The Imperial Guard is the primary ground-based frontline military force of the Imperium of Man. The Imperial Guard has a long history and has been actively defending and expanding the Imperium for ten thousand standard years. Its immediate predecessor, the Imperial Army, fought on many fronts during the Great Crusade and Horus Heresy as the most important support and garrison force for the far more powerful but far less numerous Space Marine Legions. But following the end of the Horus Heresy and the Reformation of the Imperium undertaken by the Primarch Roboute Guilliman, the decision was made to break up the Space Marine Legions into the much smaller Chapters and refocus them on planetary assault missions. As a result, the Imperial Army was reorganised into the Imperial Guard and transformed into the Imperium's frontline ground force, recruiting its troops from different worlds and human cultures across the galaxy.
During the early years of the Great Crusade in the 31st Millennium, the Emperor of Mankind and his Space Marines were the only forces necessary for the conquest of worlds and the construction of the Imperium. Many of the Primarchs, the genetically-engineered superhuman leaders of the Space Marine Legions, tried to win the fealty of human populations isolated during the Age of Strife through non-military tactics if possible, and often incorporated soldiers that had initially been arrayed against them into the fight as part of the Imperial Army to liberate and unite even more worlds.
Auxiliary human military forces were initially used to assist the Astartes in the liberation of their own homeworld from aliens, daemons, or all-too-human tyrants, and these troops would remain after the campaign and the institution of Imperial Compliance as an Imperial garrison force. Later, as the scope of the war escalated beyond the sole control of the Space Marines, these forces would be brought in to mop up any remaining resistance from Space Marine assaults, and take control of the freshly conquered world, freeing up the Space Marines to move onto other campaigns. Eventually, the expansion of the Imperium of Man reached the point in the 31st Millennium where the Astartes, fighting on every front, were spread too thinly. It was at this time the Imperial Army truly came into its own, deploying at the forefront of the Great Crusade.
The Imperial Army
The original Imperial Army boasted great variety amongst its forces; some utilising mutant troops while others had access to anti-grav vehicles. Each unit drew on the strengths and technology of its homeworld, and variations were dependent on a world's religious beliefs, level of technological advancement, social customs, and specialist industries. This led to the development of regiments recruited from a single homeworld, as men from different worlds would have trouble understanding each other's dialects, social customs, tactics or specialist equipment.
Because of the sheer variety of worlds and subsequent Imperial Army regiments, a measure of standardisation was deemed necessary to allow the Army to function as a whole, although the sheer size of the expanding Imperium by this time prevented any true standardisation. The treatise that came to be known as the Tactica Imperium was penned and distributed, and a cadre of political officers, the Masters of Discipline (precursors of the Commissars), were developed to ensure proper and effective integration and co-operation between regiments. The role of these political officers later expanded to include maintaining the quality and morale of the regiments they were assigned to, and they became almost compulsory after the Horus Heresy - the first action of many traitors would be to kill any Commissars.
The Horus Heresy
During the Horus Heresy, many Imperial Army regiments turned to the side of Horus and Chaos. Regiments seconded to Space Marine Legions that turned traitor followed their superhuman masters into rebellion, sometimes out of fear or blind faith. Regiments were raised on worlds controlled by both sides, and were thrown into the bloodbath of combat. The control over both fleet and army elements held by a relatively limited number of people meant that the faith or corruption of a few determined on which side a force would fight. Thousands of small empires were carved out by ambitious generals with no loyalty to either side, and many regiments did little more than stay out of the way, waiting for a clear victor in the galactic civil war before they committed to a side.
The Horus Heresy changed the galaxy forever, both during its wars, and after, when the Imperium made all efforts to remain in control of its vast territories. Regiments of the Imperial Army still loyal to the Emperor assisted the Space Marines in their efforts to maintain control and suppress secondary rebellions.
The Imperial Guard
At some point after the end of the Horus Heresy, the Imperial Army underwent a mass reformation. The Army was split into two separate entities, the Imperial Guard and the Imperial Navy. What aspects of the Army that could be standardised were, and practical traditions from the days of the Army, such as the size of regiments designed to fit inside cruiser-size starships and the recruiting of a regiment's members from a single homeworld, were retained.
As part of the reformation, many of the units used by a regiment were cut down, and specialist regiments, such as those made up of Rough Rider cavalry or Ratling snipers, were split into detachments and divided amongst the standard regiments, to provide support in special skills and tactics. Commissars became centrally trained by the Ecclesiarchy and Schola Progenium. These measures almost completely limited the ability for Imperial Guard regiments to be led astray by a few corrupted officers, and further measures limiting the direct power of the Ecclesiarchy that came into effect after the Age of Apostasy have shaped the Imperial Guard into what it is during the 41st Millennium; a byword for loyalty and honour.
Although similar to the regiments of the post-Heresy years, the Imperial Guard has not remained completely stagnant. The universal Lasgun was only introduced in the late 32nd millennium. Prior to this, the Imperial Guard relied on solid ammunition Autoguns (and still do in some parts of the Imperium), and some elite soldiers still prefer to use them in combat. Armoured vehicles are often modified for a single campaign or operation, when the situation calls for it, but the usefulness of some modifications results in their full-scale production by certain Forge Worlds.
- Codex: Imperial Guard (5th Edition), pg. 6
- Codex: Imperial Guard (3rd Edition, 2nd Codex), pg. 4
- Codex: Ultramarines (2nd Edition), pp. 12-15
- Horus Heresy: Collected Visions, pp. 64, 118-123, 180, 303, 361
- Imperial Armour Volume One - Imperial Guard and Imperial Navy pp. 9-11, 31, 62, 67, 79
- Imperial Armour Volume Three - The Taros Campaign
- Imperial Armour Volume Seven - The Siege of Vraks - Part Three, pg. 119
- Imperial Munitorum Manual (Background Book), pp. 6-8
- White Dwarf 316 (US), "Index Imperialis - Firstborn Sons of Vostroya"
- White Dwarf 242 (AUS),"Glorious Battles of the Imperial Guard – Part 3"
- White Dwarf 241 (AUS),"Colonel Schaeffer’s Last Chancers"
- White Dwarf 125 (UK), "Epic Imperial Guard," by Jervis Johnson, Karl Tebbutt & Roger Gerrish, pp. 49-50, 52-54, 56-61
- Codex: Catachans (3rd Edition)
- Horus Rising (Novel) by Dan Abnett, pp. 93, 262-263
- False Gods (Novel) by Graham McNeill, pp. 71, 193, 130, 322, 326
- Fulgrim (Novel) by Graham McNeill, pp. 47, 51, 197
- Legion (Novel) by Dan Abnett, pp. 5, 10, 49, 52, 88-89, 93-94, 105, 113, 120, 140, 158-159, 170-171, 177, 185, 192, 197, 199, 203, 208, 255, 265, 290, 301, 314, 318
- Battle for the Abyss (Novel) by Ben Counter, pg. 64
- Mechanicum (Novel) by Graham McNeill, pp. 309-310, 320
- Tales of Heresy (Anthology), pp. 41, 44
- Fallen Angels (Novel) by Mike Lee, pp. 55, 77, 91, 106, 119, 127, 180, 208, 210, 283
- A Thousand Sons (Novel) by Graham McNeill, pp. 46, 89, 93, 99, 106, 115, 117, 174, 191, 201, 216, 218
- Proserpo Burns (Novel) by Dan Abnett, pp. 31-33, 157, 159-161, 163, 198, 293
- Age of Darkness (Anthology), "The Iron Within," by Rob Sanders, pp. 264-265, 273, 285
- The Primarchs (Anthology), "The Sergeant Beneath," by Rob Sanders, pg. 388
- Know No Fear (Novel) by Dan Abnett, pp. 12, 18, 119, 139, 191, 199
- Angel Exterminatus (Novel) by Aaron Dembski-Bowden, pp. 37, 68, 200
- First and Only (Novel) by Dan Abnett
- Guns of Tanith (Novel) by Dan Abnett
- Double Eagle (Novel) by Dan Abnett
- The Chapter's Due (Novel) by Graham McNeill, pp. 19, 203-204