"It is of paramount importance that the worlds of the Imperium shall look to their own defence as well as the shield of Terra to protect them. They shall also look to the defence of the wider Imperium, and the prosecution of such wars as the Emperor in His wisdom shall decree. To this end, it will be the prime responsibility of every Imperial Commander to ensure the raising of a militia according to the resource, rank and nature of their world..."
- — From the Preface to the Lex Imperia Munuimenta, Vol. 1
The Imperialis Militia was the primary auxiliary Imperial military force to the much larger regular Imperial Army, the Excertus Imperialis, and was a part of the early Imperial military division known as the Imperialis Auxilia. It was comprised of normal men and women who served in those forces that were the direct ancestors of the Planetary Defence Forces of the late 41st Millennium. The primary duties of the Imperialis Militia were to secure and hold those worlds brought into Imperial Compliance as garrison forces, by acting in various capacities such as providing security for Imperial officials or serving as the planet's local law enforcement officers and its standing military force.
The Imperialis Militia was the bulwark against recidivism and disorder on newly Compliant Imperial worlds. In times of emergency, local Imperialis Militia forces were often called up to bolster the front line forces, swell the numbers of an Imperial army or serve as part of a sector-wide force tasked with defending against a specific threat.
The Old One Hundred
Amongst the very first irregular forces raised by the Imperium, and considered the forerunners of the Imperialis Militia, were those of Old Earth who served alongside the Emperor and His Thunder Warrior regiments in the long Unification Wars of Terra, or in some cases contested them in battle. When these wars finally ended and the Great Crusade began, the surviving irregulars and militias were integrated into the newly constituted Imperial Army, forming the core of the Imperialis Auxilia, and in time they came to be known as the "Old One Hundred".
These much-storied regiments, including the Geno Chiliads, the ConFed Gun Brotherhoods and the Thorosian Voltigeurs, gained numerous battle honours as the Great Crusade pressed ever outwards, and many of the Old One Hundred were in time granted settlement rights on worlds they had conquered, which would eventually produce a second generation of militia regiments from whose ranks the best would be conscripted into the Imperial Army to fight in the Emperor's ongoing wars.
Some regiments however, such as the Lucifer Blacks and the Ouranti Draks, were, for a variety of reasons, judged in some way not deserving of such rights and were tasked with a ceaseless crusade across the stars. These itinerant armies strove to replace their losses by press-ganging from worlds they liberated in an ad hoc manner, transforming over time often into a pale shadow of their former strength. Others, whose service was ultimately indentured rather than given freely, were denied even this mode of recruitment. Such regiments effectively died a slow death of a thousand cuts on a hundred battlefields, and by the time of the Horus Heresy were no more than legends.
The Great Crusade
During the time of the Great Crusade, across the entire Imperium, it fell to the Imperialis Militia to secure and to hold those worlds brought to Compliance by the Expeditionary Fleets of the Great Crusade. Part of the vast, sprawling body of the Imperium's regular military and its support structure, known collectively as the Excertus Imperialis, they formed what was in effect the lowest and most common run of the wider Imperial Army, or the Imperialis Auxilia as it was more precisely known.
Once a world was declared Compliant and under the full control of an appointed and independent Imperial Commander, part of that Commander's principal duty was the raising of an Imperialis Militia to protect their domain and sustain their control. It was the duty of this militia to act in no small part as the enforcers of the Imperial Truth if needed, and its protectors should the Iterators and cadres of administrators of all stripes who were left behind to oversee the long transition to full concordance with the Imperium become threatened or meet resistance. When the Crusade hosts departed, such worlds stood alone once more. Should rebellion ignite, the world's own militia was to crush it. Should xenos raiders or outcasts attack from without, the world's own militia was to hold until help arrived. The Imperialis Militia were therefore the first, and in many cases, the only bulwark against recidivism and disorder, and its forces, whether they were raised as conscripts from a hive city's teeming masses, the yeomancy of a feudal order or tribal levies, stood guard as watchmen against the return of the disorder of the Age of Strife.
Since the earliest days of the Great Crusade, it was common for the local forces of conquered or Compliant worlds to also provide forces to strengthen the Crusade armies themselves as they progressed. These provided auxiliary or support troops to the Legiones Astartes raised from the Sol System. Other than their disconnection from the world which gave them birth, there was in effect very little to distinguish these Imperialis Auxilia units from the better armed or more culturally warlike divisions of the militias.
The distinctions between the two often blurred, particularly in times of emergency when local Imperialis Militias were called up to bolster front lines, swell an invasion army or form a sector-wide defence force against a specific threat. Such was the case at the muster at Calth where numerous Imperialis Militia regiments from the Five Hundred Worlds of Ultramar served as the core reserve for the huge army being raised to attack the Orks at Ghaslakh. Additional regiments of the same type were also in attendance in the retinue of the Word Bearers, but these concealed their degeneration into what amounted to little more than armed Warp Cults, subordinate to their Legion's occult designs.
Order of Battle
The Imperialis Militia was not a coherent, galaxy-spanning organisation by any means, and, except in times of great emergency, was not subject to the chains of command of the Excertus Imperialis. Rather, each was a colonial defence force raised from a planet's own populace and its commander-in-chief was the Imperial Commander of the world it defended. Some were ancient martial classes with their roots in the Age of Strife, while others were irregular levies activated only in direst need, and in most cases the template of command structure and hierarchy imposed by the Imperium's Principia Militaris was adhered to only in its generality.
Just as varied were the standards of equipment and pattern of accoutrement of individual Imperialis Militia regiments. Some wore gaudy dress uniforms or burnished plasmesh chainmail, others furs and hides, while some few might be clad in advanced armour such as local pattern semi-powered Carapace Armour; all as dictated by the culture, environment and technological base of their homeworld, and so also the diversity of their weapons, within a minimum standard issued them from the Principia Command. In theory at least, Imperialis Militia armies were irregular in nature, each member first and foremost a subject of their world only called to arms in times of emergency, but many worlds with large populations required a standing army simply to maintain civil order as well as deal with any outside threat.
In practice, many Imperialis Miliita forces consisted of a core of professional soldiery, supplemented by an influx of men and women called upon or drafted to serve a period under arms as needed, after which they returned to their previous lives. The core of professional commissioned and non-commissioned officers of the Imperialis Militia were usually career soldiers, even on worlds without a strongly martial culture, and in many cases, these were often older Imperial Army veterans retired from the Great Crusade into a position where their skills and experience might still serve the Imperium. The darker side of this coin was the indenture of prisoners, recidivists and thousands considered to be members of Abhuman mutations below the standard of the accepted human norm, but not so debased as to warrant immediate extermination. These forces often served the Imperialis Militia entirely unwillingly, their control guaranteed by cadres of veteran Discipline Masters, and the ruthless application of lethal sanction and chemical inducement where and when needed.
In almost all cases, Imperialis Militia supply lines were limited to their own world, and barring an initial bequest of equipment during their founding, it was unusual for them to be regularly equipped by off-world sources unless the Imperial Commander of the militia's homeworld was prepared to negotiate supply compacts with nearby Forge Worlds or other centres of industry. Given the widely divergent technological levels of worlds across the Imperium, it was impossible to standardise supply of Imperialis Militia forces, necessitating local manufacture and provision in the vast majority of cases.
When it came to heavy vehicles and armour however, most Imperialis Militia were equipped from the Imperium's strategic reserves as very few worlds then or now are capable of manufacturing such fearsome engines of war as the Leman Russ main battle tank or the Baneblade super-heavy tank. Thus it was not uncommon to see a tribal warrior in warpaint and animal skins manning the command cupola of a Baneblade, or a company of warriors wearing primitive chainmail armour charging from the troop bay of a Gorgon armoured assault transport, Mars-manufactured Lascarbines in hand -- such was the diversity and span of the Imperium in both its earliest days and even now.
The Imperium Split Asunder
"Three generations of my line have defended this world; our blood has watered this land, in return it has fed us. The Warmaster shall not have this place, while any man or woman here yet lives."
- —Yeoman-Commander Yusavus, one hour before the Scouring of Delta-Valaxa VI
With the coming of the Horus Heresy, the Imperialis Militia was as split in its loyalties as was every other division of the Imperium's armed forces. When the Warmaster Horus' betrayal was revealed, so too was that of countless planetary commanders, forcing those not sharing their master's conspiracy to choose whether to follow or oppose them. Many militias were brutally purged in the opening solar months of the Horus Heresy and officers who could not be trusted to follow orders were replaced, while on many worlds where loyalty was divided, open civil war erupted, internecine strife that would in some places last out the length of the Horus Heresy and into the years of the Scouring until it was resolved.
Nor was this phenomenon limited to Traitor worlds, for the betrayal was greeted with such shock that a wavefront of paranoia spread out across the Imperium, unevenly, as world after world was gripped by the horror of what had occurred at Istvaan III, or some more local outbreak of open conflict was revealed to the population, who feared it might be visited upon them too. As battle lines were drawn and allegiances determined, the Imperium's worlds looked to raise more armies, either to defend themselves from the oncoming hosts of the Warmaster or to take up arms alongside them. Initially, entire planetary defence militias were swept up on both sides in unprecedented numbers, both in the Warmaster's campaign of Dark Compliance, and in Rogal Dorn and Malcador the Sigillite's desperate response to the imminent threat of the invasion of the Sol System.
In the forces of the Traitors, many militia regiments soon discovered that they were ranked the lowest of all the Warmaster's servants, and the lives of billions were callously spent as disposable cannon fodder. Only the most ambitious and determined warriors survived such battles and soon the Traitor armies were served by a host of bloodthirsty helot-soldiers, who in many cases worshipped their Legiones Astartes masters as demi-gods. Others were infected en masse with Warp-worshipping cult beliefs by their masters, particularly where the Word Bearers were concerned, succumbing irrevocably to mutation and insanity.
The Loyalists too were soon mobilising the Imperialis Militia to oppose the Warmaster's betrayal, in particular across those sectors along the route Horus would take from the Istvaan System to Terra. Entire worlds were often stripped of their defence forces, multitudinous hosts of ill-prepared but determined citizen-soldiers fed into the inexorable grinder of war so that the Warmaster's advance might be slowed as Terra prepared for the inevitable final confrontation.
Throughout the so-called "Age of Darkness" that defined the Heresy, mortal warriors from countless worlds fought in the shadow of the mighty Legiones Astartes, often unseen, invariably unheralded. The actions of the Loyalist Imperialis Militia were rarely recorded or celebrated, while those of the Traitors must remain unspoken. Though the Horus Heresy was undoubtedly a war of brother against brother, the war was fought for mastery of the entire human race, and no people and no world would survive it untouched.
Standard Issued Weaponry
Such was the vast diversity in origin and supply of the Imperialis Militia that no attempt to fully standardise their weapons and wargear had ever been attempted before the Horus Heresy, nor was it entirely warranted. Instead the Officio Militaris, the organisation which was given the unenviable task of overseeing the unknown billions of armed men and women flung across the Imperium, instead required only that certain minimum standards of muster and weaponry were met.
The metric for such personal arms was the Mars-Tellurian Pattern Mark II-III Lascarbine -- having a registered mortality rate of 70% when striking an unarmoured human in the central body mass with a single strength discharge in optimum firing conditions, and mounting a rechargeable liquid metal core magazine able to fire eighty discharges before reloading was needed. Such weapons had been mass-produced in the tens of millions by centralised Forge Worlds, but even this was but a drop in the ocean to the logistical demands of the Imperialis Militia, particularly as such Forge Worlds had calls upon their attention of far greater importance by forces ranging from the Imperialis Armada to the Space Marine Legions.
Aside from certain elite formations and the bodyguard regiments of wealthy Imperial nobles and high-ranking generals -- who had access to a higher standard of weaponry up to and including Bolter types in some cases to meet this shortfall -- it had long been the practice for militia weapons to be largely locally supplied where possible. Many worlds with advanced manufacturing capacity did manage to meet or even exceed the expected metric, either with las weaponry of their own or with a variety of high-velocity, chemically propelled-slug Autoguns functionally identical in firepower if not durability and capacity.
A great many however, were forced to make do with more primitive arms, be they low-powered charge lasers, "numo" flechette guns, hand-actuated stub rifles, and in the most extreme cases of ferals and regressives inducted into the armies of the Imperium, even black powder and crossbow weapons were not unknown. These weapons were gathered under the broad designation of "Auxilia Arms", and while still lethal, especially in skilled hands, they were found time and again wanting in power to combat the frontline enemies of the Great Crusade, being far from effective against Ork xenoforms, and against rarer though infinitely more dangerous creatures such as the nightmarish Khrave, they were effectively useless.
With the advent of the brutal civil war of the Heresy, what had proved true against the most dangerous alien life was to prove cruelly also true against the transhuman Legiones Astartes, both Traitor and Loyalist alike. In such battles, human militia, cult troops and local levies found themselves woefully under-armed to fight Space Marines which, after all, was perhaps also intended, given the role of the Legiones Astartes as destroyers of rebellion as well as conquerors of the stars.
- The Horus Heresy - Book Five: Tempest (Forge World Series) by Alan Bligh, pp. 176-183, 186-187, 190-197, 207