Great Schism

The Great Schism, also known as the Great Betrayal among the Sangheili, was a major conflict between the forces loyal to the Covenant and an association of separatists, which split apart between October 2552 and November 2552. During the Schism, three major factions fought for victory: humans and their Sangheili-led separatist allies, the remnants of the Covenant, led by the San 'Shyuum and the Jiralhanae, and the Flood, which took over the Covenant holy city of High Charity in an effort to assimilate the other warring factions.

Background
"We are strong, Kig-Yar. That is why we sit at the right hand of the Prophets."

- Thel 'Vadamee

"One day that shall pass."

- Reth

Even long before the start of the Great Schism, the High Prophet of Truth had plans of replacing the Sangheili with the Jiralhanae, as he saw them as more willing and unquestioning pawns, while he despised the Sangheili, doubting their reliability and loyalty. However, he kept these schemes strictly to himself, and waited until the time was right. After Truth learned the location of the human homeworld, he secretly amassed a fleet of over five hundred warships, entirely commanded by the Jiralhanae, at the command-and-control station Unyielding Hierophant. However, this station and fleet was destroyed by a group of SPARTAN-IIs who learned of its purpose; putting a temporary halt to Truth's plan.

Soon after, the High Prophet of Regret made a move Truth did not expect: having discovered a significant Forerunner artifact on Earth on his own, but unaware of the fact it was humanity's homeworld, he took a fleet of fifteen ships to Earth. Immediately after Truth learned of this, he sent the remnants of the fleet he had amassed to Earth, and gave his Jiralhanae troops orders to forcibly overtake the Sangheili in command of the remaining forces on and above Earth. When Regret retreated from Earth, his Sangheili-led ships in orbit still continued to engage the UNSC, but now without a leader. Only moments after, the Jiralhanae forces within the fleet and on the ground killed any remaining Sangheili forces, as Truth had ordered. They continued to hold New Mombasa until the reinforcements sent by Truth arrived. As High Charity and the rest of the Covenant fleet were now on their way to the newly-discovered Delta Halo, Truth's move remained a secret from the rest of the Covenant. Regret, who would arrive at Delta Halo some time before High Charity and the rest of the Covenant, would serve yet another purpose for Truth; the final stage in his plan.

Conflict erupts
After the Prophet of Regret was killed by John-117, the two remaining Hierarchs, most notably the Prophet of Truth, reorganized Covenant society. Claiming that the Sangheili could no longer protect the Prophets due to Regret's death, they replaced the Sangheili with the Jiralhanae as the Prophets' protectors. The Sangheili viewed this as a violation of the principle that the Covenant had been founded upon and threatened to resign from the High Council. They also argued that if Truth had not withdrawn the Phantoms that had been en route to Regret's location, the Prophet's death would have been prevented and the "Demon" would have been killed instead.

However, Truth further demoted the Sangheili by giving command of their Covenant fleets to the Jiralhanae. Once enough Jiralhanae were in place, Truth instigated the war by secretly ordering the assassination of many ruling Sangheili. Truth also freed all of the Jiralhanae prisoners and pardoned them after release. This was perceived as a Jiralhanae insurrection against the Covenant to the Sangheili, the Mgalekgolo and many Unggoy. Loyalist factions were led to believe it was a Sangheili revolt. After the assassination of the Sangheili councillors, war broke out on High Charity, with the two sides polarizing quickly. However, this was at first a debate over orthodoxy and leadership. The true split occurred when the Sangheili discovered the truth about the Halos and the Great Journey. Their disillusionment changed their goal from reforming the purity of the Covenant to actively fighting against it.

During the original conflicts aboard High Charity, the species of the Covenant took distinct sides. Loyalists followed the Prophets and included Jiralhanae, Kig-yar, and Yanme'e. Separatists primarily consist of the Sangheili, with Mgalekgolo and most Unggoy also breaking away initially.

Sangheili-human alliance
The shaky alliance between the separatists and their former foes, the humans, was forged entirely out of necessity. The UNSC forces were on the defensive, and they had two common enemies that wished for their extinction: the Covenant and the Flood. The alliance of circumstance was forged in the control room of Delta Halo, when Johnson and the Arbiter were coerced to unite in order to prevent Tartarus activating Delta Halo and firing the Halo Array, which would have killed every sentient being in the galaxy, including himself and the rest of his race, while he, and the rest of the Covenant Loyalist's, still believed the array would only start the Great Journey. However, the Arbiter persuaded the rest of the Sangheili to join the humans. While the two factions experienced some friction because their alliance is based on the the enemies of my enemies are my friends concept instead of good political relationships they learned to work with one another for their mutual survival.

The devastated UNSC fleet in Earth's space were glad to accept the help of the superior separatist warships. The Sangheili understanding of enemy equipment and Covenant's combat tactics proved invaluable in the Battle of Installation 00, where a fleet three times larger than that sent by the Sangheili was completely destroyed by the smaller force. Recurrently the heroes of battle, the Arbiter and the Master Chief fought alongside one another many times in the desperate and long conflict. Together the two made an incredible team and the Prophet of Truth was finally killed by the Arbiter with the Master Chief deactivating the Halo Array.

Later, when the Chief destroyed the Flood-controlled High Charity and the Ark was severely damaged when he fired the new Halo ring created above the Ark which also destroyed the Flood. Shipmaster Rtas 'Vadum stated that Truth's fleet was destroyed at the battle over the Ark. Had any of them survived on the Ark or in the ruins of their fleet, they were either taken by the Flood or killed when the unfinished Installation 04B fired and destroyed itself and heavily damaged the Ark.

The Arbiter's presence at memorial services for the Master Chief indicated that in the wake of victory, Sangheili and humans remained friendly to one another as they both had made tremendous sacrifices in order to stop Truth and his forces, as well as the Gravemind and the Flood. However, the peace between the two species remained on shaky ground, with many Sangheili factions still despising humanity and even openly inciting war against the Arbiter and his allies for collaborating with humanity.

With Truth dead, High Charity destroyed, and the Ark essentially lost after Installation 04B's firing, the remaining loyalist forces remained active in at least some form, rebuilding their tattered strength to continue their war of conquest. Even though the Jiralhanae eventually collapsed into various civil wars, they still posed enough of a threat that Sangheili commanders kept up the offensive against them for at least another six years, even as they continued to lose irreplaceable warships and the war continued with no end in sight.

2552
September 22: October 20: October 21st: November 2: November 2-3: November 3: November 8, 2552: November 17, 2552:
 * Installation 04 is destroyed by human forces. Its Monitor, 343 Guilty Spark, escapes to Threshold. The Fleet of Particular Justice is wiped out. The Fleet's secondary flagship, the Ascendant Justice, is captured by humans.
 * A Covenant Artifact Retrieval team led by Sesa 'Refumee is investigating the Gas Mine of Threshold when he encounters 343 Guilty Spark. Inspired by Spark's words, 'Refumee rebelled against the High Prophets of the Covenant.
 * Thel 'Vadamee, the Supreme Commander of the Fleet of Particular Justice, is tried before the High Council and sentenced to death.
 * The High Prophet of Truth overrides the council and makes the Supreme Commander an Arbiter of the Covenant. Truth then sends him to kill Sesa 'Refumee and destroy his rebellion, which they dub "Heresy".
 * Start of the Battle of Earth. The defeated High Prophet of Regret is forced to retreat to Installation 05, pursued by the.
 * Moments after Regret's withdrawal, Jiralhanae within the Covenant forces quickly kill all Sangheili and seize control of the Covenant forces on Earth.
 * The Arbiter and a Spec Ops Commander, Rtas 'Vadumee lead a team of Spec Ops forces to Threshold, where they kill Sesa 'Refumee and successfully crush his rebellion.
 * Jiralhanae-led reinforcement fleet arrives to Earth and starts the excavation of the Portal.
 * The Master Chief tracks down and kills the Prophet of Regret on Installation 05.
 * High Charity and its Defense Fleet arrives at Installation 05. Witnessing the assassination of the Prophet of Regret, the Prophet of Truth replaces the Honor Guard Sangheili with Jiralhanae. Insulted, the Sangheili Councilors threaten to resign from the High Council.
 * The Arbiter and Tartarus are sent to retrieve The Index. The Arbiter successfully takes the Index, but Tartarus tries to kill him and takes the Index from him, as he was secretly ordered to do so by the Prophet of Truth.
 * Most of the Sangheili Councilors are taken to the surface of Installation 05 where they are murdered by the Jiralhanae.
 * The Prophet of Truth transfers command of the Covenant Navy to the Jiralhanae, while Tartarus declares war on the Sangheili, and the First Battle of High Charity begins. Fighting between Sangheili and Jiralhanae forces breaks out all over High Charity. As word spreads, fighting takes place throughout the Fleet, as Sangheili and Jiralhanae controlled ships attack one another.
 * The Flood invade High Charity in the middle of the battle by jumping through Slipspace in the into High Charity and then afterward through the use of Pelicans to invade the rest of the city. The High Prophet of Mercy is attacked by an Infection Form and abandoned by the Prophet of Truth
 * Tartarus attempts to activate Halo.
 * Thel 'Vadamee, with the help of Sergeant Johnson and Rtas 'Vadumee, kills Tartarus.
 * Miranda Keyes aborts the activation of Halo, placing the entire Halo Array on standby status. 343 Guilty Spark reveals the existence of the Ark.
 * The battle in and around High Charity ends with the Flood in control of the city. The Jiralhanae and Sangheili fleets remaining around the city-station continue to destroy each other.
 * The Prophet of Truth departs High Charity for Earth aboard the Forerunner Dreadnought, to join the Battle of Earth. The Jiralhanae forces there are already engaged in fighting with the UNSC. Unbeknown to the Prophet of Truth, the Master Chief is on the ship, with the intention of "Finishing the Fight".
 * The UNSC Prowler Dusk witnesses the battle between Sangheili and Jiralhanae held ships above Delta Halo, both fighting each other pausing only to escalate an immediate outbreak of the Flood from the ringworld and links up with Battlegroup Stalingrad.
 * Sangheili Major Voro 'Mantakree kills his Ship Master, Tano 'Inanraree, while he is attempting to allow the Flood to enter the ship. He then assumes command of the ship, the Incorruptible, and becomes the de facto leader of the Second Fleet of Homogeneous Clarity. He leads his new forces to Joyous Exultation, pursued by the Jiralhanae Gargantum and his ships.
 * Imperial Admiral Xytan 'Jar Wattinree plans to lure the Jiralhanae leadership to Joyous Exultation and plans an ambush for them. He deploys the newly-promoted Fleet Master Voro Nar 'Mantakree to Onyx to recover the Forerunner technologies there and prevent the UNSC forces there from claiming it themselves. A NOVA bomb captured by the Supercarrier Sublime Transcendence devastates the planet moments after Mantakree's forces leave, the survivors joining Mantakree at Onyx.
 * The Forerunner Dreadnought arrives in the Solar System, and proceeds on course towards Earth at near-relativistic speeds.
 * The Forerunner Dreadnought enters Earth's atmosphere, and John-117 bails out, landing in the jungles near Mount Kilimanjaro.
 * Battle of Voi. The Prophet of Truth uses the Forerunner Dreadnought to activate the Portal, and the Loyalist fleet follows it through the portal, marking the end of the Battle of Earth. The Flood infested Voi, only to be contained by the Sangheili fleet.

Late November through December, 2552
 * Battle of Installation 00.
 * All Loyalist vessels destroyed over the Ark by the Separatist Fleet.
 * The Prophet of Truth, the last Hierarch of the Covenant, is executed by Thel 'Vadam with help from the Flood. The Master Chief stops the Ark from firing the Halo Rings.
 * High Charity is destroyed by the Master Chief after he overloads the city's main reactors causing them to detonate.
 * Rtas 'Vadum evacuates all humans and Sangheili via the Shadow of Intent, back to Earth.
 * The Battle of Installation 04B. Thel 'Vadam, Master Chief and Johnson battle to the control room of Installation 04B. Guilty Spark runs rampant, killing Johnson before being subsequently destroyed by John-117. Installation 04(II)'s Sentinels turned on the Master Chief, Thel 'Vadam, and the Sangheili. The unfinished Installation 04 is activated, damaging the Ark and destroying the entire local Flood infestation including the Gravemind. These final events ended the Human-Covenant War.
 * The Master Chief, Cortana, and Thel 'Vadam escape but only Thel 'Vadam gets back to Earth. The Master Chief and Cortana are drifting in space due to the portal closing. They are inside the back half of Forward Unto Dawn, and while technically MIA, are presumed KIA by the UNSC.

2553
March 3:
 * The humans and the Sangheili make a memorial near Mt. Kilimanjaro on planet Earth to commemorate those who were lost in the Battle of the Ark, while also serving as a small memorial for those lost in the war.

2559

 * The Great Schism rages on. While the Jiralhanae wage their own internecine wars, the Sangheili themselves are gradually losing their fleets in a war of attrition, having lost the technical expertise kept exclusively by the Prophets.
 * A Sangheili Shipmaster returns to the partially-glassed human colony of Kholo.

Trivia

 * A "schism" is a division within a religious sect or denomination. The term "Great Schism" is sometimes used to describe real-world internal religious debates, most notably the East-West Schism, which led to the establishment of the Eastern Orthodox Church, and the Western Schism, which caused a split within the Roman Catholic Church.
 * In Halo: Contact Harvest, the Prophet of Truth foreshadows the Great Schism, noting that if the Sangheili ever separated from the Covenant, the entire government would collapse.
 * According to the Bestiarum, the Unggoy were too indecisive to continue the rebellion and the true political motivation of the Mgalekgolo, if any, remained a mystery.
 * In Halo 3: ODST, it is implied that the Office of Naval Intelligence was aware of internal conflict within the Covenant from early on. During the level Tayari Plaza which takes place on October 20, Buck asks Dare about the Sangheili bodies lying around the city, as they appear to have been killed by the Jiralhanae; Dare responds that said information is classified.

List of appearances

 * Halo 2
 * Halo 3
 * Halo 3: ODST
 * Halo: Ghosts of Onyx
 * Halo: Evolutions - Essential Tales of the Halo Universe
 * The Return
 * Halo: Uprising
 * Halo Legends
 * Origins
 * Halo: The Thursday War