The Imperial Army 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 invasions1. The Imperial Fleet was a subset of the Imperial Army and was subordinate to it2, and both of them were subordinate to the Astartes command structure1.
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 Crusade3.
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 Crusade2.
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 Imperial Guard and the Imperial Navy; the link between them was severed. Imperial Guard commanders now have no authority over the Navy personnel transporting them and are forbidden from being able to command a navy force2.
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 demanded1.
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 attached3. 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 Fleets4. Expedition Fleets each had several ships and a large amount of Army personnel for ground warfare.
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, with additional support units attached at the battalion and regimental level6.
The organisation of individual regiments varied immensely because of the massive lack of standardization within the Imperial Army2, and this could cause issues including friendly fire, and it would not be until the introduction of the Departmento Munitorum that these issues were partially resolved5.
Organisation of the Imperial Army Fleet
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)4, First Master11a and Master (other ship officers)11a.
Some fleets, like the Saturnine Fleet, have retained their old ranks from before the Unification Wars.
The Imperial Army of the Great Crusade was much more well-equipped than the Imperial Guard of the 41st Millennia.13 Vehicles that are considered rare or lost technology in M41 were common, such as the Leman Russ Vanquisher13a, Leman Russ Executioner13b, and Destroyer Tank Hunter13c. Entire divisions were equipped with Baneblade Super-Heavy Tanks and Valdor Heavy Tanks14, vehicles now considered a rarity.13d In the ten thousand years since the Great Crusade and Horus Heresy, STC's have been lost, Forge Worlds destroyed, and the technology has been treated with far more superstition by the Adeptus Mechanicus.13e
- Archite Palatines - attached to the 28th Expedition Fleet7
- Byzant Janizars - attached to the 63rd Expedition Fleet, rebelled during the Horus Heresy4
- Calth 2nd Irregulars1
- Calth 3rd Infantry1
- Fasadian 45th Infantry1
- Fellesian 5th Airborne Auxiliary1
- Fellesian 23rd Airborne1
- Geno Five-Two Chiliad - attached to the 670th Expedition Fleet; part of the Old Hundred4
- 13th/5th Hylgar's Hell-Raisers - a formation likely to be from early M31.2
- Lacunan Lifewatch - present during the Ark Reach Secundus campaign9
- Lucifer Black - attached to the 670th Expedition Fleet3
- Ouranti Draks - attached to the 47th Expedition 9
- Outremar - attached to the 670th Expedition Fleet and most other fleet, backbone of the Imperial Army3
- Phyrian 2nd Auxillary1
- Prospero Spireguard - attached to the 28th Expedition9
- Sallan 11th Infantry- rebelled during the Horus Heresy1
- Tanagran Dragoons - formed the garrison of the Forge World of Diamat10-p.74
- Urslavik 12th Infantry1
- The 63rd Expedition Fleet - the Expedition Fleet of the Warmaster4
- The 28th Expedition Fleet - the Expedition Fleet of Fulgrim and the Emperor's Children7
- The 12th Expedition Fleet - the Expedition Fleet of Roboute Guilliman*7
- The Saturnine Fleet - a war fleet remaining from Old Night8
- 1 Horus Heresy: Visions of Darkness
- 2 Codex: Imperial Guard (3rd Edition, 2nd Codex)
- a: p. 4
- 3 Legion
- 4 Horus Rising
- 5 Imperial Munitorum Manual (Background Book)
- 6 White Dwarf 125 (UK)
- 7 Fulgrim
- 8 Battle for the Abyss
- 9 A Thousand Sons Chapter 13
- 10 Fallen Angels by Mike Lee, p.74
- 11 Know No Fear by Dan Abnett
- a: p. 187
- b: p. 270
- 12: Horus Heresy Collected Visions pg.64
- 13: Imperial Armour Volume One - Imperial Guard and Imperial Navy
- 13a: pg.31
- 13b: pg.62
- 13c: pg.67
- 13d: pg.79
- 13e: pg.9-11
- 14: Imperial Armour Volume Seven - The Siege of Vraks - Part Three pg.119