Damocles Crusade

The Damocles Crusade (also known as the Damocles Gulf Crusade) was a major campaign against the Tau Empire by the Imperium of Man.

It began as a response to first contact between humanity and the Tau, during what the Tau called their Second Phase Expansion. The Tau's colonization efforts brought them across the Damocles Gulf, and they encountered human worlds on the other side. These frontier planets were easy to subvert, as they felt little attachment to the Imperium, and even when there were loyalist factions, they were in the minority.

the Imperium was not happy to be losing worlds to Tau rule, so they sent a Crusade to toss them out, and eventually crush them all the way back to their home. They were so serious about this Crusade that they printed a special edition of the Imperial Infantryman's Uplifting Primer just for Imperial Guard Regiments going to the Damocles Gulf. They were initially successful, but as the Tau brought more of their armies to the front, the Imperial advance slowed and eventually halted on the planet of Dal'yth Prime.

The Imperium probably would have kept throwing regiments and Titans at the Damocles Gulf, but they needed every guardsman they had to repel the invasion of Hive Fleet Behemoth, so they persuaded the Tau to let them go and defend the Imperium. The Tau responded by re-taking all the worlds they had lost to the Imperium, and later launched the Third Phase Expansion to take further advantage of the Imperium's retreat.