According to the "official" histories of the Imperium, the precise details and dates of the Age of Strife are not known. This is due to the data having been lost over vast amounts of time, the sheer chaos of the period, and censorship by various Imperial authorities who seek to prevent the people of the Imperium from learning more about the rampant uncontrolled psykers and daemonic Chaos incursions that marked many of the human worlds of the period. It was in reaction to the terrible tragedies and loss of life that befell Mankind during the Age of Strife that the Emperor of Mankind first decided to unite all of humanity beneath His enlightened rule, building the foundation of what eventually became the Age of the Imperium.
The Dark Age of Technology is the name given to the period of human history that immediately precedes the Age of Strife. It began in the 15th Millennium AD and ended in the 25th Millennium, and was marked by rapid fundamental advances in science and technology that first transformed humanity into a true interstellar civilisation. The Dark Age of Technology saw the development of the specialised psychic mutants known as Navigators and the earliest use of the Warp for interstellar space travel and Astropathic communication by human beings. The greatest extent of human interstellar space exploration and colonisation across the galaxy took place during this period after the development of the Warp-Drive that allowed faster-than-light space travel. Other notable developments include the first appearance of the Standard Template Construct (STC) system and the artificially-intelligent Men of Iron, powerful, fully self-aware robots and artificial general intelligences. The Age of Technology ended in inter-human war and anarchy, at the start of an era known as the Age of Strife, when massive Warp Storms cut off interstellar travel and communications across the whole of the human-settled galaxy. The STC systems lapsed into disuse or the knowledge they contained was fragmented or lost outright. The Adeptus Mechanicus of the 41st Millennium have made finding a complete, untainted STC system their ultimate goal and are willing to go to any lengths to find even the smallest fragment of lost technology contained within an STC database.
During the so-called "Dark Age of Technology" humanity had reached its technological peak. The Standard Template Construct database library, or STC, had been perfected by human scientists and engineers to allow rapid colonisation of new worlds using the standardised designs for a common suite of advanced technologies held within it. Its invention was followed by an unprecedented expansion of humanity throughout the galaxy. One of the reasons humanity was so successful at conquering a large part of the galaxy during the Dark Age of Technology was the development of the artificially intelligent humanoid constructs known in ancient texts as the Men of Iron. These powerful and fully autonomous labour and combat robots won many wars for humanity, but for some reason turned against their organic masters at the end of the Dark Age of Technology. The war against the thinking machines was eventually won by humanity, but at great cost. The damage to human interstellar civilisation was catastrophic and shattered much of humanity's power across the galaxy. Unfortunately, this was only the beginning of mankind's misfortunes. As humanity became widely dispersed across the galaxy during the Age of Technology, the ancient Aeldari Empire began its decline; the great success of the advanced Aeldari race had led to decadence and hedonism on a grand scale. Within the immaterial, psychic universe of the Warp, the spiritual corruption of the Aeldari civilisation was reflected in the forming of a new Chaos God, Slaanesh, which in turn caused massive disturbances in the Warp; parts of the galaxy became isolated by these Warp Storms, making Warp travel and communication impossible. This cut off many human colony worlds from one another, including those of the Sol System. Towards the end of the Age of Technology psykers first appeared among humanity. While persecuted on many backwards, regressive human worlds as witches, in enlightened and progressive societies these psykers were at first protected and accepted. The intolerance shown towards psykers would seem prescient, as many human worlds ultimately fell during Old Night to the dominance of daemons and other Warp creatures using possessed psykers as gateways into the Materium. Only those worlds which had rigorously suppressed psykers ultimately survived relatively unscathed. The Age of Strife followed the Age of Technology, as human civilisation collapsed in widespread insanity, daemonic possession, anarchy and interstellar civil war. Terrible weapons of the golden age of technology were unleashed, devastating many human colonies and turning once-verdant worlds such as Baal and its twin moons into irradiated desert planets. Many isolated and vulnerable human-colonised worlds also became prey to hostile alien races, such as the Orks. In a relatively short span of time, the once galaxy-spanning human civilisation was brought to its knees, and was forced to endure nearly five Terran millennia of anarchy, terror, war, genocide and slavery. Other than tales of great suffering, little information has survived this dark time to be known to the men and women of the Imperium.
Earth, increasingly called Terra by its people, and the rest of the settled worlds of the Sol System were surrounded by terrible Warp Storms, isolating the human homeworld for several thousand standard years from the rest of the galaxy. Control of the Sol System shifted constantly between the worlds of Terra and Mars during the first half of the Age of Strife. By the 28th Millennium almost all traces of advanced technical civilisation on Terra were long gone; instead, techno-barbarians battled one another over the scraps of the ancient human interstellar society that remained on the world. Little information still exists about this dark time, but it is known that tyrants such as Kalagann of Ursh (northern Asia/Russia), Cardinal Tang, and "the half-mad, half-genius" Narthan Dume of the Panpacific Empire ruled during this era. Other known Terran nations of the time period include the Yndonesic Bloc, the Urals (a centre of industry), Jermani, Gyptus, Merica, Afrique, the Terrawatt Clan and Alba. Another ethnicity mentioned in fragmented records from this time are the "Nordyc" people. Mars underwent a very different transformation. After a brief period of anarchy at the end of the Age of Technology that saw the destruction of the Red Planet's once pristine terraformed ecosystem, the Tech-priests of the ancient Mechanicum emerged victorious over the psyker mutants and rampaging artificial intelligences that had claimed the surface of Mars. They slowly unified their homeworld beneath their theocratic rule. The Tech-priests then visited Terra but were appalled by the destruction and anarchy that ruled there and saw nothing worth saving. Instead, the Martians studied the Warp and after many lifetimes learned to detect "lulls" in the raging Warp Storms. At the same time the immense robotic fighting machines known as Titans were created to defend the Red Planet. For over a thousand standard years the Cult Mechanicus watched and waited. Whenever a break in the Warp Storms occurred, an expedition was sent, complete with a full Titan Legion and thousands of Servitors and Tech-priests. Some of these expeditions were lost in the Warp or died on faraway worlds. Others succeeded in establishing Forge Worlds -- industrial replicas of Mars. Broken Astropathic messages were transmitted to Mars, but it was not until the time of the Great Crusade that the Forge Worlds and Mars would be reunited.
Eventually, an immensely powerful human leader and psyker known only as the Emperor of Mankind divined that the final birth of the new Chaos God Slaanesh was nearing, as well as the effect this birth would have on the turmoil afflicting the galaxy. He began to accelerate His preparations for this history-changing event. From the beginning of the Age of Strife the Emperor had no longer been content with guiding humanity from the shadows under a myriad of different historical identities, some of them quite well-known even in the 41st Millennium. The Emperor had come to believe that if His species was to survive, He would have to take charge Himself of mankind's destiny, openly and without subterfuge. Within His underground gene-labs beneath the Himalazian Mountains (Himalayas), the Emperor began to experiment on the human genetic code. Through deeds of great valour and His own personal charisma, the Emperor recruited from the techno-barbarians that wandered Terra's surface and genetically altered them, turning them into the Thunder Warriors and the other transhuman supersoldiers who would comprise his first armies. Once He had gathered sufficient military forces, he conquered Terra and later its satellite of Luna. Even before the completion of these Unification Wars, the Emperor initiated the great project of genetic engineering necessary to create the Primarchs using His own DNA as a template. From the Primarchs' genomes and the genetic blueprints of his prior super-soldiers, the Space Marine Legions were His final creations. Upon the final birth of Slaanesh sometime in the 29th Millennium, the crippling Warp Storms ended, which allowed for easier galactic travel and astropathic communication between worlds. The Aeldari species was nearly annihilated in what they termed "the Fall" and the Aeldari Empire's hold over much of the galaxy disintegrated. In this environment, the Emperor began to carry out His plans to reunify Mankind under His rule. With the Emperor's ascension and the creation of the Imperium of Man in a political and military alliance between Terra and the Mechanicum of Mars, the Age of Strife was finally ended and a new age, the Age of the Imperium, begun. This age of conquest, heroism and unity began with the Emperor's Great Crusade to reunite all the scattered worlds of humanity beneath the rule of the Imperium. It would in turn be forever tainted and corrupted soon after its inception by the terrible tragedy and original sin of the massive interstellar civil war now known only as the Horus Heresy.
The Age of Strife had a tremendous impact on human civilisation. Humanity barely survived the period and the knowledge of the previous golden era was lost, much of it irrevocably; in particular, the loss of the STC system is seen as a serious setback for the technological and scientific advancement of the Imperium. The Adeptus Mechanicus regards the STC as a holy artefact of the Machine God and constantly searches for remnants of STC systems. Although much lost information has been recovered by the efforts of the Mechanicus, by focussing on recovering knowledge from the ancient STC and spending little energy on basic research and development, the Imperium advances at only a snail's pace technologically. This is further compounded by the centralisation of knowledge in the hands of the Adeptus Mechanicus, preventing the dissemination of known science and technology across the galaxy that might speed up the process of innovation. Humanity as a whole became more superstitious and distrustful because of the horrors perpetrated during the Age of Strife, something which the Imperial government goes to great lengths to encourage. In particular, the Imperium's people distrust mutants, aliens and psykers; this distrust runs so deep that the Imperium has a powerful secret police force, known collectively as the Inquisition, devoted to finding and destroying all aliens, daemons, mutants, unsanctioned psykers and "Heretics" (anyone deemed to be straying from official Imperial doctrine). Despite the general distrust of psykers, they still perform many critical functions for the Imperium, most notably the maintenance and powering of the Astronomican, so they are often rounded up by Imperial agents and forced into service. The Adeptus Ministorum, the official state church of the Imperium, enforces a strict dogma including absolute devotion to the God-Emperor of Mankind, who is treated as the embodiment of Jehovah, Jesus Christ, Allah, the Buddha, Brahma and every other divinity ever worshipped by humanity, all rolled up into a single personage. In short, the authoritarian, often harsh Imperium of Man possesses its unique cultural characteristics in large part as a reaction to the great suffering and near-apocalypse endured by humanity during the Age of Strife.
The First Heretic (Novel) by Aaron Dembski-Bowden, Ch. 17
Galaxy in Flames (Novel) by Ben Counter, pg. 410
Warhammer 40,000 Rulebook (6th Edition), pg. 167
Warhammer 40,000 Rulebook (5th Edition), pg. 122
The Outcast Dead (Novel) by Graham McNeill, pg. 332
Mechanicum (Novel) by Graham McNeill