The Sangheili-Jiralhanae war was a series of scattered conflicts between the Sangheili and the various Jiralhanae factions after the events of the Human-Covenant War and acted as a continuation of the Great Schism.
Tensions and rivalry already existed between the Sangheili and Jiralhanae long before the Human-Covenant war. However, it was the Changing of the Guard that eventually lead to actual battles between the two species when the Prophet of Truth instigated the Great Schism as part of his greater plans. The Great Schism ended with the defeat and collapse of the Covenant, but tensions continued to exist between the Sangheili and Jiralhanae, leading to the various factions continuing their conflict even after the fall of the Covenant.
Fighting continued directly after the end of the Great Schism. Initially the Jiralhanae were winning due to the remaining Prophets providing organization and resources, but the tables turned when they went into hiding. The Jiralhanae started fighting against each other giving the Sangheili the advantage.
The Covenant had settled the Jiralhanae on resource-rich worlds. However, since the Jiralhanae are "decades behind the other Covenant races technologically," they had to steal supplies from the Sangheili because they did not have the ability to mine and use the resources they already possessed. Since the Jiralhanae had little communication within their kind, these attacks were highly disorganised.
Massacres Against the Jiralhanae
At the start of the war, some fanatical Sangheili embarked upon a genocidal campaign hunting down and killing all Jiralhanae they came across. One radical group of Sangheili were the Silent Shadow squad under the command of Resa 'Azavayl, fueled by vengeance they sought to slaughter any Jiralhanae they encountered. They had wiped out Jiralhanae forces and their subordinates on an armory near Victors' Truth, the corvette Adherent, the northern highlands of Thalia, and a substation orbiting Sansar. 'Azavayl's Silent Shadow squad even eradicated the former Chieftain of the Jiralhanae Tartarus' crew aboard the Covenant cruiser Rapid Conversion.
The Silent Shadow squad went to attack the SDV-class heavy corvette Elegy's Lament, but they were unaware that it was lead by Atriox and the Banished. After killing several Jiralhanae forces, the Silent Shadow squad encountered Atriox who tried to talk to them into a peaceful resolution. Atriox offered to hire the Sangheili and claimed he had no affiliation to the Jiralhanae responsible for massacring the Sangheili during the Great Schism. As a result, the Silent Shadow Squad began to view Atriox as "other". Despite this, 'Azavayl was not forgiving of Atriox and tried to kill him. Before he could attack Atriox, 'Azavayl's own Silent Shadow squad betrayed him and stabbed him with their energy swords. The remaining members of the Silent Shadow squad remarked that old hatreds were holding them back from a better future, and were then recruited into the Banished as mercenaries.
Massacres Against the Sangheili
In 2553, two hiding San'Shyuum had many Jiralhanae forces working for them. The two San'Shyuum: Boru'a'Neem and Prelate Tem'Bhetek, sought revenge against the Sangheili species for the destruction of High Charity and the Covenant Empire. 'Bhetek's family was killed in the Flood invasion of High Charity, and he held Shipmaster Rtas 'Vadum and the Fleet of Retribution responsible for it.
'Bhetek and his Jiralhanae forces attacked Rahnelo killing many Sangheili civilians in hopes to draw out attention from 'Vadum's CAS-class assault carrier Shadow of Intent. 'Vadum with the help of his crew and a Rahnelo Scion Sangheili female named Tul 'Juran, pursued 'Bhetek when they were ambushed at Duraan. Another battle ensued between 'Vadum's Sangheili forces and 'Bhetek's Jiralhanae forces. 'Bhetek was captured but escaped and regrouped with 'a'Neem only to be betrayed by him. 'a'Neem ordered Yanme'e forces to open fire and mortally wound 'Bhetek. Bleeding and slowly dying, 'Bhetek listened as 'a'Neem revealed his plan to use a miniature prototype Halo ring to commit a genocide of the Sangheili homeworld and its moons, effectively setting the species back for ages. These plans were foiled as 'Vadum and 'Juran invaded the miniature Halo ring and mortally wounded 'a'Neem who attempted to fire the installation and wipe out the Sangheili forces in the system in a last act of defiance. Devastated by his betrayal, 'Bhetek forgave 'Vadum and sacrificed himself by blowing up the miniature Halo as the Sangheili escaped. A scan of the debris field confirmed that nothing of the installation survived while the faction was presumably completely destroyed in the process.
Following the conflict, 'Vadum had grown tired of war. He was debriefed by his comrade, friend, and now superior: Arbiter Thel 'Vadam. 'Vadum told 'Vadam that he will seek out more hiding San'Shyuum, but this time he would determine which ones are innocent and which ones are guilty to prevent more needless suffering.
Enemy of an enemy
There was some overlap between the internal Sangheili conflicts and the internal Jiralhanae conflicts. In September 2557 the tensions between the Sangheili factions on the Outer Colony of Carrow, now in one of the Joint Occupation Zones, turned into all-out civil war. Thars 'Sarov, leader of the more extreme Sangheili faction, temporarily forged an uneasy alliance with a powerful Jiralhanae Chieftain named Hekabe. The Jiralhanae Chieftain was met with much resistance and backlash for this decision on his homeworld. However, unknown to Thars, Chieftain Hekabe had ulterior motives to control the hundreds of thousands of Sharquoi in a hive hidden under the planet's surface and use them to seize power. Hekabe had secretly attained the vertex several years prior during the climax of the Great Schism on Installation 00, which would allow him to command the Sharquoi and ultimately betray Thars.
After it appeared that Thars had discovered Hekabe's true intentions, Hekabe eventually turned on him. The Chieftain ordered his Jiralhanae fleet to attack Thars' Sangheili fleet, devastating much of his remaining forces. However, Hekabe lost almost all of his own fleet as well in the process.
At the same time, Spartan-II Gray Team was released from their cryo chambers aboard Rojka 'Kasaan's ship by Melody Azikiwe. Though Rojka initially sought revenge for the destruction of his homeworld by the Spartans, he eventually forged an alliance with them against the greater threat. Though the Surakan Militia Volunteers managed to destroy Hekabe's flagship, the ORS-class heavy cruiser Foebane, he was able to reach the Sharquoi hive and unleash many of the creatures, controlling them through the vertex. With the Sharquoi army at his command, Hekabe was left virtually unstoppable and planned to rule the galaxy for himself, refusing to enlist the aid of any of the other factions such as the Banished. Thars was eventually defeated and later killed by Rojka and Gray Team, but refused to have his ships destroy their slipspace drives until it was too late, allowing Hekabe to hijack the ships as a means of getting the Sharquoi off the planet.
With Hekabe's threat growing, the human Governor of Suraka Ellis Gass was able to come up with a plan to use a makeshift EMP cannon to disrupt Hekabe's control over the Sharquoi through the vertex. Gray Team, Ellis, Rojka, Melody and a squad of militia assaulted the hive, resulting in the deaths of the militia squad. However, the survivors succeeded in using the cannon on Hekabe and Ellis took the vertex for herself. Through her new command over the Sharquoi, Ellis used the creatures to kill Hekabe in revenge for his murder of her son and had the Sharquoi on Thars' ships crash them, ending the attack on the Sangheili city of Rak. Ellis recalled all of the Sharquoi into the hive and had them slaughter all of the remaining Jiralhanae they could find on the way. Once the Sharquoi were back in the hive and Gray Team, Melody and Rojka had escaped on a Pelican she summoned, Ellis detonated a HAVOK tactical nuclear weapon, destroying the hive and the Sharquoi.
In the aftermath, all three factions, both Sangheili and Jiralhanae, were left decimated. In addition, the humans suffered heavy casualties and their city suffered extensive damage, both from Hekabe's attack and the nuclear detonation. The new leader of Rak, Kaidon Akato 'Dakaj recognized the need for closer relations with the humans on the planet and Rojka agreed to act as envoy for the Sangheili to that end. A scan by the UNSC frigate UNSC Welcome to the Snipehunt confirmed that all of the Sharquoi appeared to have perished in the nuclear blast, but the UNSC planned to keep an eye on the surrounding desert in case any escaped before the detonation.
Attempted Peace Negotiations
In 2558, both sides wanted to reach some sort of agreement. Since the Jiralhanae forces were distrustful of the Sangheili, they agreed on Thel 'Vadam's proposition to have humans mediate their discussion. The UNSC and Swords of Sanghelios realised that pacifying the Jiralhanae forces were in the best interest of everyone, so it was humanity's job to make that clear to the Jiralhanae.
In order to try and establish an agreement, the Swords of Sanghelios requested several high-ranking humans of the UNSC to sit in their discussions and mediate negotiations on the neutral planet of Ealen IV. On part of the humans, Admiral Terrence Hood and Captain Thomas Lasky were chosen as negotiators, the Sangheili were represented by Kaidon Thel 'Vadam and the Jiralhanae by the Elder Chieftain Lydus. They agreed also that each faction should have a strong military force, Spartans, Sangheili Commanders and Jiralhanae Captains alike.
The negotiations were soon attacked by Banshees from Vata Gajat's Mercenary Group, killing multiple Sangheili Commandos. The Jiralhanae Lydus assumed Thel 'Vadam was behind the attack, but Thel denied this accusation and started fighting the Remnant forces as well. Despite surviving the attack, no peace agreement between the Sangheili and Jiralhanae was reached.
In 2559, some Sangheili had resumed hostility, and were winning in their genocidal campaign against the Jiralhanae as the latter had splintered and begun to fight among each other. While the lack of Jiralhanae solidarity gave the upper hand to their Sangheili opponents, ironically the Sangheili had already been fighting amongst each other for several years as well.