With the threat of the Traitor Legions held at bay in the wake of the Great Scouring in the early 31st Millennium, Roboute Guilliman turned to ensuring that such a catastrophic intra-species war could never happen again, distilling his formidable wisdom into the mighty tome known as the Codex Astartes. Thecommonly heardin-universe answer(usually made up on the spot by sexually insecure male Warhammer fans, for whom the existence of powerful, dominant, and especially combatant female characters in any fiction is inexplicably enraging) normally involves a lot of waffle about howthe Emperor’s gene-crafting processes ‘just weren’t designed’ for female bodies. As if it made any kind of sense for the nigh-omnipotent Emperor of Mankind to rule out halfhissubject human population as potential recruits. No Space Marine, not even an Iron Hands battle-brother, could have lived long in that toxic place, though the tanks of Dauuk's strike force endured, and scoured the planet clear of threats in less than a solar week. It’s a videogame that focuses almost all its energy on the audio-visual rhythm and sheer serotonin-pumping, kinetic joy of its combat gameplay – and fills in the piffling details of story, characters, environments, level design, and other such distractions, afterwards. A Chapter also includes a number of officers and specialists who exist outside of the formal organisation of the companies. These individuals are known as the headquarters or command staff, and they will often stride out to lead a strike force in battle, or provide essential battlefield support, spiritual leadership, psychic capability and destructive combat prowess.

Vanguard Space Marines are reconnaissance and infiltration experts, equipped to operate alone in enemy territory for extended periods of time and intensively trained in shadow warfare tactics and sabotage techniques. Vanguard strike forces are tasked with achieving full-spectrum superiority over the foe.Under the revised organisational scheme, each of the ten companies of a Chapter still boasts one hundred warriors, led by a captain -- a veteran of countless wars -- and now often two lieutenants as sub-company leaders. A company is still organised into ten squads of ten Space Marines, each led by a sergeant.

In the early 31st Millennium, in the wake of the calamity that was the Horus Heresy, the foundations of the Imperium of Man were reformed to make such a tragedy less likely in the future. The first High Lords of Terra, under the direction of the Lord Commander of the Imperium and Imperial Regent Roboute Guilliman, the primarch of the Ultramarines Legion, established the structure by which the Adeptus Terra operated, and described the feudal responsibilities and duties of the planetary lords.The Horus Heresy had revealed previously unknown genetic weaknesses in the gene-seed of the primarchs and the Space Marines among the original 20 First Founding Space Marine Legions, weaknesses that left the Legions in question greatly exposed to corruption by the Ruinous Powers of Chaos. In the wilderness of the Halo Stars, Captain Dauuk of the Iron Hands led just such a mechanised strike force into the Warren Worlds. There, among the caustic yield-nests of the Heretek cults, his Predators stalked the sentient mecha-fauna, blasting apart the Chaos worshippers and their daemonic allies. The Repulsor grav-tank unleashed by the Primaris Space Marines in the Era Indomitus has added further power to the armoured might of the Adeptus Astartes. Normally painted upon the hull of Space Marine vehicles and the greave of a battle-brother's armour, it is not uncommon for these badges to be incorporated into an individual's heraldry as a permanent honour somewhere on his power armour after the campaign's completion, either to commemorate exceptional deeds or as a mark of remembrance for the fallen.