Horus Heresy - Unification Wars: During the turbulent era known as the Age of Strife or "Old Night," the Sol System and the nearby star systems that had been colonised by Humanity during the Age of Technology were effectively cut off from interstellar travel or communication with each other. This was due to the massive Warp Storms that swept the galaxy as the Immaterium was roiled by the millennia-long gestation of the Chaos God Slaanesh and the turbulence that marked the decay of the Aeldari Empire before the Fall.During this dark time, Terra sometimes held sway over the Sol System, while at other times the rulers of Mars or Luna were dominant. The different worlds found themselves constantly at war. During this 5,000 standard-year-long period of anarchy, fear and violence, Old Earth's once unified planetary government had completely broken down and been divided into dozens of warring states of so-called "techno-barbarians."Continuous warfare raged across the surface of Terra for 2,500 standard years, beginning in the late 27th Millennium. Little remained of the once sophisticated civilisation of Old Earth's glorious past as the centre of a growing human interstellar commonwealth marked by advanced science, high culture and wondrous technologies. Techno-barbarian warlords and their warrior hordes continuously fought over the planet, which had become little more than a massive battleground for their wars of attrition. They made use of chemical, biological and even thermonuclear weapons of mass destruction, and slowly transformed the cradle of Mankind into a battered, post-apocalyptic wasteland across most of its scarred surface.This was a dark period for the people of Old Earth, when they were dominated by brutal rulers and despotic tyrants. It was against this backdrop of oppression, violence and casual brutality that the Emperor of Mankind first revealed Himself openly to the people of Terra. In secret, He had been planning for this moment in history for millennia, ever since the Age of Strife had fractured what remained of the ancient Human interstellar civilisation.With His massive army of genetically-enhanced warriors who comprised the first units of the Imperial Army and would serve as the prototypes for the later development of the Space Marines, the Emperor began His conquest of Terra. His intent was to reunite the warring nations of the world into a unified planetary government and then use Terra as the springboard from which to begin His reconquest of the galaxy under the aegis of an Imperium of Man dedicated to the atheistic Imperial Truth of progress and reason.Brutal rulers such as the warlord Kalagann of Ursh, Cardinal Tang of the Yndonesic Bloc, and the most infamous of all, the half-mad, half-genius Nathaniel Dume, the tyrant of the Pan-Pacific Empire, would all fall by the wayside. Once unleashed, the genetically-enhanced armies of the Emperor swept all before them like wheat before the harvester's scythe until all the techno-barbarian warlords had either been conquered outright or had agreed through diplomacy to become subservient to the Emperor's will.During this time the Emperor created a number of military organisations, such as the Imperial Army, which would become the nucleus of the armed forces that would later support His Space Marine Legions and His reconquest of the Human-settled galaxy. Amongst the early Imperial Army units that saw action during the Unification Wars were such venerable regiments as the Geno Five-Two Chiliad, which would become one of the oldest and most respected regiments within the Imperial Army. Like many of these early regiments, the geno-soldiers were created through the use of some of the Emperor's sophisticated genetic engineering techniques, developed in His laboratories beneath the Himalazian (Himalayan) Mountains and by the advanced geneticists who called Luna home.The most elite of this first generation of genetically-engineered supersoldiers were known as the Thunder Warriors, men who were physically stronger and more formidable in combat than even the later Space Marines, though they were engineered to be vicious killers and lacked many of the more noble aspects of the Astartes as well as their tremendously long lifespans. The Thunder Warriors were engineered to be the means to an end and were never intended to be integrated into the Emperor's new realm after Unity had been achieved.Through genetic engineering and selection the standard geno-soldiers displayed many of the characteristics of the perfect human warrior -- they were physically more resilient, stronger and capable of taking more damage than any of their unaltered techno-barbarian foes, including the ability to face down rogue psykers and sorcerers. These early genetically-engineered Imperial Army regiments would go on to continue serving the Emperor after the conclusion of the Unification Wars.These regiments would eventually be referred to as the Old Hundred, and would form the core of the initial military force that embarked upon the Emperor's galaxy-spanning Great Crusade in the late 30th Millennium beside his newly created Space Marine Legions, the early versions of which had taken part in the later campaigns of the Wars of Unification.When the Unification Wars were complete in the late 30th Millennium, the Emperor forged a new unified planetary government for Terra under His leadership. He next journeyed to Mars and met with the tech-adepts of the Cult Mechanicus.In return for the use of the Mechanicum's vast manufactoria, the use of its Titan Legions and the orbital shipyards to construct the weaponry and starships he would need for His Great Crusade to reunite Mankind under the banner of the Imperial Truth, the Emperor agreed to grant the Mechanicum complete autonomy on Mars and its other Forge Worlds as well as an exemption to the atheism required by the Imperial Truth.This agreement, known as the Treaty of Mars (the Treaty of Olympus to the Mechanicum), marks the true foundation of the Imperium of Man in the alliance between Terra and Mars.