Ur-Clemait Civil War
In 920.M41 a Tau delegation arrived to begin negotiations for the worlds absorption into the Tau Empire and end the civil war. To try and forge a peace the Tau ended up heavily arming one of the factions with high-technology weapons such as Pulse Rifles, allowing them to emerge victorious after centuries. Though most of the population seems content with the Tau's method of pacification, the elders and priests of the old faith say that the "Blood Tithe" must be paid. Trying to incite rebellion, the priests even launch attacks at the Tau garrison directly but are easily crushed.
The Tau nonetheless continue their mission to assimilate the planet into their empire, but before the year is out the Word Bearers arrive to enforce the neglected Blood Tithe. The heretical Space Marines attack the newly prosperous world of Ur-Clemait in terrible force, reigniting the civil war in addition to devastating the planet. The Tau Fire Caste meets the invasion head on, and by 921.M41 the situation is far worse for the people of Ur-Clemait then it has ever been.