The Siege at Kolaar was an attack on Arbiter Thel 'Vadam and his forces by Avu Med 'Telcam and the Servants of the Abiding Truth in the first major conflict of the Sangheili civil war. Thel 'Vadam, while largely victorious in the engagements during the Great Schism months earlier, was not universally popular at home, largely due to his tentative alliance with the UNSC and his refusal to finish the attack on Earth and wipe out the rest of humanity. The siege initially went well for the Servants, but involvement by the UNSC Infinity on the side of Thel 'Vadam resulted in a victory for the Arbiter. The remaining Servants of the Abiding Truth escaped the system to regroup for future conflicts.
These events are depicted in the novel Halo: The Thursday War.