The Imperial Army (also known as the Imperialis Auxilia in High Gothic) was the largest military force in the early Imperium, and was founded during the early Great Crusade to fill the need for more manpower to support the Astartes Legions, and quickly was used to garrison worlds, perform sieges, and mass invasions. The Imperialis Armada was the fleet subset of the Imperial Army and was subordinate to it[2x], and both of them were subordinate to the Astartes command structure.
The fighting elite of the Imperial Army was the Solar Auxilia, which comprised about 20-25% of the Army's strength by the end of the Great Crusade. The Imperial Army also maintained a force of auxiliaries known as the Imperialis Militia.
The genus of the armed force that was to become the Imperial Army was on Terra, in the form of the few Strife-era military formations that the Emperor allowed to continue in his service after the end of the Unification Wars. These surviving units in time became referred to as the Old Hundred, and formed the initial nucleus of human soldiery at the very beginning of the Great Crusade.
When the Emperor left Terra on the Great Crusade, it is said that he would expect the inhabitants of the worlds he conquered to maintain defences and armed forces for their own safety, against internal rebellion and external invasion; similar to the role of Planetary Defence Forces in the 41st millennium. It is out of these forces that the Imperial Army was first formed. Initially used as garrison forces, they were quickly pushed to the forefront of the Crusade[2x].
The Imperial Army's subordination to the Astartes Legions led to the Imperial Army tearing itself apart in the Horus Heresy, when half of the legions rebelled against the Emperor, and took their Army regiments with them. Thousands of Army commanders took it upon themselves to carve themselves their own empires, with no loyalty to Horus or the Emperor, and it was centuries before the Imperium was reforged.
In the aftermath of the Horus Heresy many changes were made to the structure of the Imperial armed forces. Many of these changes were made to ensure to no single individual would be powerful enough ever again to attempt to overthrow the ruling body of Imperium if he turned traitor. It was during these post-Heresy reforms that the Imperial Army was split into two organisations: the Astra Militarum and the Imperial Navy; the link between them was severed. Astra Militarum commanders now have no authority over the Navy personnel transporting them and are forbidden from being able to command a navy force[2x].
Organisation and Recruitment of the Imperial Army
The Imperial Army was recruited from each world in the Imperium unless exempted (such as Astartes home worlds, Forge Worlds, and planets in the Sol System, though some of the regiments from the Unification Wars of Terra were allowed to fight in the Great Crusade), but the size of the tithed forces was dependent on the population of the planet, and Imperial administrators determined the amount of soldiers and war materiel demanded. If all else failed or in times of desperation, the Imperial Army would utilize inducted levy militia, troops with little to no training and poor equipment.
The best equipped and trained Imperial Army troops were known as Grenadiers.
The largest single organisational unit in the Imperial Army was the Expedition Fleet, each of which was controlled by a Lord Commander unless an Astartes officer was attached. By the time of the Ullanor campaign and the inauguration of Warmaster Horus, there were 4,659 Primary Expedition Fleets (with 372 of those in refit at the time) and more than sixty thousand Secondary Expedition Fleets. Expedition Fleets each had several ships and a large amount of Army personnel for ground warfare.
For garrisoning and occupying worlds, the Imperial Army organized local Imperialis Militia units.
Organisation of Imperial Army Regiments
The largest permanent organisational unit during the Horus Heresy for the Imperial Army was the regiment, and to take a typical example, regiments could be divided into three or more battalions, with each battalion made up of two or more companies and Platoons, with additional support units attached at the battalion and regimental level.
The organisation of individual regiments varied immensely because of the massive lack of standardization within the Imperial Army[2x]. Expedition Fleets were only equipped with a portion of their supply needs, and were expected to re-supply themselves from conquered worlds. These issues were partially resolved with the Council of Terra's introduction of the Departmento Munitorum. The Munitorum originally existed as the Corps Logisticae, a division of naval administration, but was made into a full Departmento shortly before the Ullanor Crusade.
Organisation of the Imperialis Armada
Regiments were usually assigned to individual ships, with Army personnel commanding both elements.[2a]
In the fleet, the highest rank was the Master of the Fleet, who was under the authority of the Lord Commander or Astartes officer in the Expedition Fleet. Other common ranks include Shipmaster (in control of a ship), Executive Officer (second in command), First Master[11a] and Master (other ship officers)[11a].
Some fleets, like the Saturnine Fleet, retained their old ranks from before the Unification Wars.
The Imperial Army of the Great Crusade was much more well-equipped than the Astra Militarum of the 41st Millennia.[13x] Vehicles that are considered rare or lost technology in M41 were common, such as the Testudo, Dracosan, Gorgon, Malcador, Leman Russ Incinerator, Leman Russ Vanquisher[13a], Leman Russ Executioner[13b], and Destroyer Tank Hunter[13c]. Entire divisions were equipped with Baneblade and Stormhammer Super-Heavy Tanks and Valdor Heavy Tanks and fleets were equipped with multitudes of Battleships and Grand Cruisers, all now considered a rarity.[13d] In the ten thousand years since the Great Crusade and Horus Heresy, STCs have been lost, Forge Worlds destroyed, and the technology has been treated with far more superstition by the Adeptus Mechanicus.[13e]
- Ayliet Phalanx - Subordinate to the Luna Wolves
- Archite Palatines - attached to the 28th Expedition Fleet
- Byzant Janizars - attached to the 63rd Expedition Fleet, rebelled during the Horus Heresy
- Calth 2nd Irregulars
- Calth 3rd Infantry
- Clandestine Orta - Special forces akin to Stormtroopers
- Fasadian 45th Infantry
- Fellesian 5th Airborne Auxiliary
- Fellesian 23rd Airborne
- Geno Five-Two Chiliad - attached to the 670th Expedition Fleet; part of the Old Hundred
- Inferallti Hussars - Assigned to guard Pluto
- Jovian Grenadiers
- 13th/5th Hylgar's Hell-Raisers - a formation likely to be from early M31.[2x]
- Lacunan Lifewatch - present during the Ark Reach Secundus campaign
- Lucifer Black - attached to the 670th Expedition Fleet
- Mohindas Eleventh - wiped out by the Nephratil on Diurnus in the 6th year of the Great Crusade
- Ouranti Draks - attached to the 47th Expedition 
- Outremar - attached to the 670th Expedition Fleet and most other fleet, backbone of the Imperial Army
- Phyrian 2nd Auxillary
- Malcador's Chosen - Praetorian Guard of Malcador the Sigilite
- Prospero Spireguard - attached to the 28th Expedition
- Regnault Thorns
- Saturnine Hoplites
- Sallan 11th Infantry- rebelled during the Horus Heresy
- Selucid Thorakites - troops of the Iron Warriors
- Seventh-Suckle Parthenari Shieldmaidens
- Solar Auxilia - Elite fighting force
- Tanagran Dragoons - formed the garrison of the Forge World of Diamat
- 1st Terra Tank Division[12a]
- Thorosian Voltigeurs - Took heavy losses in the Battle of Hell's Anvil
- Urslavik 12th Infantry
- Uzuran Sabreteurs
- The 63rd Expedition Fleet - the Expedition Fleet of the Warmaster
- The 28th Expedition Fleet - the Expedition Fleet of Fulgrim and the Emperor's Children
- The 12th Expedition Fleet - the Expedition Fleet of Roboute Guilliman
- The Saturnine Fleet - a war fleet remaining from Old Night
Notable Members of the Imperial Army
- Tabor Ludovicia and Haldane Ma'lon - Ruling co-Lord Militants a the end of the Great Crusade
- Pietra Veitz Gustavus - Lord Militant
- Ireton MaSade - Lord Marshal
- Teng Namatjira - Lord Commander
- Hektor Varvarus - Lord Commander
- Thaddeus Fayle - Lord Commander
- Tyana Kourion - Lord General
- Eodraki Hakon - Marshal
- Dellasarius - Imperial Commander
- Holst Lithonan - Traitor General
- Hestor Navarre - Officer of the Ouranti Draks
- Marcus Valerius - Commander of the Therion Cohort
- Hurtado Bronzi - Hetman
- Dimitar Shiban - Hetman
- Peto Soneka - Hetman
- Fatidicus - Decorated veteran later key Imperial Cult religious figure
- Ollanius Pius - Patron Saint of the Imperial Guard
- Ospheus LaBray - Admiral of the Imperial Armada
- Kaminska - Rear Admiral of the Imperial Armada
- Hasani Sabbyat - Stormtrooper Officer
- Aevos Jovian - Medicae Officer
- Khalid Hassan - Captain of the 4th Clandestine Orta
- 1: Horus Heresy: Visions of Darkness, [Needs Citation]
- 2: Codex: Imperial Guard (3rd Edition, 2nd Codex)
- 3: Legion (Novel) by Dan Abnett, [Needs Citation]
- 4: Horus Rising (Novel) by Dan Abnett, [Needs Citation]
- 5: Imperial Munitorum Manual (Background Book), pgs. 6-8
- 6: White Dwarf 125 (UK), [Needs Citation]
- 7: Fulgrim (Novel) by Graham McNeill, [Needs Citation]
- 8: Battle for the Abyss (Novel) by Ben Counter, [Needs Citation]
- 9: A Thousand Sons (Novel) by Graham McNeill, Chapter 13
- 10: Fallen Angels (Novel) by Mike Lee, [Needs Citation], pg. 74
- 11: Know No Fear by Dan Abnett
- 12: Horus Heresy: Collected Visions, pg. 64
- 12a: pg.357-362
- 13: Imperial Armour Volume One - Imperial Guard and Imperial Navy
- 14: Imperial Armour Volume Seven - The Siege of Vraks - Part Three, pg. 119
- 15: Brotherhood of the Storm (Novella), pg. 30
- 16: Unmarked (Short Story) by Dan Abnett - Mark of Calth (Anthology), pg. 270
- 17: The Horus Heresy Book Two - Massacre pg. 31
- 18: The Serpent Beneath (Novella) by Rob Sanders, Chapter Delta, Operatus Five-Hydra: Elapsed Time Ω1/-214.12//XXU
- 19: The Horus Heresy Book Three pg.150-151
- 20: Forgeworld
- 21: The Horus Heresy Book Four - Conquest pg.81
- 22: The Horus Heresy Book Five pg.195
- 22: Macragge's Honour (Graphic Novel), Dramatis Personae