Auretian Technocracy War
The Auretian Technocracy War was a campaign of the Great Crusade.
Waged after Horus secret decision to overthrow the Emperor following his experiences on Davin, the war itself seems to have been instigated by Horus after he murdered an Technocracy emissary during negotiations on the Vengeful Spirit. The war was necessary to give Horus the support of the Adeptus Mechanicus for his rebellion, as the Technocracy had possession of multiple STC's the Mechanicum greatly coveted.
The war was waged primarily by the 63rd Expeditionary Fleet and the Sons of Horus as well as the World Eater's 203rd Expeditionary Fleet and a contingent of Emperor's Children. They fought against the armored soldiers of the Technocracy, which in many ways resembled Astartes. After 10th months of difficult fighting that Horus used as cover to identify loyalist elements within his forces, the Technocracy was defeated at the Battle of the Iron Citadel. The battle was noted for its brutality, as the World Eaters under Angron slaughtered soldier and civilian alike.