The Damocles Crusade was the Imperium's first great reckoning with the T'au Empire, the young and aggressively expansionist xenos power whose spheres of influence had begun to swallow human worlds along the galaxy's eastern fringe. Alarmed by the rapid spread of the T'au and their seductive creed of the Greater Good, the Imperium mustered a crusade to cross the perilous Damocles Gulf and annihilate the upstart threat before it could grow strong enough to resist.
The Astra Militarum, Space Marines and Imperial Navy fell upon the T'au frontier worlds with characteristic brutality, and at first the crusade carried all before it, scouring colonies from orbit and shattering the septs it encountered. But the T'au proved to be nothing like the primitives the Imperium had complacently imagined. Their advanced technology, disciplined battlesuit cadres and flexible, alien doctrine exacted a steadily rising toll, and the war ground toward stalemate before the walls of their fortified strongholds. Each hard-won Imperial advance was met and answered by the precise, disciplined fire of the alien battlesuit cadres and their auxiliaries.
The crusade eventually reached the T'au heartworld of Dal'yth and laid it under siege, yet for all the fury of the assault neither side could force a clean decision. With Imperial losses mounting alarmingly and a fresh and far greater Tyranid menace stirring elsewhere in the region, the crusade's command chose to negotiate an uneasy truce and withdraw rather than see its armies bled white for a marginal and uncertain gain.
The crusade checked T'au expansion but wholly failed to break it, and the two rival powers would clash again and again across the millennia that followed. The campaign is chronicled in the account of the Damocles Crusade and the wider history of the T'au Empire.