The Imperium of Man or Imperium of Mankind is a galactic empire in the fictional universe of Warhammer 40,000. It is a theocratic, industrial, militaristic, and totalitarian regime that rules over almost all of humanity and spans more than a million inhabited worlds.
The "Imperium" is one of the original factions in the fictional background of the Warhammer 40,000 tabletop miniatures wargame; the game was initially published in 1987 by Games Workshop, and continues to be actively developed. The game system's setting is a dystopian far future fictional universe that is extensively described in numerous rulebooks, tie-ins, and other official sources.
Notwithstanding backstory development by Games Workshop and affiliates (such as the publishing imprint Black Library) that spans several decades, the core depiction of the Imperium – as a totalitarian, strife-torn galactic empire on the brink of catastrophe – changed little; a major reason is the publisher's decision to "freeze" the narrative setting to the universe's late–41st millennium (or late–M41). As of 2013[update], this setting had been utilised by the majority of related material; however, for several years prior and continuing, the publisher has also been promoting a parallel narrative thread for the fictional universe, which is taking place during the founding and early times of the Imperium.
The fictional empire is variously called "Imperium", "Imperium of Man", or "Imperium of Mankind" in official publisher sources.
27,000 years in the future, the Imperium rises from the ruins of a more prosperous, advanced, and enlightened human civilization. That civilization, shortly after being exhausted by a massive failed rebellion from man-made artificial intelligence, collapsed when warp storms cut off interstellar travel and left its worlds vulnerable to attacks by daemons and aliens during a millennia long era known as the Age of Strife. When the storms abate, the Emperor embarks on a "Great Crusade" to unite all human-settled worlds in the galaxy under his banner. Over the course of two centuries, his armies conquer more than a million worlds stretching across 50,000 light years from Earth, giving humanity a dominant position among the galaxy's species. According to Imperial propaganda, it is humanity's "Manifest Destiny" to rule the galaxy.
Early in the 31st millennium, the Great Crusade comes to a sudden halt when half of the Emperor's Space Marine Legions (along with a substantial fraction of the Imperium) convert to the worship of the four Chaos Gods. The resulting civil war became known as the Horus Heresy and nearly destroyed the Imperium. Though these rebels are ultimately defeated and driven out, the Emperor is critically wounded in the final battle and survives on life support in a persistent immobile and unresponsive state on the Golden Throne. In the absence of his guidance, the Imperium becomes a very brutal totalitarian regime.
The Imperium endures for 10,000 years. It still claims lone dominion over the Milky Way Galaxy and all humanity. It does not recognize other governments, whether human or alien, and maintains that all aliens must be exterminated and that all humans must be brought into the Imperium and made to worship the Emperor. It remains united chiefly through religious fanaticism and threats of brutal disciplinary force, while teetering on the verge of collapse due to a combination of perpetual war, technological stagnation, and bureaucratic inefficiency.
The founder and nominal ruler of the Imperium is an enigmatic and mysterious persona known only as "the Emperor of Mankind". He is a Perpetual: human psyker of immeasurable power, an immortal who has secretly guided humanity since the dawn of civilization on Earth. He was born in 8000 B.C. in Anatolia, now modern day Turkey. For some unknown reasons the Emperor left the Earth and was uninvolved for 30,000 years and returned to reunite the shattered tribes of Earth during the Unification Wars.
Shortly after founding the Imperium and the recovery of the lost Primarchs, the Emperor set out to conquer the stars on a great crusade. With the help of his sons, the 20 Primarchs, the Emperor battled hostile alien races and reunited lost human planets across the galaxy.
However, during the Horus Heresy, Holy Terra was besieged by forces of Chaos and the Emperor was critically injured in a battle with Horus and never recovered. Since then he survives on a stasis-equipped life support system while in an unresponsive and immobile state bound to the Golden Throne. What little presence the Emperor does retain is devoted to projecting a psychic beacon, the Astronomican, so that human starships may navigate the Warp for their faster than light travel. His worshipers believe he is fighting a constant spiritual battle in the Warp against the Chaos Gods.
Because of his condition, the Emperor cannot participate in affairs of state. It is believed that he communicates through vague visions of the future that trained psykers may augur through special tarot cards from time to time, but otherwise he cannot communicate his will. The High Lords of Terra govern his empire according to a distorted memory of his words and deeds from ten millennia before. Bound to the Golden Throne, the body of the Emperor is being guarded by his loyal Adeptus Custodes. The Emperor wanted to build a secular and enlightened society, but the Imperium instead became mired in brutality, superstition, and ignorance. Recent gamebooks hint that his life support system is starting to fail near the end of the millennium, and thus he may soon die.
The Emperor's seat, and the heart of Imperial Administration, is the Imperial Palace, a massive construct that covers most of what used to be the continent of Asia on Terra; the "Sanctum Imperialis", commonly referred to as the "Inner Palace", is built on top of, within, and under Mount Everest.
Though the Emperor is the nominal head of state, in practice the highest tier of government is the Council of the High Lords of Terra or the Senatorum Imperialis, which has ruled for over ten millennia in the Emperor's name. Under this top echelon is a multi-tiered hierarchy consisting of countless departments, agencies, and organisations, both military and civilian, charged with implementing the decisions of the High Lords and with the day-to-day administration of the Imperium as a whole. Recent events have had one of the Primarchs, Roboute Guilliman, return to Terra and proclaimed the Imperial Regent, the voice of the Emperor. In addition, he has reclaimed his seat as Lord Commander within the High Lords and has become their de facto head.
The Imperium of Man divides the galaxy into five distinct sections called Segmentae (Segmentum in singular), assigned by their relative galactic position from Holy Terra (Earth) on a 2-dimensional plane. They are as follows: Segmentum Solar (the Solar System, Holy Terra), Segmentum Obscurus (galactic north), Segmentum Pacificus (galactic west), Segmentum Ultima (galactic east), and Segmentum Tempestus (galactic south). Each Segmentum stretches across vast regions of space, containing hundreds of thousands of inhabited planets.
Most worlds in the Imperium are ruled by a planetary governor. The cultures and governments of the Imperial worlds are very diverse. A few are even democratic and prosperous, though these are rare islands of happiness in a generally grim empire. Generally speaking, each governor is allowed to rule his world as he sees fit provided he keeps the faith and provides the requisite tithes of conscripts, exports, and captured psykers.
Outside and above the Imperial hierarchy is the Imperial Inquisition, the all-powerful and much feared secret police and main intelligence agency, which answers only to the High Lords and has the power to investigate and persecute anyone it pleases.
Five fundamental taboos, set by the Emperor at the Imperium's founding, underlie its governance by the High Lords; their flouting results in summary capital sanction, regardless of office or rank.
In late–41st millennium, the Imperium is still broadly governed by these principles. However, it is also ruled by expediency in the face of constant threats. It is not uncommon for Imperial servants to assume discretion in their enforcement when they deem it necessary, and some among their number do this as a matter of course; this is considered heretical or traitorous by more conservative Imperial officials.
The Imperium's methods for enforcing its rule are often exceedingly brutal. Source material calls it "the cruelest and most bloody regime imaginable"; in-universe, Imperial servants commonly state, "ruthlessness is the mercy of the wise." The Imperium's galactic law enforcement agency (the Adeptus Arbites), readily uses deadly force when faced with any possibility of resistance. The Inquisition makes liberal use of torture and cares more about punishing the guilty than protecting the innocent, routinely causing excessive suffering and death of the latter. On the other hand, they rationalize that it is better for a few million innocent lives and a single system to be harmed than allow Chaos to infiltrate the Imperium to cause billions of more deaths and destruction to dozens of systems. That and given the sheer population of the Imperium in excess of tens of trillions is spread over more than a million worlds, it is not uncommon for entire systems and sub-sectors to be purged to prevent any possible taint of Chaos influence from spreading if such action is deemed necessary. Witches (rogue psykers) and heretics are publicly flogged or burned at the stake. The Imperium's mainstream military forces, such as the Imperial Guard Regiments, often have political officers (Imperial Commissars) attached. They have authority to execute out-of-hand any soldier, regardless of rank, who displays cowardice or disobedience.
Occasionally, the Imperium resorts to destroying entire worlds with weapons of mass destruction in order to suppress heresy, rebellion, or alien encroachment. An Exterminatus, the obliteration of an entire world, is only resorted to when the level of corruption a world bears is so monumental that it cannot be wiped out by any other means and leaving it intact could lead to an anchor for Chaos or mankind's other enemies to attack the Imperium.
The Imperium has three official languages, High Gothic, Low Gothic, and Lingua-Technis.
High Gothic was the only official language of the Imperium in its early days, then known as Imperial Terran. The language is now considered sacred because it is the language the Emperor spoke during the Great Crusade and is now only a hieratic language spoken by members of the Imperial Inquisition, Priests of the Ecclesiarchy, and the Tech-Priests of Adeptus Mechanicus and even for them, its only a second language.
Low Gothic is the common language of the Imperium and is spoken by every Imperial citizen, though there exists millions of dialects of it. High Gothic is represented as pseudo-Latinized English while Low Gothic is represented as 21st century English. Lingua-Technis is the official language of the Adeptus Mechanicus, characterized as binary code when written and possessing unique tones similar to digitally modified sounds when spoken.
The Imperium promotes the Imperial Cult, the worship of the Emperor as humankind's only true god and rightful master. He is the savior, the only one who can protect humanity from the dangers of the cosmos. The irony of this is that the Emperor had wanted to build an enlightened and secular society, but cults devoted to him started everywhere, and after he was incapacitated and interred in the Golden Throne this worship spread unimpeded until it became the state religion. Ironically, his promotion of a secular and logical Imperial Truth was superseded by the nascant Imperial Cult. In addition, it was his suppression of the cult with Lorgar Aurelion, one of the cult's foremost figures, humiliatingly rebuffed that led to his son turning to Chaos and planting seeds to bring the Horus Heresy into existence.
Just as all human-inhabited worlds must be brought under the Imperium's control, all humans must be made to worship the Emperor and all other religions must be expunged. The state religion, which is the responsibility of the Ecclesiarchy (the State Church), preaches that unquestioning obedience and self-sacrifice are the best ways to honor the Emperor. It teaches that mutants (with few exceptions) are vile perversions of the "holy" human form, and thus they are subject to the worst oppression. Rogue psykers ("witches") are harbingers of doom. Heresy is the worst crime of all despite the deliberate vagueness of the official definition, and those judged as heretics are stripped of all rights and punished in the most horrific fashion. To be excommunicated by the church or the Inquisition is to be stripped of your very rights as a human being; you are no longer considered part of humanity.
Religion is a central theme in all Warhammer 40,000 fiction; predominantly a more violent parody of the medieval-era dogmas of Catholicism. Most human protagonists, from warrior-monk Imperial Space Marines to Witch Hunters of the Inquisition, are fanatical worshippers of the Emperor. A very common motif in Imperial iconography is a human skull, often surrounded by a halo. This is the face of the Emperor, whose broken, half-dead body resembles a corpse.
While the ordinary citizen of the Imperium believes that the Emperor has always been venerated as the immortal and omnipotent God of humanity throughout the history of Mankind, Imperial historians and the Battle-Brothers of the Space Marine Chapters know that this was not always the case: At the beginning of the Emperor's Great Crusade in the 31st Millennium, there was no Ecclesiarchy and the veneration of the Emperor, in the form of the Imperial Cult's precursor, known as the Lectitio Divinitatus, was frowned upon and outright condemned by the Emperor himself. The Emperor secretly believed that religion was what fueled the Chaos Gods existence, and sought to starve them into oblivion. Ironically, it would be the Emperor's severe rebuff of the cult's founder, the Primarch Lorgar Aurelion, that would lead to the latter's secret defection to Chaos and sowing the seeds leading to the Horus Heresy.
The official Imperial doctrine was that the Emperor was an extremely powerful being, the rightful ruler of all Mankind, and the perfect physical, mental and spiritual embodiment of humanity, but no matter how supreme, he was still only a human being. This changed after the Emperor ascended on his Golden Throne and was visible in the Warp for all Mankind.
In the 41st millennium, the tenets of the Imperial Cult, known as the Imperial Creed, are actually highly flexible and are tailored by the Adeptus Ministorum's Missionaries to fit the native culture, existing religion, and cultural practices of whatever world it exists upon. Polytheism is even permitted, provided that the locals in question regard the Emperor as their supreme deity.
The Cult of the God-Emperor is the dominant faith, but the tech-priests of the Adeptus Mechanicus follow a parallel religion around their "Machine God", of whom they see the Emperor as an avatar, the "Omnissiah". While this does not fully please the Ecclesiarchy, this compromise is grudgingly accepted by both sides because Adeptus Mechanicus has a monopoly on all technological expertise in the Imperium but must also rely on regular importation of resources to function.
Imperial citizens are almost exclusively human; a relatively small fraction of the population consists of members of stable mutant species of human descent, called abhuman in Imperial classification and are recognized as citizens (if second-class). Common citizens of the Imperium have very little say in how, and by whom, they are governed. They have no influence in the selection process of the High Lords or any other Imperial official, and are not allowed to question Imperial decisions or participate in the decision-making. Even worlds with democratically elected Planetary Governors are under the absolute veto of Imperial Authority.
Imperial society galaxy-wide is characterized by religious intolerance, superstition, xenophobia, militarism, and antiscience. Critical thinking is considered a waste of time, and may even be dangerous. Ignorance of anything beyond one's place in society is common and, for many, desirable. Some even know next to nothing about worlds or societies beyond their homeworld unless recruited by members of the Imperium's various military branches and bureaus. Though no discrimination on basis of color or gender exists in the Imperium, any divergence in political or religious beliefs, even slight, is often deemed nigh-heretical, and disagreement with such beliefs is a capital offense. Unquestioning obedience to authority is the greatest virtue.
There are at least five varieties of human worlds: Agri-worlds are highly rich in fertile land with farming and animal husbandry being the main ways of life. These worlds are usually pre-industrial in nature and are havens of idyllic lifestyles for retiring members of the military; they supply food and basic fabrics to whole sectors. Death worlds feature wild, often extremely hazardous, landscapes with numerous dangerous flora and fauna species. The few human inhabitants of these worlds are usually rugged survivalists with unflinching natures, ideal recruitment centers for the military. Shrine worlds are the final resting places of extensively decorated or martyred holy individuals, also serving as vast memorial sites and grand monuments for epic events in the imperial history. Though the local populations are minimal, millions of people engage in pilgrimages to shrine worlds from every corner of the Imperium each year. Hive worlds, also known as Imperial Worlds, are extensively developed planets with towering city structures that stretch for many hundreds of kilometers in every direction; tens of billions of citizens live in basic hab-blocks with often substandard living conditions while the local aristocracy and ranking officials wallow in extreme luxury and decadence. These worlds form the fortified capital planets of the Imperium where millions of citizens are recruited as hard laborers or soldiers in the Imperial Guard and other military organizations. Finally, Forge worlds are the exclusive planets of the Adeptus Mechanicus; highly industrialized worlds with colossal factory structures blanketing over most of their landmasses. Thick toxic smog and corrosive chemical fumes permeate the atmosphere where only cybernetically altered individuals can survive without protective clothing. These Mechanicus worlds supply the Imperium with vast building materials and of course its innumerable war machines and weapons.
A sixth type is unofficially classified as Feral worlds, for worlds that have not even reached Iron Age development. Whether by simple clerical negligence or by intentional imperial direction, the inhabitants of these planets are usually hunter-gatherer societies with primitive cultures and beliefs (though missionaries from the Imperial Church will often erect rudimentary places of worship to promote beliefs centered around the Emperor). In these cases, the planetary governor often only resides in orbit and only interacts with locals to obtain tithes in resources or manpower for the Imperium's armies. In many cases, these worlds are protected by a Space Marine chapter, for they are ideal places of recruitment. Their reasons behind this are that their ways of life produce hardy peoples with excellent physical prowess; whose bodies are more able to survive the unforgiving biological augmentations of a space marine and whose minds are more easily indoctrinated into a warrior-monk existence. Some Chapters even use these worlds, alongside Hive Worlds (some known for their violent, gang-infested undercities) and Death Worlds as recruitment worlds or even Chapter home worlds.
Virtually all types of human worlds are at or somehow participating in the Imperium's wars. Even those worlds that are not battle-zones feel the weight of war on them through heavy taxation and conscription.
Mutants are humans that display physical deviations from the norm, the result of exposure to mutagenic chemicals, alien experimentation or Chaos corruption. In general, mutants are hated and feared by the common citizens, seen as a sign of mankind's slow degeneration away from the holy human form. On worlds where they are not summarily exterminated, they are used as slaves for dangerous and menial tasks or banished to fringes of society.
Psykers, psychic mutants who draw their powers from the dimensional realm known as the Immaterium or the Warp, are too dangerous to remain at liberty on the Imperium's myriad worlds; their minds are susceptible to daemonic possession as their strong presences in the Warp can serve as gateways into the material universe for all kinds of Warp entities if they are weak or undisciplined with their powers. Uncontrolled, they can intentionally or unintentionally cause the destruction of entire worlds and have done so in the pre-Imperium past, such as during the terrible days of the Age of Strife. Humans found to be bearing the psychic mutation, if they are not simply killed, are sent to Terra aboard the infamous Black Ships which endlessly traverse the Imperium. There, they are usually sacrificed to sustain the Emperor's Golden Throne, but those who are strong enough and can control their powers are trained to aid various branches of the Imperium's military-albeit under constant guard.
Although humans have access to sophisticated technology, they do not understand the scientific principles behind it, for scientific knowledge among the majority of Mankind was replaced by superstition, and those who do possess some scientific understanding the technology's inner workings are likely to have fragmented knowledge almost universally diluted with mysticism. Advanced machinery is built and maintained by the tech-priests of the Adeptus Mechanicus, who believe that all machines possess an animistic spirit. They believe that, long ago, a being they call "the Machine God" gifted humanity with all the technical knowledge there is to know, including the designs of every device that has been created, exists right now or would be created in the future. According to this dogma, tech-priests often see it that all technology was already invented and have it have to be just re-assembled at Adeptus Mechanicus forge-worlds. This dogma also leads to general reluctance among the tech-priests to invent or improve technology via original research and experimentation. However, when new inventions are made by the members of the Adeptus Mechanicus (this happens, albeit at very slow rate) they go through lengthy and laborious (sometimes taking hundreds of years) process of testing and verifying the creation's compliance with Cult Mechanicus' dogma and strictures (for example, some areas of research are considered forbidden and heretical, like the development of artificial intelligence). Majority of the tech-priests also see alien technology as corrupt and refuse to copy it, even when such technology surpasses the Imperium's (as is sometimes the case). Such conditions ensure that most common way for Imperial technology to actually progress is when somebody discovers a lost blueprint that dates back to pre-Imperium civilization, which the tech-priests trust are the pure, unadulterated revelation of the Machine God. When building or repairing machinery, tech-priests are somewhat inflexibly follow the "rites" described in their manuals, and these rites are as likely to include prayers and libations as much as welders and screwdrivers.
Aside from their reluctance to innovate the tech-priests are also very inefficient at distributing their technology, as they will not allow anyone outside their faith to share and profane their secrets. Some worlds in the less developed portions of the Imperium even live at pre-industrial levels of development.
The Warp is a chaotic and maddening place where conventional navigation instruments are useless, so humans make use of a special breed of psyker called "Navigators". With the third eye in their foreheads, they can gaze upon the Warp and perceive the Astronomican. The Astronomican is a psychic beacon that is projected from Earth by the God-Emperor's spirit. Like a lighthouse, it provides a fixed reference point for ships to navigate by. Beyond the range of the Astronomican, Warp travel is slower and more dangerous, and this sets the Imperium's effective borders. What the Imperium unfortunately does not know is that it is the Astronomican that's bringing the extra-galactic Tyranids into The Milky Way.
The Imperial Space Fleet is divided into three distinct branches - the Battlefleets of the Imperial Navy, the Merchant Fleets and the Civil fleets, and whilst the inquisition are able to requisition the ships of other Imperial organisations, they also maintain vessels of their own. The Space Marines Chapters possess their own fleets of warships that stand apart from the Imperial Navy, purpose-built for both the transport of their warriors and providing aid in their style of warfare.
Merchant fleets usually refers to Chartist captains, who are given a charter allowing them to ply a given warp route or sector, usually in slow going ships that usually lack a navigator. Instead, these trading vessels make use of short "hops" through the warp based upon cognitor algorithms and some degree of risky guess work in every given hop. These trips between systems can take anywhere from months to years, even centuries. As such, these chartist vessels tend to be gargantuan in scale, compensating with sheer capacity and often decent armaments for their lack of speed and attendant escort vessels by being too large and requiring the application of overwhelming amounts of firepower to raid effectively.
Rogue Traders may also be considered part of the merchant fleets, though they are often more akin to privateers. Their charters are usually more expensive to acquire as they afford them greater range and freedoms in conducting their trades. Although classified as civilians, more wealthy rogue trader captains will sometimes own small private military forces and possess one or more merchant vessels that mount considerable armor and weaponry. The nature of rogue trader charters are often mistakenly interpreted by these captains as licenses to explore and trade in any part of space they deem fit and are not beyond negotiating with alien races in areas outside of Imperial territory. The Imperium will sometimes overlook these transgressions as long as the captains adhere to the tenets of the Imperial Creed, they must also report all their acquisitions to the Administratum for tax purposes and pay heavy tithes or risk losing their charter as well as inviting imperial censure, which is often lethal.
For planetary communications, the Imperium makes use of a device known as a Vox, a form of advanced radio communication. For interstellar communications, the Imperium uses Astropaths, who are powerful telepaths extensively trained to send and receive messages across interplanetary distances. This latter method of interstellar communication is not especially reliable or efficient, but it is faster than sending a messenger ship. To send messages across especially great distances (i.e. from one Segmentum to another), astropaths relay messages, forming astropathic choirs to boost their range. As part of their training, every astropath is brought before the Emperor for him to soul-bind, by which he boosts the astropath's power and guards his mind against daemonic influence. However, they are usually rendered blind in the process due to the sheer magnitude of the Emperor's power.
Both these factors make the Emperor's spirit the linchpin of the Imperium's infrastructure. Should the Emperor finally die, the Imperium could immediately collapse. The Emperor's tenuous grip on life is sustained by the daily sacrifice of a thousand psykers so that his psychic light may continue to shine.
The Imperium commands the largest military in the galaxy, honed in millennia of almost unending war (the prevalent theme of the Warhammer 40,000 universe): at any time, there are numerous conflicts engaging the Imperium, across the Imperial space and beyond. Because of the scale of the distances involved, and the number and severity of threats, Imperial military commanders have great autonomy into how and when they prosecute campaigns within their areas of responsibility.
The core military force of the Imperium is the Imperial Guard, also known as the Astra Militarum. It is made up of professional soldiers, who may be volunteers from all walks of life or conscripts drawn from each imperial world's planetary defense force. The basic Guard unit is a self-contained formation called a Regiment, a fighting force that range from being tens of thousands strong to just a few thousand; there are millions of active Imperial Guard Regiments. Due to their diverse backgrounds, regimental specializations and types cover every conceivable kind of warfare in the galaxy, regardless of theater or fighting conditions.
A special subsection of the Imperial Guard are the elite storm troopers or Tempestus Scions. Though not as physically powerful as Space Marines, they are more extensively trained and better armed than their guardsmen counterparts. The Tempestus Scions function in much the same capacity as modern day special forces, deploying in smaller numbers and are invariably tasked to accomplish important strategic goals during a military campaign, conducting their operations with surgical precision often far from friendly lines. Due to their elite training and specialist skills, some regiments are permanently seconded by the Imperial Inquisition as Inquisitorial Storm Troopers.
The Imperial Navy, the military arm of the Imperial Fleet, commands millions of spacecraft, aircraft, deep-space weapons platforms, and military space stations. It is organised into five major Battlefleets: Battlefleet Solar, Battlefleet Obscurus, Battlefleet Tempestus, Battlefleet Ultima, and Battlefleet Pacificus; each of which is further divided into a multitude of Battlegroups and specialized armadas. The Navy also operates a vast and constantly busy transport network of gigantic spaceships used to ferry troops, supplies, and equipment to and from the Imperiums's numerous warzones across the galactic theater.
The Space Marines, also known as the Adeptus Astartes, are genetically modified transhuman supersoldiers and serve as the Imperium's elite shock troops. Standing approximately seven feet in height (eight feet when wearing their iconic Power Armour) and possessing superior musculature, their augmentations are derived from the genome of one of the twenty Primarchs, the genetically engineered sons of the Emperor who were made to be his generals, and are organised into nearly 1,000 Chapters that act as autonomous units. As dictated by their own battle tenets, Chapters are usually about 1,000 Marines-strong, though a handful of Chapters deliberately maintain slightly larger numbers. All Chapters have considerable resources at their disposal, including their own space and vehicle fleets, auxiliary personnel, and other support elements. Most Chapters operate in specific areas, near the Imperium's most troublesome spots, or at its borders. Originally the Space Marines were organized in a score of Legions that served as the Imperium's primary fighting force (while the Imperial Guard's predecessor served as a reserve, auxiliary, and garrison force). Composed of tens to hundreds of thousands of Astartes, each was led by a Primarch, and were virtually unstoppable. However, after half of them turned to Chaos and nearly destroyed the Imperium in the resultant civil war (the Horus Heresy), it was decided that the remaining Legions would be divided to ensure that no single leader can command such power and be such a potential threat to the Imperium.
All Space Marine Chapters are expected to pay a tithe of 5% of their gene-seed every year to the Adeptus Mechanicus so that the Mechanicus' Tech-priests (and in many cases, the Inquisition) can monitor the health of each Chapter and facilitate the Founding of new Chapters from existing gene-seed lines whenever the High Lords of Terra decree that the Imperium has a need for new Chapters of the Adeptus Astartes.
The Adeptus Mechanicus, apart from being responsible for the production and maintenance of most Imperial weapons and military equipment, does field specialised military forces of its own. These include formations of colossal, heavily weaponised, bipedal war machines called Titans, smaller but more common walkers known as Imperial Knights sourced from feudal worlds, elite cybernetic breeds of warriors called Skitarii, and more advanced iterations of the spacecraft they provide the Imperial Navy. Mechanicus forces may often be under the strategic command of Imperial officers, or be tactically seconded to Imperial Guard Regiments and Space Marine Chapters. In rare cases, such as in Mechanicus Explorator Fleets, their expeditionary forces would be under the direction of an Arch Magos, a high-ranking member of the Tech-Priesthood.
Each planet may also be defended by local militias or a more structured Planetary Defence Force. In most cases, the latter fills its ranks with short-term and part-time soldiers, who do not have the professional training and equipment of the Imperial Guard. Planetary defense forces of the more important systems may be complemented by a "System Fleet" of sub-light speed spacecraft.
The Ecclesiarchy, though banned from keeping men under arms, also maintains its own military force. As a loophole to the ban, these are made up of Adepta Sororitas, more commonly known as the Sisters of Battle. They are a group of all-female, militant religious orders headed by an Abbess at the highest echelons and commanded on the field by Canonesses. They utilize weapons and power armor similar to that of space marines, though they are not genetically modified as their strict tenets forbid biological enhancements.
Outside of the normal military command structure are other paramilitary organisations, several of which are classified or operate covertly; they usually deploy in much smaller units, and undertake highly specialized missions. The Inquisition also possess their own private military organizations such as the Inquisitorial Storm Troopers, as well as more specialized units called Chambers Militant which can often take the form of seconded space marines and sisters of battle, receiving extensive re-training and advanced specialized equipment. These are tasked with carrying out clandestine missions that are far too dangerous (physically and often spiritually) even for the vast numbers of the imperial guard or the brutal firepower of the space marine chapters. Such is the unconditional authority of the Inquisition that they may even take direct command of an entire planet's defense forces as well as any Astra Militarum regiments that happen to be on it in order to root out a potential catastrophic threat to the Imperium. Meanwhile, the Officio Assassinorum dispatches elite assassins against enemy targets such as Chaos leaders and renegades while answering directly to the High Lords of Terra. Meanwhile, the Sisters of Silence, a militant female order of anti-psykers, acted as a form of secret police for the Emperor in regards to psychic activities and secretly remain active in the present era. Additionally, the Inquisition will sometimes utilize rogue traders; their freer charters allowing them to traverse in and out of imperial space and engage in clandestine operations more easily. Rogue Traders by themselves will also, in rare instances, be called to war by the Highlords of Terra to act in defense of Imperial interests.
Last but not the least, the Adeptus Custodes, also known as the Custodian Guard, are an elite group of superhuman warriors who once marched as the bodyguards of the Emperor when he led the Great Crusade. They now serve as guardians of the Imperial Palace as a whole. With superior biological enhancements and martial training even to that of Space Marines, these golden armored warriors are second only to the Primarchs in single combat. In addition, their allegiance to the Emperor is unshakable with their loyalty gene-coded into their own bodies since their selection as infants. Unfortunately, the process required to create them is even more demanding and difficult than the Astartes, meaning their numbers have never exceeded their peak of ten thousand during the Great Crusade. Three hundred of their number, sworn to silence, are directly assigned to protect the Golden Throne while the rest guard the palace grounds.
In total, the Imperium of Man can call upon incalculable military resources to battle against any individual foe. However, such is the nature of conflict in the 41st Millennium that humanity faces opposition from all sides, even from within, and the forces of the Imperium are nearly always stretched thin enough that there are times when whole solar systems must be sacrificed just to slow enemy movement.
The Imperium of Man as portrayed in the Games Workshop universe resembles various totalitarian institutions from throughout fiction, history, and literature. Some of the inspirations including commonplace works such as Brave New World and 1984. Dune, a novel by Frank Herbert, had some influence on the overall design of the Imperium, and the "Galactic Empire" from the book series Foundation, by Isaac Asimov, also bears strong resemblance to the Imperium, portraying a loosely governed empire where extreme persecution is carried out on those who do not align to the governmental beliefs.
Since it originally was created as a sci-fi spin-off of the Warhammer Fantasy battle game, the 40K game world contains many elements of the fantasy genre, for example, the concept of magic and adapted versions of classic fantasy races.
The inspirational sources for the 40K universe include classic and contemporary sci-fi, horror, and fantasy movies and television series and the works of renowned genre authors such as Isaac Asimov, Frank Herbert, H. P. Lovecraft, Michael Moorcock, J. R. R. Tolkien, and Robert Heinlein (Heinlein's novel Starship Troopers inspired many elements such as elite marines in powered armor, and drop pods in which encased Space Marines and equipment are fired from orbiting ships down to the battlefield); medieval, baroque, and surrealist art (especially H. R. Giger), popular depictions of historical settings, such as the World Wars, Victorian Britain, Imperial Rome, The Inquisitions, Nazi Germany, and the Soviet Union:
The Imperium bears several strong resemblances to the totalitarian Stalinist regime of the Soviet Union, which was exceptionally brutal and intolerant of differing ideologies, with its citizens frequently imprisoned, tortured, or even executed if believed to be dissenters. The Commissariat of the Imperium is similar to the ranking structure and purposes of the Red Army Commissariat during World War II. Commissars of the NKVD would frequently execute Soviet soldiers to punish cowardice or to simply coerce soldiers to fight as Imperial Commissars do within the Warhammer 40,000 universe. Early-War Red Army soldiers were utilized with high expendability via human wave attacks supported by overwhelming artillery bombardment, identical to several Imperial combat doctrines and their reliance on heavy artillery.
The cult of personality and complete state control of media that is shown within the Imperium also bears similarities to those in nations such as contemporary North Korea, where much of society is taught to believe their leader is an eternal, omnipotent god, while in the Warhammer 40K Universe, the Emperor is indeed a supreme being, his mind continuing to be a beacon for humanity, his soul a god in the Warp waiting to be born.
The primary inspiration, however for the Imperium is most likely the Byzantine Empire, as the inspriation for the Double-headed eagle comes from the Byzantine Aquila of the Palaiologoi Dynasty. Other influences include the classical Roman Empire, with the Imperium's various "lesser gods" viewed as subservient to the Emperor of Mankind, much like the Romans' pantheistic religions believed their emperor to be. The language of High Gothic appears to be badly mangled Latin, and many organizational, military, and even personal names bear strong Latin influence. Much Imperial architecture is designed after the Romanesque Gothic architecture of Medieval Europe, and Space Marine chapters are most likely based on historical military orders such as the Teutonic Order or the Knight's Templar.