- "The skies themselves burn, and we burn with them, yet we fight. This is our planet and ours alone."
- — Vladimir Rex, Castellan of Hellhive Crag
A Planetary Defence Force (usually abbreviated to PDF) is the primary military force of an individual Imperial world. The PDF is under the control of the local planetary government rather than direct Imperial control; the raising, training, arming, leadership, etc., of such units is entirely in the hands of the world's Planetary Governor. According to Imperial law, all Imperial planets must maintain a standing army in order to maintain the planetary government's (and thus the Imperium's) control over the world against any possible invasion or insurrection.
The PDF, as essentially the local military of an Imperial world, has no standardisation and can take any form; some, such as the PDF of Hive Worlds, may be no more than legitimised gangs supported and armed by the local planetary government to keep down the more anarchic elements. When facing any internal or external threat, it is the responsibility of the PDF to engage the enemy until Imperial reinforcements such as the Astra Militarum or Space Marines can be mustered by the nearby Sector Command.
Thus, although rarely respected within the broader Imperial military due to their relative lack of experience, the PDF are widely considered crucial to the Departmento Munitorum as the first line of defence for Imperial worlds against any form of invasion or rebellion. Also important is the PDF's role as the primary source of recruits for the Imperial Guard. Whenever required, the best soldiers in a PDF are transferred into the regiments of the Imperial Guard, receiving further training as they are shipped to their destined locations elsewhere in the galaxy. However, the loyalty of many PDF units is not always true. Many PDF regiments on certain worlds will betray their Planetary Governor if the opportunity -- and the right incentive -- arises.
Larger planetary populations may permanently support professional PDF corps, including elite special forces troops. These elites may be House troops belonging to a particular noble dynasty, under the command of the Planetary Governor of the world, or even Kill Squad Storm Troopers.
During the era of the Great Crusade, it fell to the Imperialis Militia to secure and to hold those worlds brought to Imperial Compliance by the Crusade's Expeditionary Fleets. Part of the vast, sprawling body of the Imperium's military and its support structure, known collectively as the Excertus Imperialis, these regiments 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 become threatened or meet resistance. When the Crusade hosts departed, such worlds stood alone once more. Should rebellion ignite, the world's own militia were to crush it. Should xenos raiders or outcasts attack from without, the world's own militia were 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 Old Night.
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 Imperial Army. 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. The standards of equipment and pattern of accoutrement of individual Imperialis Militia regiments were just as varied. 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. Each militia's kit was dictated by the culture, environment and technological base of their homeworld, and so also was the diversity of their weapons, within a minimum standard issued them from the Principia Militaris 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 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.
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 Abhumanity 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 needed.
Although the PDF automatically defaults all command to any Imperial forces that arrive in the event of an emergency, there are many possible reasons the PDF or elements of it may join in rebellion against Imperial authority; Chaos or alien (particularly Genestealer) infiltration of the planet can cause divided loyalties as PDF regiments turn Traitor or are psychically enslaved by their Genestealer relatives.
The sheer ambition of corrupt Planetary Governors and other organisations in control of the planet can mean the PDF under their control turns against the off-world Imperial forces. This concern is a cause of continual mistrust in PDF forces by the Imperium in general, and further reason for Imperial military commanders to prefer to rely on non-local defences when possible. Imperial Inquisitors are particularly known for a lack of trust in the local PDF of a world they are investigating, and are often quick to have nearby Imperial forces available, particularly elements of the Chambers Militant of their respective Ordo, should the need arise.
Notable Planetary Defence Forces
- Agripinaa Home Guard - The PDF of Agripinaa, the Home Guard participated in the defence of Agripinaa against the Forces of Chaos during the 13th Black Crusade in 999.M41.
- Armageddon Defense Force - The PDF of Armageddon participated in the defence of that Hive World during the multiple campaigns fought against the Forces of Chaos (First War for Armageddon) as well as Ork forces (Second War for Armageddon and Third War for Armageddon).
- Baran Siege Masters - The PDF of Baran fought alongside the Raven Guard Space Marine Chapter against the remnants of the Ork WAAAGH! of Arbuttz the Incredible. Baran is a fecund world of rolling blue oceans, expansive forests, and rolling steppe land. It was originally an Eldar Exodite world, but was conquered by the Orks of WAAAGH! Arbuttz in the late 40th Millennium. By 224.M41 an Imperial Crusade passed over the world with the intention of liberating it from its Ork overlords. The first assaults quickly established the Imperial forces as masters of the steppe-lands. The Feral Ork tribes fled to the hinterlands. General von Paulus, commanding General of the Death Korps of Krieg's 127th Infantry Regiment, lacked the troops to threaten one area without exposing another, so he had a system of fortifications set up to contain the Orks instead. As the only regiment assigned to the crusade, the 127th Infantry Regiment were given the Right of Settlement of Baran. As human settlers began to arrive, these fortified positions became the basis for Imperial cities, with a curious blend of civilian structures and military fortifications. As generations passed the original conquering regiment died away and was replaced by home grown Planetary Defence Forces. Eventually, these regiments were inducted into the Imperial Guard as the Baran Siege Masters.
- Cadian Interior Guard - The Cadian Interior Guard defended its homeworld of Cadia against the encroachment of the Forces of Chaos, most notably during the 13th Black Crusade.
- Erebus Defence Legion - The Erebus Defence Legion defended their homeworld of Tarsis Ultra alongside the Ultramarines 4th Company and the Mortifactors Chapter during the invasion of Hive Fleet Leviathan in 999.M41. This conflict left a terrible mark upon the world, but Tarsis Ultra was finally transformed into an uninhabitable Dead World by the actions of the Iron Warriors' Warsmith Honsou, who unleashed a terrible biological weapon known as the Heraclitus Virus into Tarsis Ultra's Tyranid-altered biosphere.
- Kaerls - The Kaerls of Fenris fought alongside their feral overlords, the Space Wolves Space Marine Chapter, during the First Battle of The Fang against the Daemon Primarch Magnus the Red and his Thousand Sons Traitor Legion in the 32nd Millennium.
- Maccabian Militia - The Maccabian Militia of Maccabeus Quintus are a capable fighting force, possessing a proud military tradition which can be traced back to the Angevin Crusade. Their finest warriors are often chosen to serve in the ranks of the Maccabian Janissaries Astra Militarum regiment.
- Tartarus Planetary Defence Force - The Tartarus Planetary Defence Force defended their homeworld of Tartarus when it was besieged by Orks sometime during the 41st Millennium. At one point during this conflict, multiple different factions clashed on the planet which included the Orks, the regiments of the Imperial Guard, the Chaos Space Marines of the Alpha Legion, the Eldar of the Biel-tan Craftworld, and the Space Marines of the Blood Ravens Chapter.
- Vervun Primary - The Vervun Primary defended their homeworld of Verghast during the Imperium's Sabbat Worlds Crusade against the Forces of Chaos.
- Ultramar Auxilia - Ultramar possesses a formidable Planetary Defence Force that has a long and proud military tradition that stretches back to the dark days of the Horus Heresy. They have played an instrumental role in the defence of the Realm of Ultramar, most notably during the Battle of Macragge and the Invasion of Ultramar by the Iron Warriors, led by the Warsmith Honsou.
- Codex: Armageddon (3rd Edition), pg. 2
- Codex: Eye of Terror (3rd Edition), pg.24
- Codex: Imperial Guard (4th Edition)
- Codex: Tyranids (5th Edition), pg. 13
- Codex: Ultramarines (2nd Edition), pp. 12-15
- Only War: Core Rulebook pg. 50
- The Horus Heresy - Book Five: Tempest, pp. 176-177
- Battle of the Fang (Novel)
- Nightbringer (Novel) by Graham McNeill
- Necropolis (Novel) by Dan Abnett
- Cain's Last Stand by Sandy Mitchell
- For the Emperor (Novel) by Sandy Mitchell
- Death or Glory (Novel) by Sandy Mitchell
- Warriors of Ultramar (Novel) by Graham McNeill
- Dead Sky Black Sun (Novel) by Graham McNeill
- The Heraclitus Effect (Short Story) by Graham McNeill
- Defenders of Ultramar (Graphic Novel) by Graham McNeill
- The Chapter's Due (Novel) by Graham McNeill
- Games Workshop: Swordwind Pt. 3, The Baran War