Great Schism

The Great Schism, also known as the Covenant Civil War, was a major conflict between two factions, the Covenant Loyalists and the Separatists, which split apart in October 2552. During the Covenant Civil War, three major factions fought for victory: Humans and their Separatist allies, Covenant Loyalists, led by the San 'Shyuum and the Jiralhanae, and Gravemind with the Flood, which took over the Covenant Holy City of High Charity in an effort to assimilate the other warring factions.

Conflict Erupts
After the Prophet of Regret was killed by Master Chief, the two remaining Prophets, most notably the High Prophet of Truth, reorganized Covenant society, seeing that the Elites could no longer protect the Prophets due to Regret's death, they replaced the Elites with the Brutes as the Prophets' protectors. The Elites 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 They also argued that if Truth had not withdrawn the Phantoms that had been en route to Regret's location, the Prophet's death would have been prevented and the Demon would have been killed instead.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 Elites by giving command of their Covenant Fleets to the Brutes. Once enough Brutes were in place, Truth instigated the war by secretly ordering the assassination of many ruling Elites. Truth also freed all of the Brute prisoners and pardoned them after release (Seen on the Halo Timeline Edict of the High Prophet of Truth). This was disguised as a Brute insurrection against the Covenant to the Elites, the Hunters and most Grunts. Loyalist factions were led to believe it was an Elite revolt. After the killing of the Elite Council members, war broke out on High Charity, the two sides polarizing quickly. However, this was at first a debate over orthodoxy and leadership. The true split occurred when the Elites learned 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.

The split was not cleanly divided by either race or faction. Loyalists followed the Prophets and included Brutes, Jackals, Drones, a small number of Grunts and some Hunters. Separatists included Elites, with most Hunters and the majority of Grunts also breaking away initially. The Great Schism seemed to be more associated with intellect. The Elites' reasoning, logic and good intentions allowed them to see through the abuse of religious authority enforced by the Prophets.

The only Grunts who actually remained loyal to the Prophets were the most devout followers of the Covenant religion (ex. Deacons). This might also suggest that the Grunts' "new found courage" in Halo 3 might have been the result of fanaticism rather than cowardice. There is some evidence to support this since there are a number of Grunts who commit acts that are anything but cowardly (ex. Kamikaze Grunts). It's also possible that the reason Separatist Grunts are never seen in Halo 3 is that shortly after the Great Schism, they returned to Balaho to lead a rebellion against the Deacons and other Loyalist forces. The Separatist Hunters might have done the same on Te, but it is still unknown.

Sangheili-Human Alliance
The shaky-to-steady alliance between the Separatists and their former foes, the Humans, was forged entirely out of necessity. The UNSC forces were big but 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 experience some friction because their alliance is based on the the enemies of my enemies are my friends concept instead of good political relationships they learn to work with one another for their mutual survival. During the events of Halo 3, the Elites fight alongside humans in battle. The Elites decided to save the humans from destruction.

The devastated UNSC Fleet in Earth's space were glad to accept the help of the superior Separatist Warships. The Elites' understanding of enemy equipment and Covenant's combat tactics proved invaluable in the Battle of Installation 00. Recurrently the heroes of battle, the Arbiter and Master Chief fought alongside one another many times in the desperate and long conflict. The Arbiter's presence at memorial services for the Master Chief indicated that in the wake of victory, Separatists and humans remained friendly to one another as they both had made tremendous sacrifices in order to stop Truth and the Loyalist forces, as well as the Gravemind and the Flood. The Elites and humans will probably continue to be allies and will benefit greatly from this alliance in the future.

With Truth dead, High Charity destroyed, and the Ark essentially lost to the reborn Installation 04's firing, it is unknown what exactly the remaining Loyalist forces did. According to the Bestiarum in Halo 3's Limited Edition, the Grunts were too indecisive to continue the rebellion and the true political motivation of the Hunters, if any, remained a mystery. Ship Master Rtas 'Vadum stated that Truth's Fleet was destroyed. Had any of the Covenant survived on the Ark or in the ruins of their Fleet, they were either taken by the Flood or killed when the unfinished Installation 04 (II) fired and destroyed itself and heavily damaged the Ark. At the end cutscene of Halo 3, victorious Elites depart for their homeworld. With the Covenant disbanded and the Flood threat ended, each race returned home to rebuild their homeworld.

2552
September 22: October 20: November: 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 UNSC In Amber Clad. On the same day, the Jiralhanae forces sent to Earth kill the remaining Sangheili stationed there.
 * 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.
 * The Master Chief tracks down and kills the Prophet of Regret on Installation 05.
 * High Charity arrives at Installation 05. Witnessing the assassination of the Prophet of Regret, the Prophet of Truth replaces the Honor Guard Elites with Brutes. Insulted, the Elite 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 Elite Councilors are taken to the surface of Installation 05 where they are murdered by the Brutes.
 * Tartarus declares war on the Elites, and the Battle of High Charity begins. Fighting between Elite and Brute forces breaks out all over High Charity. As word spreads, fighting takes place throughout the Fleet, as Elite and Brute controlled ships attack one another.
 * Flood invade High Charity in the middle of the battle by jumping through Slipspace in the In Amber Clad into High Charity and then afterwards through the use of Pelicans to invade the rest of the city.
 * 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 Brute and Elite Fleets remaining around the city-station continue to destroy each other.
 * The Prophet of Truth takes the Forerunner Dreadnought to Earth, to join the Battle of Earth. The Brute 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 Elite and Brute 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.
 * Major Domo 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 Brute Gargantum and his ships.
 * Imperial Admiral Xytan 'Jar Wattinree plans to lure the Brute 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 Kenyan jungles.

Late 2552 to Early 2553

 * Battle of Voi
 * The Flood infest Voi, only to be contained by the Fleet of Retribution
 * Battle of Installation 00.
 * All Loyalist vessels destroyed over the Ark by the Separatist Fleet.
 * The final Covenant religious leader, the Prophet of Truth, is executed by the Arbiter 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 04 (II). Thel 'Vadam, Master Chief and Johnson battle to the control room of Installation 04 (II). 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 Elites. 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 between Earth and The Ark due to the portal closing. They are inside the back half of Forward Unto Dawn, and no one on Earth knows what happened to them.

2553
March 3:
 * The Humans and the Elites 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 Human-Covenant war. The memorial has the number 117 engraved on it in the corner. It might have been engraved by a Combat Knife, or even Thel 'Vadam as a mark of friendship.

Trivia

 * The term "Great Schism" is taken from human religious debates over orthodoxy and power, most notably the East-West Schism, the Western Schism, and the Believers Schism. The Sunni-Shia split is also often called a Schism.
 * In Contact Harvest the Prophet of Truth foreshadows the Schism, noting that if the Elites ever separated from the Covenant that the entire government would collapse.
 * Throughout the Halo 3: ODST Mombasa Streets levels, you can find dead Sanghelli bodies in various corners of the map. In particular, during the first Buck flashback mission, he comes across some dead Sanghelli and comments to Dare about it. Oddly enough, seeing as how the game begins just six hours after the Prophet of Regret slipspace-jumps away, this would mean that either the entirety of sans-Earth Halo 2 took place in the period of six hours, or that Jiralhanae had started killing Sanghelli before the Schism took place. However, Buck's story takes place only minutes after the drop.

Internal Links

 * Covenant
 * Elites
 * Brutes
 * Prophets