Imperial Guard Regiment
The Regiment is the largest semi-permanent unit of organization of the Imperial Guard, consisting of a few hundred to a few hundred thousand Guardsmen and their associated equipment and vehicles.
Space Marines are all well and good, but the implantation of gene-seed is a risky, prolonged, and expensive process. Even at the peak of the Great Crusade, there were never more than a few million Marines in the entire galaxy. The Emperor and his growing Imperium needed more soldiers to take and hold planets, and so the Imperial Army was founded to act as an auxiliary force -- while the Space Marines would smash fortresses and other hard targets, the Army would follow behind to hold the line.
To improve force cohesion, they were under the direct authority of the Space Marines with whom they served, but the Horus Heresy proved how dangerous such an arrangement could be. When Horus fell, not only did he take his fellow traitor Primarchs and their Legions with him, but also his associated ships and soldiers. Therefore, after the Heresy was put down, the surviving leaders of the Imperium divided the three military branches so that no traitor could strike such a blow at the Imperium ever again.
The forces themselves were separated into smaller pieces, also to reduce the ability of heretics to cause trouble. The Legiones Astartes were split up into Chapters, the Imperial Navy was divided into Battlefleets, and the newly-named Imperial Guard was divided into regiments. Each regiment in theory could only contain a few hundred to a few thousand Guardsmen (but often times there can be regiments of several million Guardsmen although they are very rare and expensive), and was limited to a single specialty (Infantry, Artillery, Armor), so that if even an entire regiment turned traitor, they would not last long -- without the Navy, they could not travel to other planets (or bomb the shit out of them since the navy also got the aircraft), and without the support of other regiments, they would lack most tactical options. The Commissariat was also founded to be an independent watchdog to root out heresy wherever it grew.
The Administratum evaluates every planet in the Imperium, analyzing what its resources are and what portion of those resources it owes to the Imperium. For many planets (especially Hive Worlds), one such resource is manpower. Worlds are literally tithed to supply a certain number of a particular kind of regiment. After a regiment is founded, it is shipped off from one battlefield to another according to the designs of the Departmento Munitorum, and may periodically be resupplied with new recruits and equipment by the same, or just left hanging until they are almost totally routed, and then folded into another regiment. Because the information used by the Administratum is often out-of-date (by up to millennia, theoretically), a planet's requirements may be unreasonable, whole regiments can be entirely forgotten or called to battle long after their destruction, and regiments may be deployed to "battlefields" whose wars have long been ended. The fact that designation numbers and equipment often get recycled after the regiment that owns them is destroyed does not help matters. On occasion, a regiment can get a lucky break from bureaucratic inertia (e.g. the Valhallan 597th, of Ciaphas Cain fame, was formed by the merger of the half-strength 296th and 301st regiments, but is still listed in official records as the 296th and 301st working together, so they get double supplies), but that's not very grimdark so it only happens very rarely.
Each planet's regiments are different with a variety of names for commanders and subordinates, but we'll use the modern day system. In general, a commanding officer sits at the top, with a rank of colonel (or the equivalent), who is assisted by a command staff. The second-in-command may be titled the "executive officer" and often is a major.
The Regiment is divided into several companies, the exact number of which depends on the nature of the regiment:
- Each company is led by a captain and a command squad and contains two to six Platoons (because a standard 40K army -- which is about the right size for a company -- has between two and six units of Troops)
- Each platoon is led by a lieutenant and a command squad and has around five, (but sometimes more) squads of infantry.
- A squad is led by a sergeant who is sometimes assisted by a corporal (if this is the case, the squad can then be divided into two fireteams). Infantry squads nominally contain ten Guardsmen each, though heavy and special weapon squads only have six, and of course numbers can fluctuate as Guardsmen are recruited and killed.
- Each platoon is led by a lieutenant and a command squad and has around five, (but sometimes more) squads of infantry.
Of course, different types of regiment do things differently but are generally consistent with the levels of authority delegated to their commanders: For example, Armoured Companies still have vehicle squadrons led by sergeants, but with each individual vehicle within the squadron is commanded by a corporal, and thus Armoured Companies have a higher proportion on NCOs compared to standard Infantry Regiments. Meanwhile, Artillery Companies are less likely to have corporals at all.
The Uplifting Primer does include provisions for other ranks not usually found on the tabletop or encountered in the fluff, which might actually be consistent with their in-universe function: lieutenant colonel in real-world militaries command battalions of multiple companies which are usually homogeneous in structure, (A RL regiment is usually multiple battalions of various compositions, or a single battalion with added support units) but in 40k there is actually little need to deploy separate battalions, as it would be inefficient for the Navy to transport such small units individually and without external support, so the rank might only find use in specialist or headquarters units, rather than in a typical regimental system.
Further, while the Imperial Navy uses the rank of warrant officer, the Imperial Guard does not appear to have a standard use for it. This is most likely because a naval starship has a much larger number of men under its command than a regiments does (tens of thousands as opposed to thousands) and also the fact that regiments do not often have a fixed headquarters and have a higher turnover of manpower due to casualties, therefore field commissions directly from sergeant to lieutenant would be more practical than awaiting a newly commissioned officer to arrive from an academy.
That's not to say that senior sergeants cannot be found in staff roles outside of squads, colour sergeants, gunnery sergeants and staff sergeants are known to exist in-universe but are not mentioned in the Primer, but the factional division of the military into Engineering, Adminstration or Medical branches probably makes such staff appointments less common.
Although the purpose of a RL regimental system is to ensure an unbroken chain-of-command where each trooper has one sergeant who himself has one lieutenant who in turn has one captain and so on, the 40k universe as with most things tends to be almost medieval in its execution: the Uplifting Primer as well as most fluff on the matter is quite emphatic about when you get issued an order from anyone above you, you comply without question. That attitude may be helpful if you anticipate a high turnover of unit commanders mid-battle (this is the Guard, so of course) where it becomes impractical to double-check orders with your direct (or even intermediate) superior, but it would cause nightmares for intelligence and logistics officers trying to keep track of a battlefield when officers subvert the units of their allied commanders, and would probably cause confusion and mixed messages to filter down the chain of command and contribute to even more casualties. (this is the Guard, so again, of course) The reliability and effectiveness of this system depends on the pre-campaign planning and inter unit training, regiments and their soldiers are told who are their allies, who is in command of their allies, and such stuff, of course this is not always possible, sometimes regiments are newly drawn up and then immediately thrown into a nearby war with only basic training, sometimes they're diverted from one campaign to another, and other times the Munitorum and their allies are just being assholes. Of course, experienced field commanders and strategic officers would encourage policies to make sure they don't end up killing their own allies.
To inhibit the spread of Chaos, regiments are usually confined to a single specialization each -- that way, even if an entire regiment falls to Chaos, it will be severely restricted in its tactical options, making it easier for loyalist regiments (who have enough different specializations to complement each other) to put it down. The specializations are:
- Armoured. Armoured regiments supply the tanks to a war effort. They may operate as a single force if there is a single, massive target to be destroyed, or if the war is being fought on multiple fronts (e.g. urban warfare), they may be split up to support infantry units or crack vital enemy defense lines.
- Artillery. Artillery regiments have ALL the big guns. They are deployed whole for massive sieges or defensive actions, often with siege infantry regiments to create defenses and trenches; for a more mobile siege, they may be parceled out into batteries that accompany infantry regiments.
- Drop infantry. Drop infantry are space paratroopers -- they drop onto enemy weak points for surprise attacks. Unfortunately, this limits the heavy firepower available to them, as artillery pieces and tanks can't be carried by Valkyries. The Elysian Drop Troops are an example of drop infantry regiments.
- Light infantry. Light infantry regiments are frequently used for reconnaissance and stealth work, as they aren't as tied to a supply chain as more mechanized regiments, and are less noisy (again, because of the reduced mechanization and supply chain needs), though they lack the heavy firepower required to do much else. However, when played to their strengths by smart commanders light infantry can kick all the ass, as demonstrated by the Tanith First.
- Line infantry. Line infantry regiments supply the raw manpower that is emblematic of the Imperial Guard.
- Mechanized infantry. Mechanized infantry regiments are probably the closest thing to an all-rounder combined-arms force that the Imperium has. Mechanized regiments generally give each squad their own Chimera, and often receive detachments of tanks from armored regiments working in the same theater for some extra punch -- or, if there aren't enough Chimeras available to justify keeping them all in one place, they may be split up into "Armoured Fist" squads and given to armored regiments to hold objectives, or infantry regiments who need an armored spearhead. The Valhallan 597th is an example of mechanized infantry regiment.
- Siege infantry. Siege infantry regiments are similar to line infantry, in that they have lots of men, but they receive extra training in construction of fortifications, trenches, and defense lines, and are usually disciplined and trained to fight and die over the course of months or years for even a few yards of advance. The Death Korps of Krieg are an example of siege infantry regiments.
The aforementioned restrictions on regiments means that they are not very useful on their own, so there is also a super-regimental command structure used for campaigns.
- Regiments that complement each other in a useful way can be combined into a battlegroup commanded by a General,
- Several battlegroups on the same front will be overseen by a Lord General
Back in the third edition of Warhammer 40,000, the Imperial Guard Codex included a "doctrine" system to represent different types of regiments in the Imperium. Some options would allow certain units to be taken in larger numbers, while others would allow some units to purchase special abilities. The Catachan Jungle Fighters also received their own mini-dex in Third and Fourth Edition.
The doctrine system was removed in the fifth-edition codex, though some characters can modify the army's abilities a little, and Forge World has published army lists for the Death Korps of Krieg, Elysian Drop Troops, and a generic "Armoured Battle Group" (tanks to the max) in their Imperial Armour books.
We have tacticae on the various Imperial Guard army lists that have been published:
- Imperial Guard, for light infantry or combined-arms regiments (especially Cadian Shock Troops and Catachan Jungle Fighters).
- Elysian Drop Troops, for drop regiments
- Death Korps of Krieg, for siege regiments
- Armoured Battlegroup, for armored regiments or mechanized infantry
- Solar Auxilia, for the elite of the Imperial Army during the Heresy era
- Imperial Militia and Cults, for the miscellaneous military units that formed the bulk of the Imperial Army during the Heresy era
- Imperial Guard Regiment Creation Tables, a set of rolltables for generating a regiment.
- Only War, a roleplaying game made by Fantasy Flight Games, which contains rules for generating and role-playing an IG regiment.