Astra Militarium

"Men, we are the first, last and often only line of defence the Imperium has against what is out there. You and that fine piece of Imperial weaponry you hold in your hands is all that is keeping humanity alive. Most of you will probably not live to see your second year in the Guard and most of you will probably never see your homeworlds again, but I can guarantee you that when you do fall, with a prayer to the most high and mighty God-Emperor on your lips, you will have earned the right to call yourself a man!"
— Staff Sergeant Vermak, 12th Cadian Shock Regiment

The Astra Militarum, also known as the Imperial Guard in colloquial Low Gothic, is the largest coherent fighting force in the galaxy and serves as the Imperium of Man's primary military force and first line of defence from the myriad threats which endanger the existence of the Human race in the 41st Millennium. It is comprised of countless billions of men and women -- hundreds of thousands of different regiments, supported by a vast array of light and heavy armoured vehicles that provide the Imperial Guard's primary offensive punch. The Astra Militarum is usually the first Imperial force to respond to a threat if a world's Planetary Defence Force (PDF) fails to suppress it. They also garrison major locations of strategic or cultural interest to the Imperium and are often found in defensive roles. Supported by legions of heavy armour and thundering artillery, the Imperial Guard fight a never-ending war for the survival of Mankind in an unrelentingly hostile universe.
The primary combat tactic of the Astra Militarum is to overwhelm the enemy with their endless numbers, while at the same time hammer them into submission with devastating artillery and crush them with powerful main battle tanks. As a result, the Astra Militarum is often referred to as the "Hammer of the Emperor" -- the sheer amount of force that the Imperial Guard can bring to bear on the enemy is devastating, but is not as direct or as precise as their Space Marine allies in the Adeptus Astartes, who are described as the "Scalpel of the Emperor" and specialise in planetary assaults, special operations and decapitation strikes.
There is no universal uniform or regimental command hierarchy in the Astra Militarum, although it is compulsory for every regiment to have at least one commissar to maintain the discipline and morale of the troops while watching for any signs of corruption or heretical taint in the ranks. However, it is worth noting that a number of worlds copy the uniform and equipment used by the regiments of the Cadian Shock Troops, who are viewed as being the most effective of the Astra Militarum's infantry units.
The origins of the Astra Militarum date back to the Great Crusade of the late 30th Millennium, when the Emperor of Mankind conquered the stars and forged the Imperium of Man. On the front line of this mission of expansion and reclamation were the Space Marine Legions -- the Adeptus Astartes -- the finest warriors Humanity had ever created, each the equal of a dozen normal men. Despite their formidable battle prowess, the forces of the Space Marines were not limitless, and the relentless demands of building a galactic empire pushed the Legions further apart. Separated by countless thousands of light years, their presence became ever more scattered and dilute. 
The Emperor required more manpower to ensure the momentum of the Great Crusade did not falter, and so the Imperial Army was created, known in High Gothic as the Imperialis Auxilia. The Imperial Army was a vital part of the expedition fleets sent out to claim the stars in the Emperor's name. Gargantuan numbers of brave troops -- millions growing to billions of troops, ranks of armoured battle tanks and mighty armadas of capital-class starships that were a part of the Imperial Army's subsidary Armada Imperialis -- were raised, all subordinate to the Legiones Astartes. At first, the Imperial Army was employed for garrison duties and to mop up resistance in the wake of the Legions' initial assaults, utilised where the back of an enemy was broken and Compliance to the Imperium required only a watchful presence. Eventually, certainly by the time the Great Crusade reached the Eastern Fringe of the galaxy, the troops and starships of the Imperial Army were deployed at its forefront, fighting alongside the power-armoured super-soldiers of the Legiones Astartes. But everything changed when the galaxy was ripped asunder by the treachery of the Warmaster Horus, triggering a cataclysmic interstellar civil war that engulfed the newly-founded Imperium.
In the aftermath of this so-called Horus Heresy, the organisational structure of the forces of the Imperium were revised significantly. To prevent the possibility of large-scale rebellion occurring again within the Imperial armed forces, the titanic armies of the Imperium were divided. The nine remaining Loyalist Space Marine Legions were split into Chapters. The Imperial Army, as it was, ceased to exist. The link between fleet and army was severed; never again would ground commanders be given direct control over interstellar warships. From its ashes were born the Imperial Navy and the Imperial Guard -- known officially as the Astra Militarum.
The Astra Militarum was reorganised into planetary-sized units known as "Militarum Regimentos," which in turn were divided into smaller regiments. Commissars of the Officio Prefectus, based on the Imperial Army's former "masters of discipline," were introduced to ensure loyalty and discipline. The interdependence of the newly formed Astra Militarum with the now-separate Imperial Navy ensured that, should a regiment turn against their oaths to the Emperor, they would not be able to spread their treachery beyond a single world, and should a Navy fleet mutiny then they would not have the ability to re-supply or deploy ground troops.
The Imperium had learnt a painful lesson from the dark days of the Horus Heresy, one its rulers were determined never to repeat in the grim days to come.

Parent Organization

Imperium of Man

Related Species

• Human