Warhammer 40k is first and foremost a tabletop wargame, the key term here being “war.” So in the grim darkness of the far future where there is nothing but, let’s give some consideration to how the Imperium conducts its wars.
It can hardly be news to anyone that the Imperium has the greatest number of armies out of any 40k faction. However, far from being mere clones of each other, each of the Imperium’s numerous ground forces tends to be quite distinct, and seems to fill a very different battlefield role. In spite of many individual differences between specific units, most of the Imperium’s armed forces can be grouped into some general categories.
Imperial Guard: The primary military service of the Imperium. Tactically they rely on a combined arms approach to warfare, mutually supporting elements creating a synergistic whole. Strategically they are suited to holding actions as they have the manpower to garrison captured territory. The various different tactical elements at their disposal mean that they are the most flexible military organization in the Imperium, they can conduct any and all actions that a modern real world ground force can, which means nearly any and every military action that we can imagine. They are ponderous, but only in comparison to some of the other armed forces at the Imperium’s disposal, there is every reason to believe that they can by quite mobile when the situation calls for mobility.
PDF: Defensive military which operates only on a single planet. Roughly analogous in nearly every way to the Imperial Guard, though generally with slightly inferior equipment and training. Capable of most of the same actions as the Guard, albeit on the scale of a single planet. Because they are never intended to leave their world, from a strategic standpoint they are almost completely defensive in nature, as their engagement with the enemy pre-supposes said enemy has come to them.
Imperial Navy (in atmosphere): Broadly analogous to modern air-forces in nearly all respects including all associated strengths and weaknesses. Very high mobility and powerful long ranged direct and indirect offensive capacity, non-existent territory holding abilities, and very limited defensive capacity. When working in concert with the Imperial Guard they round out the latter’s abilities to include any available to a modern nation’s armed forces, such as “amphibious” actions and aerial insertions.
Astartes: Highly autonomous self contained elite military forces. Tactically they are extremely flexible with most units excelling in a single capacity while at the same time being capable of multiple combat roles. Strategically ill-suited to any actions requiring them to occupy extensive amounts of captured territory as any diminishment of forces for occupation rapidly erodes their combat effectiveness. Extremely high mobility (both on the ground and intra-system) coupled with nearly complete autonomy and rapidly diminishing combat effectiveness after few losses makes them well suited to first response and hit-and-fade actions. Very high individual durability means that they can brave enormous amounts of enemy firepower for brief periods. Very effective when employed as shock-troops and super-commandos, less effective when employed as line troops, however due to generally very high marks for training, moral, discipline, equipment, and martial skill they are able to act effectively in nearly any role for a limited time.
Adepta Sororitas: Highly motivated elite religious armed forces. Tactically they are very well equipped for primarily short ranged engagements. Strategically unique in that they are deployed primarily on the orders of the Ecclesiarchy (or the Inquisition) and so the strategic goals they have may actually be non-military, quite capable of small scale territory holding action. Somewhat less mobile than Astartes but at least as mobile as the Imperial Guard. Most suited for both limited defensive and offensive actions, particularly in urban environments, and for deployment as guard details for senior Ecclesiarchy official or holy sites. High individual durability coupled with less risk of diminishing combat capacity than Astartes makes them more suited to a frontline deployment as hyper-elite traditional forces than the afore mentioned Astartes. Distinct limitations in their equipment make them most effective when fighting alongside other military forces.
Fratris Militia: Part-time all volunteer paramilitary force under the Ecclesiarchy. Tactically they are poorly equipped, poorly trained, and undisciplined, though often highly motivated. Strategically they are very limited, they lack dedicated mechanized support and heavy weapons. As with the Adepta Sororitas their strategic goals will be set by the Ministorum (exclusively so, unlike the Sororitas) and may not be strictly militarily motivated. They act as guards for Ecclesiarchy holdings and uphold Ministorum law and interests on shrine-worlds. In actual military engagements they would function as secondary troops and/or as rudimentarily trained and equipped foot-soldiers, their greatest asset being their eagerness to martyr themselves in holy service to the emperor. They are unreliable as soldiers, and cannot and would not persecute military action effectively without other forces.
Titan Legions: Massive self contained land fighting vehicles with ultra-high damage capacity. Tactically geared entirely towards large-scale conflict, they are able to bring truly overwhelming amounts of direct firepower down on a relatively small area. Strategically incapable of holding territory with no capacity to garrison captured territory, and not at all suited for limited warfare. Because they are the military arm of the Mechanicus their strategic operation may often be predicated on the Mechanicum’s goals. Deployable only in situations where collateral damage is deemed totally acceptable, the aftereffects of their very operation ensure the devastation of whatever area they operate in, and they cannot function effectively in extremely rough or broken terrain. Perfectly suited for siege operations and effective in extremely large scale military operations over open ground. Due to their operational limitations they must carefully choose the time and place of their deployment or they will become impotent and a waste of resources.
Skitarii: Essentially the same as the Guard in terms of capabilities, though generally with better equipment through the Mechanicus. Generally employed as Guard analogue in support of Titan actions to hold ground and guard their flank, and as bodyguard for senior tech-magi.
Adeptus Arbites: Unilateral legal enforcers, empowered to both administer and enforce judicial actions. Tactically equipped for non-lethal action and riot control, limited military capability, particularly in urban environments. Strategically limited, very small numbers and limited military orientation means that they cannot function in anything but small scale actions. Their primary concern is with keeping the peace, a role for which they are well equipped and trained. Because they are highly disciplined and already onsite they function as the first line of defense in military actions, particularly in cases of widespread insurrection.
Inquisition: Supra-legal completely autonomous operatives. There is no way to generalize the Inquisition’s military capabilities. Military capacity would vary from Inquisitor to Inquisitor, with some commanding virtual private armies and others nothing more than a handful of field agents. In addition in the event that they have to act in a military capacity they would generally choose to induct forces from the other military arms of the Imperium, so their military capabilities can be judged in terms of the other military branches capabilities.
Now when we break it down like this, what clearly emerges is a fact that has been mentioned in several of the books; the Imperium’s armed forces are not meant to fight independently, they never have been. Each organization covers the flaws of the others, while they can strike quickly and with great power the Astartes will be overwhelmed by greater numbers and cannot hold large amounts of territory, the Guard by contrast is comparatively ponderous (though powerful) but they can hold vast swathes of territory. Both forces might batter themselves to nothing against a fortress, but a handful of titans could shred those defenses in mere heartbeats, those same titans would fall to dedicated infantry without the support of their own infantry. Once inside an occupied city the Guard might be woefully outmatched at close quarters, while the Astartes could be isolated and picked off piecemeal, and titans would either destroy everything they were trying to reclaim or be so hamstrung by their environment that they would be useless, however the Sororitas with their dedicated close ranged weaponry would be able to cleanse the dense environment with relative ease. All of these forces could be annihilated with impunity from the air without the Navy air-support, while Navy fighters themselves would be destroyed on the ground if their airfields were unprotected.
In truth the “face” of the Imperium at war would not be Astartes, or endless columns of Guard, or the zealous Sororitas, or even mighty Titans, it would be all of these. When only one of these forces goes to war in reality the Imperium is fighting with one armed tied behind its back. In the very few cases we have actually been shown the Imperium truly commit itself to war (such as the engame of the Vraks Siege, or the final push on Taros) it has employed all these forces and more; Astartes make lightning strikes and establish beachheads that the Guard then occupy and advance from, when they encounter fortresses the Titan legions, with infantry guarding their flanks, rend holes in their defenses which the Astartes once again take with lightning speed until the Guard can move in to hold it, all the while dodging about the ankles of Titans dueling enemy war engines, all under the watchful umbrella of the Navy.
This is why the Imperial warmachine is really so successful, not the endless legions of the Guard, nor the matchless heroism of the Astartes, or even the earthshaking Titan Legions, it is all of these working in concert. This is why the Imperium has come to dominate the galaxy, their combined forces are all but irresistible in the battlefield.
All of the Imperial codex books
Imperial Armour Volumes 3,5,6, and 7
How do you see all of the Imperium’s forces working together in the field of war? How about those forces like the PDF or the Inquisition that I didn’t specifically talk about on the field, where and how do they fit in? Do you see different roles for than those that I have identified? If so, what are they?
If you have a favorite corner of the 40k lore that would like to see featured, or just a lore question you think would interest the community, let me know. Ideas are always welcome.