Auretian Technocracy War
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 Sons of Horus's 63rd Expeditionary Fleet, the World Eater's 203rd Expeditionary Fleet, and a contingent of Emperor's Children; against the Technocracy's Brotherhood. The war was one of most difficult the Astartes had faced against a non-Imperium human civilization, thanks to the Technocracy being in possession of two functional STCs. These STCs allowed them to produce power armour and bolters which rivaled that of the Astartes war gear. While the Brotherhood soldiers were not gene-enhanced like the Astartes, they could produce soldiers, weapons, and power armour in the millions. After 10 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.