late 3rd Edition development.

Most Imperial worlds that contribute to the Astra Militarum will contribute more than one type of regiment (Catachan supplies light infantry and armoured regiments, for example, Cadia provided infantry, armour, superheavy armour and artillery regiments, etc), so if two or more regiments end up in the same warzone it's more likely that one traitor can bring two or more regiments with them.

Tanith provided a single undersized light infantry regiment before being destroyed - had that not happened I'm sure it would have supplied armoured regiments too (in fact, that might have been a better way for them to contribute as an agri-world with limited population).

Something that's interesting is that the Tau form their large units on an even more ad-hoc basis. Cadres are their only permanent military organization. There is a very wide variety of cadres, but the Hunter Cadre is the most common, and it is roughly the size of a company. Everything bigger than a cadre is formed to fulfill a mission and then disbanded once that mission has been completed.

With how regimented the Tau are I can see this not being as big of a problem for them as it would be for the Imperium.

(The example Hunter Cadre in the current codex is relatively massive compared to older examples. Old examples would fit easily within the old FoC but the current one wouldn't fit in a Brigade. From what I can tell most people still go by the older examples.)

 Dakka Flakka Flame wrote:
Something that's interesting is that the Tau form their large units on an even more ad-hoc basis. Cadres are their only permanent military organization. There is a very wide variety of cadres, but the Hunter Cadre is the most common, and it is roughly the size of a company. Everything bigger than a cadre is formed to fulfill a mission and then disbanded once that mission has been completed.

With how regimented the Tau are I can see this not being as big of a problem for them as it would be for the Imperium.

(The example Hunter Cadre in the current codex is relatively massive compared to older examples. Old examples would fit easily within the old FoC but the current one wouldn't fit in a Brigade. From what I can tell most people still go by the older examples.)

Interesting fact there: a cadre by the old fluff is roughly the transport capacity of a Manta. Coincidence? Maybe, but I think it's a strong hint fluff-wise at why the cadre system exists.

What gets me is if a Guard regiment is the size of a modern one (3-5000 soldiers) It can't really safely secure a medium sized town, let alone a continent or planet. It cannot independantly be air mobile (without the Imperial Navy support) unless some cunning fellow has realised Helicopters/conventional aircraft are not space-worthy and are thus are okay with regulations.

Even air mobile you're going to be without heavy support and can't hold a regional area (against medium enemy activity).

I personally, to borrow a phrase, headcanon, a GW regiment (or anyone else's unless stated otherwise) to be a combined force (mechanised, light, armoured, artillery, engineer etc.) capable of holding maybe a continent the size of Europe.

Therefore when you have a battle and elements(or completely) of three regiments are holding a major hive or city it is really a big deal and not like 15000 combatants.
Which to add a tad of grounding context to this British fatalities of the first day of the Somme numbered 19,240.

Even with a regiment big enough to secure a small continent it is still really quite strange to think that a galactic empire is worried about such a small, and expendable force betraying it. So much so that it actively handicaps its own army's organization and structure. Seems bloody hysterical.

Its like worrying about spending petty cash when you're a multi billionaire.


Besides isn't the guard, like many of its vehicles, meant to be a big clunky warmachine that chews up men and women (in their trillions) and spits them (in a much reduced quantity/quality) out.

Would make better sense if forces were huge and to hell with heresy just send more. They die like flies and breed like rats anyway.

I would just like to add my own regiments are at a strength of 3-5000 soldiers, they're just part of a bigger permanent force and thus quite insignificant in the grand scheme of things.

 OldMate wrote:
What gets me is if a Guard regiment is the size of a modern one (3-5000 soldiers) It can't really safely secure a medium sized town, let alone a continent or planet. It cannot independantly be air mobile (without the Imperial Navy support) unless some cunning fellow has realised Helicopters/conventional aircraft are not space-worthy and are thus are okay with regulations.

It's tricky because the size of IG regiments varies greatly. Some have hundreds of thousands of soldiers.

It's also pretty normal for them to send multiple regiments (or elements of multiple regiments) to accomplish even relatively small tasks.


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 Peregrine wrote:

Spoiler:

 Dakka Flakka Flame wrote:
Something that's interesting is that the Tau form their large units on an even more ad-hoc basis. Cadres are their only permanent military organization. There is a very wide variety of cadres, but the Hunter Cadre is the most common, and it is roughly the size of a company. Everything bigger than a cadre is formed to fulfill a mission and then disbanded once that mission has been completed.

With how regimented the Tau are I can see this not being as big of a problem for them as it would be for the Imperium.

(The example Hunter Cadre in the current codex is relatively massive compared to older examples. Old examples would fit easily within the old FoC but the current one wouldn't fit in a Brigade. From what I can tell most people still go by the older examples.)

Interesting fact there: a cadre by the old fluff is roughly the transport capacity of a Manta. Coincidence? Maybe, but I think it's a strong hint fluff-wise at why the cadre system exists.

That would make sense.

I suggest you all break out your copy of Codex Astra Militarum. It has many answers to your questions on Regiments like:

Each Militarum Regimentum comprises multiple regiments, all which come from the same planet.

...officials of the Departmento Munitorum often interchange the term 'Militarum Regimentum' and 'regiment'

Regiments are typically raised with a strength of several thousand soldiers, but the precise numbers can vary enormously.

So when you wonder how a regiment can hold a world, it because they are really talking about several regiments of a Millitarum Regimentum, which is 10's of thousands of troops.

 Dakka Flakka Flame wrote:

 Kosake wrote:
The Regiment being the main organizational unit for the IG makes a whole lot of sense.
The battalion is a monotypic unit formation, i.e. a tank btn has only tanks, a mech inf btn has only mechinf and so forth.
A regiment by contrast is the first unit that has different assets. It contains infantry, armor, engineering support, medical staff, recon and artillery, sometimes augmented by more specialized assets. It is the first tactical element, that can be autonomous on the battlefield and this is the smallest unit you can send of to any warzone with the reasonable expectation of doing something valuable on their own.

Imperial Guard regiments are typically portrayed as being specialized as an anti-heresy measure. So if, for instance, the commander of an armored regiment turns traitor they wouldn't have infantry or artillery support unless commanders of other regiments also turned traitor.

There are probably examples of combined arms regiments in the fluff though, so if a person wanted to do something like that for their own IG army I wouldn't yell at them for doing it wrong. The first I remember reading about regiments being specialized was in the Gaunt's Ghosts novels, so I'm not sure if this anti-combined arms policy existed prior to then or if it is a late 3rd Edition development.

I think you are mixing stuff up a bit here. There's a divide between Guard, Navy and Astartes, yes. And yes, regiments do have a focus, true enough. But in general, a regiment should be mixed assets.

According to Lexicanum they do get split between armour regiments, artillery regiments, infantry regiments and so on. So they aren't mixed assets at all which seems like it does more harm than good to me.

IG regiments are more like battalions. IRL how many battalions include infantry, armour, and artillery.

I think part of the problem is the differences in the British Regimental system (the commonwealth uses) and the Continental system (the rest of the world uses). Combined with the shrinking of modern military sizes.
In the world wars a Regiment would consist of multiple battalions. The Grenadier Guards Regiment started WW1 with 3 Battalions and at the end of WW2 it had 6. These battalions often didn't fight together however, as they were in different Divisions. Each battalion was assigned to a brigade depending on where manpower was needed.
The Regiment was important in keeping tradition and heritage because no matter what Brigade, Division, etc, your unit was assigned to you would always belong to your Regiment.
Your Division is temporary you Regiment is permanent (mostly).

IMHO (I've probably said this before) that the IG Regiments should regarded more as Battalions. The 801 Cadian Regiment is the 801st Battalion of the Cadian Regiment. Whatever Division it was attached too could change from Campaign to Campaign.

It is also probably why in 2nd Ed a "Regiment" had 3 companies.

Three companies was the minimum in 2nd edition. It was entirely theoretical at that point, since a game of 40k would be unlikely to use more than a platoon or two anyway.

 Haighus wrote:

 Hawky wrote:
It kinda makes sense, as in 40k, regiments are raised from a single world, perhaps even from a single region or a city. Regiments are then formed into brigades, divisions etc based on the current state of the front line in a grand scale.

Spoiler:

In the British Army, for most purposes, the regiment is the largest "permanent" organisational unit. Above regimental level, the organisation is changed to meet the tasks at hand. Because of their permanent nature, many regiments have long histories, often going back for centuries: the oldest British regiment still in existence is the Royal Jersey Militia, established in 1337 although historically the Jersey Militia are referred to as a regiment it is disputed that they are in fact a corps. The Buffs (Royal East Kent Regiment), formed in 1572, was the oldest infantry regiment. It now forms part of the Princess of Wales Royal Regiment.[9] - Wikipedia

IIRC, Regiment is also the largest unit with a single specialisation and given how regiments in 40k work, each regiment has a specialisation, be it heavy armour, line infantry, artillery etc. That means an armoured regiment has several companies consisting of nothing but tanks (or armoured vehicles in general, be it a Hellhound, Salamander or some other design), with a one or more support or logistic companies that are not (should not be) engaged directly in combat.

But I agree that the word "regiment" is too overused and any higher organisation unit is usually lacking.

Exactly this- it only makes sense within the British context of what a regiment is. Regiments are largely a peacetime organisation within the BA- they represent a common heritage and culture to create a sense of cameraderie within the unit, which has a specific role and typically contains multiple battalions (although now most regiments are markedly shrunk in size or amalgamated with other regiments). As such, you get infantry, light infantry, cavalry/tank, paratrooper, etc. regiments. The regiments being permanent means they store the history of all their constituent formations, regardless of what units those formations were attached to during a campaign. Regiments don't have a fixed size, ranging from a single battalion to many battalions.

However, the British Army deploys battalions, not regiments, and has used brigades as the roughly equivalently-sized formation to regiments in battle since at least WWII. British brigades contain a number of battalions (usually three to four) which could be drawn from various regiments. This allows for greater flexibility of deployments and a degree of combined arms (although these were typically assigned from the divisional level).

If you look at this example, you have a brigade formed of 4 battalions and a brigade HQ. Two battalions are drawn from the Royal Tank Regiment, one from the 4th County of London Yeomanry (which had a single battalion), and one motor battalion from the Rifle Brigade (confusingly named- it is actually a regiment). The overall formation is composed of elements from three different regiments, but forms one approximately regiment-sized unit.

If you look at how Imperial Guard forces are deployed, they match this deployment style- an infantry force will have attached artillery and tanks support from different regiments. It is quite clear that typical Imperial Guard deployments are combined-arms brigades under ideal circumstances, not single-arms regiments. However, as the regiments are the permanent adminstrative structure, the battle honours and records are accorded to the regiment, not the brigade (or higher ad-hoc structures like armies). All regimental assets in an engagement would get credit though, so the 4th Krieg artillery regiment would be mentioned in the battle report of the 713th Valhallan light infantry regiment halting a renegade assault, even if it was one company of artillery supporting three companies of light infantry.

Therefore, a single regiment can be wildly inconsistent in size (1000 to 100000+ soldiers), yet still be broken up into companies and battalions and mixed into useful combined-arms formations on the front.

I am going to bump this. 40k regiments match British Army regiments. This is the form British Army regiments take.

We don't see regimental deployments very often in the background (outside some BL books)- we see mixed battlegroups formed from elements of multiple regiments deployed to the planet. Clearly regiments are not intended to fight as one unit under nornal circumstances, but mixed in with the other specialists they are deployed with. This is deliberate! An IG army on the table would be boring if they were only tanks, or infantry, or artillery, with no specialist units attached. The battlegroup system of several units drawn from different regiments into one whole is perfect for creating mixed armies on the tabletop.

As such, brigades, divisions etc will likely exist in 40k, but are purely temporary formations only used in a specific campaign. Whereas regiments are the permanent building blocks that persist between campaigns and maintain the histories and traditions of the regiment in question. Just like the British Army.