“You need to do more than overthrow the Arbiter. You need to wipe both him and Vadam from the face of Sangheilos, because as long as that state exists, it will remain loyal to him, and it will exert its power and influence.”
The seeds of the Sangheili Civil War began immediately after the end of the Great Schism and the Human-Covenant war. In January 2553, the ArbiterThel 'Vadam began to visit the various states to propose a peace treaty with humanity. He argued that too many Sangheili had been lost in the Great Schism, and that Sanghelios had to be rebuilt. This proposal was not well received by all, and civil unrest ensued. Dissident Sangheili began to join the Servants of Abiding Truth, an old orthodox religious group who opposed the Arbiter led by Avu Med 'Telcam.
In 2553, the United Nations Space Command's Office of Naval Intelligence, with the help of a civilian consultant named Evan Phillips, made contact with Avu Med 'Telcam. They agreed to arm them with Sangheili weapons in exchange for the Servants leaving humanity alone if they seized power. ONI's true goal was to keep the Sangheili administrative and military authorities weak and disorganized until humanity could rearm and eliminate the Sangheili race altogether.
After a Jiralhanae uprising in Ontom, right outside the Servants' headquarters, 'Telcam decided the time had come to begin the rebellion. He and his supporters gathered their ships, vehicles and troops and attacked Vadam. The Arbiter was winning at first, since fewer keeps joined the rebellion than 'Telcam had thought. The tide turned when the Arbiter allowed Kilo-Five to land on Sanghelios to search for Evan Phillips in the Temple of Abiding Truth. Many Sangheili viewed this as a desecration and joined 'Telcam's side. The Servants quickly surrounded Vadam Keep and began a siege. The tide turned once more when Admiral Hood arrived in orbit with the UNSC Infinity and offered the Arbiter aid, which he grudgingly accepted. The Infinity shot down 'Telcam's ship Defender of Faith and fired a MAC round directly into the besieging forces outside Vadam Keep. The siege quickly became a rout as the Arbiter counter-attacked. 'Telcam and the Servants were able to escape with three frigates intact, along with as many troops, vehicles and supplies the ships could hold, to New Llanelli. The rebellion was crushed for the time being, and the Arbiter remained in power on Sanghelios, however in a weakened state.
By 2555, the war had seemingly ended, with the Swords of Sanghelios securing the allegiance of Sanghelios and several of its colony worlds. However, the Swords of Sanghelios continued to face opposition from Covenant remnant groups including Jul 'Mdama's faction and Vata 'Gajat's mercenary group.