- The Covenant were at the peak of their power and reach in 2552, with major assaults on the Human Inner colonies and an invasion followed by a prolonged engagement on their home planet, Earth.
- During the battle at Installation 04, the ominous Flood were released for the first time in over a hundred thousand years.
- Over two months later, Installation 05 was discovered, and the flood was released once more.
- The war finally drew to a close at Installation 00, known more commonly as the "Ark," with the Humans and Sangheili attempting to assassinate the Prophet of Truth, before he could activate the remaining Halo Installations. The alliance was successful in their mission against the Covenant.
- With the help of the Sangheili, the UNSC eventually won the Human-Covenant war.
- A team of ODSTs (Including Kojo Agu and Taylor H. Miles) descend onto Ariel (a small science colony) in response to a distress call from the planet in Operation: FIRESIDE.
- Insurrectionists on Harmony disable the planet's primary communications relay and steal two drydocked freighters.
- Battle of Fumirole: Noble Team member Thom-A293 destroys a CCS-class battlecruiser but kills himself in the process, and Catherine-B320 loses her arm. She subsequently receives a robotic prosthetic replacement.
- SPARTAN-B312 takes part in a counter-insurgency operation on Mamore. The Spartan's skills as a Sabre pilot are exhibited during the op, prompting Colonel Urband Holland to have the Spartan transferred to Noble Team.
- The Slipspace probes from the Archimedes encounter the Covenant ships sent to attack Sigma Octanus IV
- The UNSC Iroquois is tasked with patrolling the Sigma Octanus Star System.
- The Battle of Sigma Octanus IV begins.
- The UNSC win a victory during the battle at Sigma Octanus IV. Commander Jacob Keyes, of the UNSC Iroquois, plays a critical role in this success when he devises the Keyes Loop. Keyes is subsequently promoted to Captain by Vice Admiral Hieronymus Michael Stanforth. Unbeknown to the UNSC, the Covenant place a tracker on the hull of the Iroquois. The importance of Reach to the UNSC is revealed to the Covenant when the Iroquois returns to that world.
- Two months after the death of Thom-A293, SPARTAN-B312 joins Noble Team as Noble Six.
- Noble Team is sent by Colonel Urban Holland on a mission to investigate a downed communication relay at the small farming village of Visegrád on Reach.
- The Covenant begin their invasion of Reach.
- Noble Team discovers the Covenant's presence on Reach for the first time, learning that it was they who sabotaged the Visegrád Relay. Planetary directive WINTER CONTINGENCY is declared.
- Sword Base, an important ONI research facility comes under attack by a SDV-class heavy corvette. The UNSC Army with the help of Noble Team successfully repel the attack and the Corvette is destroyed by orbital support.
- After discovering Reach, the Covenant move on to glass the other planets in the system, including Tribute, resulting in the prolonged Battle of Tribute.
- During the Battle of Tribute, Spartan-II Team Black is sent to Verge to destroy a Covenant Beacon supplying helium-3 to the ships laying siege on Tribute. The team successfully destroys the beacon.
- After the mission on Verge, Fireteam Black is assigned to Operation: BLOWBACK, a covert mission in Covenant-occupied space. Their ship, as well as a Covenant carrier, crash-land on an unidentified moon, igniting the Battle of Line Installation 1-4.
- Margaret-053 is captured by Forerunner Gatherers. Team Black and the Covenant forces on the moon enter into a reluctant alliance to rescue the Spartan.
- Fireteam Black encounters the Forerunner monitor 686 Ebullient Prism en route to rescuing Black-One. The monitor briefly causes rifts between the team to prevent them from rescuing their comrade. Black-One, however, escapes anyway, aided by Reff 'Talamee, and the rest of Team Black stop fighting amongst themselves.
- Team Black manages to enter the control room of Line Installation 1-4, eventually destroying 686 Ebullient Prism with the installation's main gun; they also simultaneously bringing the entire Covenant fleet crashing down. They manage to fight off the surviving members of the Clarity of Faith, then commandeer the vessel and escape the planet.
Early August (sometime from the 6th to the 12th)
- The UNSC Apocalypso crashes in lunar orbit, a Slipspace Anomaly knocking out communications within the Solar System for Seven seconds.
- John-117 and Corporal Harland are debriefed by ONI at Camp Hathcock, Reach, on the events on Côte d'Azur during the Battle of Sigma Octanus IV.
- Battle of Viery: UNSC forces assault an amassing Covenant invasion force in the Viery Territory on Reach, managing to destroy two AA Plasma Cannons and a Covenant Spire. However, a cloaked CSO-class supercarrier (the Long Night of Solace) that spearheaded the Covenant invasion is revealed; the supercarrier destroys the frigate UNSC Grafton, which had been providing local firing support.
- 60% of the remainder of the UNSC fleet is recalled to Reach, destined to arrive within the next 48 hours.
- Operation: UPPER CUT is initiated, as Noble Team retakes the Sabre Program Launch/Research Range at Farkas Lake and joins the battle in orbit, eventually capturing the Covenant corvette Ardent Prayer and using it to deliver the UNSC Savannah's Shaw-Fujikawa Translight Engine, used as a makeshift bomb, to the Long Night of Solace . The Solace is destroyed at the cost of Jorge-052, who sacrifices himself to detonate the drive manually after the timer fails. Moments after the carrier's destruction, a massive Covenant fleet enters in-system.
- Noble Six participates in the Raid on New Alexandria on Reach. Six rescues civilians by helping them escape and managing to severely damage one of the Covenant corvettes laying siege to the outer section of the city by use of a pair of Anti-Aircraft missile batteries. Six eventually meets up with the rest of Noble Team, and the Covenant begin glassing the city. As Noble Team is on their way to a fallout shelter under the Olympic Tower, Catherine-B320 is killed by sniper fire from a Sangheili Field Marshal.
- Dr. Halsey and Cortana choose UNSC Pillar of Autumn, with Jacob Keyes as its commander, for a secret mission Halsey is planning.
- The remainder of Noble Team is evacuated from the destroyed New Alexandria via a Pelican.
- All available Spartan-IIs gather at the FLEETCOM Military Complex on Reach, where they learn of their next mission: to acquire a Covenant vessel, navigate to the Covenant home world, and return with captured Covenant leadership in what was designated Operation: RED FLAG.
- John-117 receives a neural interface upgrade, Mjolnir Mark V armor, and is introduced to Cortana.
- Noble Team is ordered to destroy Sword Base in a torch-and-burn op. During the operation, Noble Team makes their way inside Sword Base where they encounter Dr. Halsey, who asks them to protect her lab. Noble Team is then given direct orders to escort a fragment of Cortana holding crucial data from the Babd Catha Forerunner complex to the Pillar of Autumn. Halsey is then escorted back to CASTLE Base by Jun-A266.
- Cortana has ONI James Ackerson illicitly transferred to the front. This is her revenge for his attempts to eliminate both her and John-117.
- Due to the course of the battle on Reach, Operation: RED FLAG is postponed and the Spartan-IIs are prepared for imminent deployment.
- The UNSC Pillar of Autumn deploys John-117, Linda-058, and James-005 to Reach Station Gamma to destroy the navigation database of the ONI Prowler, UNSC Circumference, docked in Bay 9, after complications arose for the local AI to implement the Cole Protocol.
- The rest of the Spartan-IIs aboard the UNSC Pillar of Autumn are designated Red Team and deployed to the surface of Reach to defend the power generators for the Orbital Defense Platforms. Red Team's pelican sustains heavy damage from attacking Seraph fighters and the Spartans are forced to leap from the dropship as it explodes. Red Team smashes into the surface of Reach at terminal velocity resulting in the deaths of four Spartans.
- First Lieutenant Jake Chapman, along with most of Charlie Company is killed during the Fall of Reach.
- The Covenant, despite sustaining heavy losses, wins a crushing victory at the Fall of Reach. Many of the remaining Spartan-IIs are killed.
- The surviving Spartan-IIs on Reach are separated into two groups. One recovers Vice Admiral Whitcomb and goes into hiding at Camp Independence. The others are admitted into CASTLE Base by Dr. Halsey and receive parts for the Mjolnir Mark V upgrade and new weapons.
- Dr. Halsey learns of Colonel Ackerson's mysterious plans that seem to involve "her" Spartans.
- The UNSC Pillar of Autumn engages a Covenant Supercruiser operating as a flagship in the space battle over Reach, and was able to successfully destroy it where much more powerful vessels had failed.
- By this point, the UNSC Pillar of Autumn had recovered John-117 and a critically wounded Linda-058 from Reach Station Gamma, then proceeded to land at the ship-breaking yards in Aszod to retrieve a fragment of Cortana from the Spartans B312, Carter-A259 and Emile-A239.
- The three surviving members of Noble Team arrive at Aszod; Carter's Pelican becomes too badly damaged to continue carrying them all, so Noble Six and Emile-A239 deploy to the ground.
- Battle of Aszod: The Covenant forces throughout Aszod attempt to stop Noble Six and Emile from reaching the Pillar of Autumn, going so far as to deploy Scarab Walkers against them. Carter sacrificially collides his damaged Pelican into a Scarab engaging Emile and Six, destroying it. Emile and Six proceed to push towards the ship breaking yards (now almost totally controlled by the Covenant, who begin making preparations to destroy it) and rendezvous with the scant UNSC forces still attempting to defend the facility. After clearing most of the facility, Six, Emile, and several Trooper squads meet up with Captain Jacob Keyes, successfully delivering the fragment of Cortana. However, Emile is killed by a Sangheili Zealot's Energy Sword.
- SPARTAN-B312 destroys a CCS-class battlecruiser with a mass driver to allow Pillar of Autumn to escape Reach. The Spartan makes a last stand against several Sangheili warriors soon after, during which he met his end. With Spartan-II Red Team currently unrecoverable, the Autumn jumps away from the system, with Cortana using Forerunner navigation data from the Sigma Octanus IV artifact as well as the Babd Catha Forerunner Complex.
- SPARTAN-B312 is killed by the Covenant.
- The planet Coral is reported glassed by the Covenant.
- The Battle of New Jerusalem takes place. After a chain of events involving a group of ODSTs attempting to steal gold from a bank vault (Treason), the 26th Marine Expeditionary Force is nearly wiped out, leaving the Rookie as one of few survivors. Eventually, the UNSC forces cut their losses and fall back to Earth.
- Lieutenant Wagner returns to Earth and reports on the evident destruction of Reach to Colonel James Ackerson, Admiral Hood, and Major General Strauss at HIGHCOM Facility Bravo-6 in Sydney, Australia.
- The UNSC Pillar of Autumn arrives at Alpha Halo.
- The Battle of Installation 04 begins.
- John-117 is awoken from cryo sleep and tasked with preventing Cortana's capture by the Covenant.
- John-117 and Cortana escaped the ship aboard a Bumblebee escape pod, which later crashed on the ring.
- The UNSC Pillar of Autumn is forced to crash land on the ring. A significant portion of its crew survives to continue the fight on the ground.
- Captain Keyes and the others in his escape pod are captured by the Covenant. Only Keyes escapes summary execution and is taken to CCS-class battlecruiser, the Truth and Reconciliation.
- John-117 leads a Marine force to board the Truth and Reconciliation. They recover Keyes and escape aboard a Spirit Dropship. Keyes orders John-117 to find the "Silent Cartographer," a map room that will reveal the location of the Control Room. Keyes leaves to lead an assault on a Covenant weapons cache.
- Having found the map room and the pertinent information, Cortana directs a Pelican to take them to the area where the ring's Control Room is. Upon reaching the Control Room, Cortana transfers to the Control Room's computer and directs John-117 to stop Keyes from accessing the 'weapons cache', really a Forerunner research and quarantine facility on the Flood.
- John-117, without Cortana, enters the facility but is unable to locate Keyes. Instead, he discovers the "Flood" and is forced to retreat. He links up with marines and takes command of the surviving members of Keyes' team and together they fight their way to the extraction point. However, the Spartan is conscripted by the "Monitor of Installation 04" and teleported away before he can be evacuated with the surviving members.
- The Monitor takes John-117 to the ring's "Library" to retrieve the "Activation Index," a device needed to activate the ring's primary weapon. Having retrieved the Index, the Monitor teleports the Spartan-II back to the Control Room where an irate Cortana confiscates the Index and prevents the ring from firing. John-117 and Cortana flee a now-hostile 343 Guilty Spark and Sentinels.
- To forestall the ring from being activated manually, John-117 disables the ring's three Pulse Generators. This is to buy time to destroy the ring by activating the auto-destruct on the UNSC Pillar of Autumn. Only Keyes neural interface can activate the ship's self-destruct, and John-117 embarks once more to rescue the captain.
- Cortana teleports the Spartan to the Flood-overrun Covenant cruiser where Jacob Keyes is being held. Alas, the pair arrives too late to save Keyes, who has been assimilated into a Proto-Gravemind. John-117 retrieves the neural interface after plunging his fist into Keyes' skull. Cortana downloads the necessary codes, and the duo returns to the UNSC Pillar of Autumn on a captured Banshee.
- 343 Guilty Spark overrides the auto-destruct codes, forcing John-117 to manually destabilize the ship's still-functioning fusion cores. John-117 and Cortana then escape the resulting explosion and the fragmentation of the ring aboard a Longsword.
- Staff Sergeant Johnson, Lieutenant Haverson, Warrant Officer Shiela Polaski, Corporal Locklear also escape the destruction of the ring aboard a Pelican.
- John-117 and Cortana rendezvous with Johnson's group in Installation 04's debris field. They also recover a set of cryotubes, one which contains Linda-058.
- The Battle of Installation 04 ends. Gunnery Sergeant Marcus Stacker and PFC Chips Dubbo survive, but it is unknown how they do.
- Skirmish over Threshold: As the Longsword and Pelican are not jump capable, the UNSC survivors assault and capture the Ascendant Justice, a Covenant warship and apparently the flagship of the Covenant Fleet sent to Installation 04. They then set course back to the fallen world of Reach.
- Jersey Morelli confronts his mother about having an affair.
- A Time/Date Record Anomaly continues from this point in the narrative, resolved below on Sept 7, 2552.
After the recovery of a Forerunner crystal on Reach, the members of an organized resistance comprised of Spartans from Reach and survivors of the Battle of Installation 04 are in some sort of time anomaly, causing them not to be in the "correct timeline." What is known is that the events taken place with the possession of the device take place after September 7.
- The Time/Date Record Anomaly of Sept 23, 2552 most likely rejoins the correct timeline.
- Dr. Halsey and members of Red Team discover an enigmatic shard in a Forerunner installation below CASTLE Base. They are besieged by Covenant forces who seek to possess the shard.
- Dr. Halsey initiates Operation: WHITE GLOVE, destroying CASTLE Base, thus preventing the Covenant forces above from capturing any UNSC and/or Forerunner technology. Halsey and members of Red Team are protected from the resulting detonation when they hide in the caves of Menachite Mountain beneath the base.
- John-117, Cortana, Sgt. Johnson, Lieutenant Haverson, Warrant Officer Polasky, and Corporal Locklear aboard the captured Covenant flagship Ascendent Justice make a slipspace jump to Reach, a temporal anomaly sending them over two weeks into the past, twelve days before the UNSC Pillar of Autumn carrying John-117 would emerge from slipspace at Installation 04.
- John-117 recognizes the Oly Oly Oxen Free tune detected by Cortana being broadcast from the surface, and is aware of the Spartan survivors on Reach. John and the other survivors enact a harrowing rescue attempt initiated by an inter-atmospheric slipspace jump based on knowledge Cortana gleaned from the systems of Ascendent Justice.
- With the assistance of Huragok teams aboard the ship, the Ascendent Justice is connected to the damaged UNSC warship Gettysburg for additional reactor energy to facilitate the frequency between slipspace jumps.
- The remaining Spartans, Admiral Whitcomb, Dr. Halsey, Sgt. Johnson, Lieutenant Haverson, Warrant Officer Polaski, and Corporal Locklear jump to Slipspace aboard the Covenant/Human hybrid ship UNSC Gettysburg-Ascendant Justice, and discover that the shard seems to have the ability to bend both space and time. The crystal forms a massive bubble around the ship within slipspace, a furious battle ensues inside the bubble with weapon guidance all but impossible.
- A group of Sangheili hidden on the ship sabotage the power conduits, Polaski, Anton-044 and Li-008 are presumably flash-vaporized along with the Sangheili when a large plasma globule splashes down on top of their position.
- The Gettysburg/Ascendant Justice arrives for refit and provisioning at the United Rebel Front base in the Eridanus system.
- Vice Admiral Whitcomb secures a meeting with rebel leader Governor Jacob Jiles, attempting to reason with him that the Covenant would most likely arrive to claim the Forerunner Crystal. When several Covenant warships emerge from Slipspace, Jiles agrees to help the UNSC forces.
- Dr. Halsey sedates Kelly-087, and absconds with her on Governor Jiles' personal ship, the Chiroptera-class Beatrice. Before she goes, she gives the Forerunner crystal to Corporal Locklear, with implicit instructions to destroy it.
- A large Covenant fleet enters the Eridanus system, forcing the hand of Admiral Whitcomb; he pulls out and retreats, as does the Gettysburg/Ascendant Justice, leaving the rebels to their doom.
- Governor Jiles is killed and the Eridanus Rebels are destroyed during the Massacre at Eridanus Secundus. Eridanus Secundus itself is destroyed by the Covenant.
- UNSC forces aboard the Gettysburg/Ascendant Justice win a crucial victory at the Battle of the Unyielding Hierophant. Grace-093, Lieutenant Haverson of ONI and Vice Admiral Whitcomb are killed. The survivors, now aboard the singular UNSC Gettysburg and with a captured Covenant slipspace drive, depart for Earth to warn of the impending Covenant invasion.
- An Edict from the Office of the High Prophet of Truth orders that in the virtue of the Jiralhanae, all Sangheili posted with the Fleet of Profound Solitude, the Fleet of Tranquil Composure, and the Fleet of Inner Knowledge to be completely replaced by Jiralhanae. Each removed Sangheili to be assigned other duties. In celebration, all imprisoned or criminal Jiralhanae were released, rehabilitated, and pardoned.
- The UNSC Red Horse is sent to Installation 04's debris field in the Soell System to scout the area for survivors. They detect the prison ship Mona Lisa and deploy Marines into the ship. Soon after, it is revealed that the ship is infested by the Flood and was used by the Office of Naval Intelligence as a laboratory to test the parasite on prisoners. The infestation is contained as the Red Horse unleashes a Shiva-class Nuclear Missile on the Mona Lisa, destroying it.
- Installation 03 is discovered by the freighter Floral Express while fleeing the destruction of Tribute.
- Battle Group Rhino finds twelve unidentified large ships broadcasting outdated civilian IFF codes near Enceladus. Eleven ships immediately jump to slipspace and Rhino engages the remaining ship. UNSC Totem Lake is subsequently lost with all hands and Rhino disengages.
- John-117 is outfitted with the Mjolnir Mk. VI aboard Cairo Station.
- The Fleet Commander of Particular Justice is deemed a Heretic by the Covenant High Council and sentenced to death.
- John-117 and Staff Sergeant (now promoted to Sergeant Major) Johnson are decorated at a ceremony aboard Cairo Station. Captain Keyes is awarded posthumously and the decoration is entrusted to Lieutenant Commander Keyes.
- The Battle of Earth begins.
- The High Prophet of Regret's fleet arrives at Earth, not expecting any human presence. The vastly outnumbered Covenant force attacks without seeking support from other Covenant forces.
- John-117 foils a Covenant attempt to demolish Cairo Station and commandeers the Covenant's antimatter charge to destroy an assault carrier.
- The High Prophet of Regret's assault carrier breaks through the orbital defenses and initiates a ground assault on New Mombasa and the surrounding area.
- Alpha-Nine, consisting of Corporal Taylor "Dutch" Miles, Lance Corporal Kojo "Romeo" Agu, PFC Michael "Mickey" Crespo, and the Rookie, led by Captain Veronica Dare and Gunnery Sergeant Edward Buck, launch into New Mombasa from the UNSC Say My Name. However, the squad is scattered after the Prophet of Regret's carrier jumps to Slipspace.
- John-117 deploys to New Mombasa in an attempt to capture the High Prophet of Regret. Before this can happen, the assault carrier initiates a slipspace jump within the city, escaping Earth and severely damaging the city in the process. The UNSC In Amber Clad, with John-117 aboard, successfully pursues the carrier through slipspace.
- Immediately after Regret's retreat, an additional Jiralhanae-led Covenant occupation force is inserted to New Mombasa, as a part of the Prophet of Truth's plan to eventually replace the Sangheili with the Jiralhanae as the Covenant's primary warrior race. For the rest of the day, UNSC and Covenant forces battle for control of the city. Meanwhile, the Covenant fleet begins the process of uncovering the Ark Portal.
- Gunnery Sergeant Buck leads the defense of Tayari Plaza. After he attempts to find the missing Captain Dare, he finds her crashed SOEIV, but no trace of her. He manages to link up with Lance Corporal Agu.
- Marine forces under the command of an unknown Colonel within New Mombasa start to regroup, managing to take back the Uplift Nature Reserve. However, the New Mombasa Orbital Elevator collapses, killing the Colonel and scattering the Marine forces into disarray.
- The ONI Alpha Site in New Mombasa is destroyed by the combined forces of the UNSC and the New Mombasa Police Department, led by Corporal Miles and PFC Crespo, in order to prevent the Covenant from accessing it. During the attack, Commissioner Kinsler activates the fire extinguishing systems in the underground data facility, flooding its sublevels with argon. The argon kills Daniel Endesha, who had been on Sublevel 9. The explosion also inadvertently awakens a large Drone hive in the tunnels below the base.
- Thel 'Vadam, formerly Fleet Commander of Particular Justice, is made the Arbiter by the High Prophets of Truth and Mercy.
- Gunnery Sergeant Buck and his squad regroup and find a way out of the city through Kikowani Station. They manage to navigate through the Covenant-occupied station aboard a captured Phantom dropship and escape the city, but abruptly turn around when they receive a distress call from Captain Dare.
- The Arbiter destroys a band of heretics and 'recovers' the Oracle during the Battle of the Gas Mine.
October 20 through November 2
- The Spartan-II Blue Team is deployed to defend Earth during the two weeks that follow the initial Covenant invasion. They participate in numerous combat missions on and around Earth, including Battle of Ross Island in the Antarctica and an underwater operation in the Yucatan Peninsula.
- Sadie Endesha and Mike Branley manage to escape New Mombasa.
- The Battle of Mombasa ends with a Covenant victory.
- In the dawn, Covenant reinforcements sent by the Prophet of Truth continue arriving at Earth to uncover the Portal.
- Alpha-Nine recovers Vergil and escapes from the data facility.
- Covenant forces attempt to capture Vergil from the Squad during the Battle of the Waterfront Highway. The Squad manages to hold out and eventually escapes New Mombasa as the city, along with its surroundings are glassed by the Covenant excavation fleet.
- The Battle of Onyx begins.
- The standby signal rouse the Sentinels of Onyx, who destroy the UNSC base on the world.
- Spartan-III Gamma Company Team Gladius is totally wiped out by the Sentinels. Team Katana is forced into Zone 67, and Team Saber loses contact with Camp Currahee.
- Kurt-051, SPCO Franklin Mendez and Petty Officers Tom-B292 and Lucy-B091 begin to form a resistance against the Sentinels.
- The UNSC Agincourt, in high orbit above Onyx, is destroyed by Sentinels.
- Chief Mendez declares emergency code Bloody Arrow over the UNSC COM.
- The Battle of Installation 05 begins.
- The UNSC In Amber Clad arrives at Installation 05, following the High Prophet of Regret's assault carrier. John-117, aided by ODSTs and Marines, pursue the High Prophet to a Forerunner temple and assassinate him. However, orbital bombardment from the escorting fleets of the newly arrived High Charity destroys the temple. John-117 is thrown into a lake and captured by Gravemind.
- The Great Schism begins.
- The UNSC Dusk arrives at Delta Halo and witnesses the final stages of the Battle of Installation 05. She retreats to Earth via a Slipspace Wake caused by the Forerunner ship.
- Dr. Halsey and Kelly-087 arrive at Onyx and are trapped along with the rest of the human survivors. Halsey sends a distress signal piggybacked on Cortana's October 28 signal.
- The Battle of Havana is fought; William-043, Linda-058, and Frederic-104 manage to stall the Covenant's forces in Havana by destroying the Orbital Elevator Tallo Negro del Maiz. They receive new orders from Dr. Halsey to redeploy from Earth to Onyx. The UNSC survivors are forced to retreat underground as a newly arrived Covenant fleet begins landing troops.
- Voro 'Mantakree is promoted to Fleet Master by Imperial Admiral Xytan 'Jar Wattinree, and is given the mission to intercept the Spartans heading for Onyx and to retrieve the Forerunner technologies said to be there.
- Danforth Whitcomb's NOVA Bomb detonates on board the Covenant supercarrier Sublime Transcendence, in orbit above the Sangheili outpost world Joyous Exultation, destroying every ship in the vicinity, and also scorching a quarter of Joyous Exultation and completely vaporizing the nearby moon Malhiem.
- The Dusk arrives at Earth after mere hours. Upon arrival, she is reassigned to Battlegroup Stalingrad. The group jump to Onyx via the captured Covenant Ships Slipspace wake.
- Blue Team arrives at Onyx aboard the captured Covenant destroyer Bloodied Spirit, and links up with Kurt-051, Kelly-087, Dr. Halsey, Chief Mendez, and the Spartan-III's. They proceed to push forward into Zone 67.
- Battlegroup Stalingrad arrives at Onyx to retrieve the survivors, but is unsuccessful due to the Covenant presence. The UNSC battlegroup is destroyed.
- After an emergency mission redirect initiated by the ONI AI Endless Summer, Kurt-051 and his forces proceed to destroy the Sentinel Factory in Onyx's polar region and retrieve Team Katana, cryogenically frozen in Slipspace Field Pods.
- Onyx is found to be an artificial Shield World built by the Forerunners.
- Frederic-104 is promoted to Lieutenant, Junior Grade by Kurt.
- Kurt-051 dies after detonating 2 FENRIS warheads inside the Onyx Shield World structure.
- Halsey and the other survivors hold off pursuing Covenant troops long enough to flee to the safety of the Shield World and seal the entrance.
- Sentinels annihilate the remaining Covenant forces as Onyx's artificial landmass disintegrates.
- The Battle of Onyx ends.
- The Arbiter is sent to Delta Halo to retrieve that installation's Activation Index. The Arbiter is successful, but is then betrayed by Tartarus, who casts the Arbiter into the depths of the ringworld. The Arbiter is subsequently captured by Gravemind.
- Gravemind sends John-117 and the Arbiter to secure the Index and prevent the installation from being activated. John-117 is teleported to High Charity while the Arbiter is teleported close to the Control Room. Both are aided by the chaos of the Covenant civil war.
- John-117 is unable to prevent the Index from leaving High Charity with Tartarus. Instead, on the advice of Cortana, the Spartan stows away aboard the Forerunner dreadnought that is carrying the High Prophet of Truth to Earth. Cortana remains behind to ensure the destruction of Installation 05, should that become necessary.
November 3 through November 7
- Cortana sends a slipspace message to Earth informing them that John-117 is en route.
- The Arbiter reaches the Control Room but is unable to prevent Tartarus from activating the ring. With the help of humans and the Sangheili, the Arbiter kills Tarturus. This allows Commander Keyes to physically remove the Index from the Core, temporarily interrupting the firing sequence.
- Due to the interruption, Installation 05 automatically sends a signal to all other Forerunner installations in the galaxy, activating and placing them in standby mode.
- The Prophet of Truth arrives in the Sol system aboard the Forerunner dreadnought, and proceeds on course towards Earth at near-relativistic speeds.
November 8 through November 17
- The Battle of Mare Erythraeum takes place on Mars, resulting in a victory for the Covenant Loyalists. Following the battle, Colonel James Ackerson, leader of the UNSC forces on Mars, is taken hostage and interrogated by the Jiralhanae Commander Lepidus on board the Covenant cruiser Triumphant Declaration, who tells them of a Forerunner device linked to the Halo Array known as the Key of Osanalan, and that it can be found in Cleveland, Ohio.
- Battle of Cleveland: The Covenant Loyalist Army invades Cleveland via the cruiser Harbinger of Piety. The Loyalist forces, under the direction of the Minister of Inquisition, are searching for the whereabouts of the Key of Osanalan (In reality, the Key of Osanalan was made up as a ploy by James Ackerson to prevent Cleveland from being glassed and to save the life of his brother Ruwan.) When Ruwan learns the reason the Covenant are invading, he devises a plan with Sergeant Twyker, involving being injected with organic tracking material. Ruwan confronts Lepidus at Chagrin Falls and tells him that he is the Key of Osanalan. Ruwan is taken on board the Harbinger of Piety to meet with the Minister of Inquisition, who vows to spare Earth in exchange for Ruwan's knowledge. Ruwan plays along until the Prophet informed him that he knew of the organic tracking material, believing it to be a rescue beacon. In reality, it was a tracking vector. With his last words, he told the Prophet to "kiss [his] ass" right before Nassau Station used its Magnetic Accelerator Cannon to destroy the Harbinger of Piety.
- Lepidus realizes that the Key of Osanalan does not exist, and returns to the Triumphant Declaration to confront Colonel Ackerson about this deception. Ackerson mocked the Jiralhanae Chieftain, saying that his belief in the Key of Osanalan was just one example of how their pompous arrogance would cost them the war. Moments later, Ackerson is beheaded by Lepidus.
- Sergeant Major Johnson arrives back on Earth and gets clearance from Lord Hood to question Vergil about the artifact the High Prophet of Truth is digging up near Voi.
- The Forerunner dreadnought enters Earth's atmosphere. On-board is Master Chief Petty Officer John-117, who ejects from the ship, using a hatch as a heat shield upon re-entry. Meanwhile, Battle of Sector Six takes place at Pawa, Afghanistan. There, numerous Marines of the 2nd ODST Battalion track John-117's descent. John-117 eventually lands in the East African jungle near Mount Kilimanjaro, going unconscious for hours.
- The Prophet of Truth's Dreadnought lands in the middle of The Artifact near Voi, Kenya. During the last few weeks, Truth's fleet of twenty-odd CCS-class battlecruisers and two CAS-class assault carriers have almost entirely uncovered the artifact and occupied the entire region surrounding it.
- John-117 leads a UNSC counterattack to regain lost UNSC ground, retaking a jungle near Mount Kilimanjaro and securing the area around Crow's Nest, an underground facility used by the UNSC Defense Force to coordinate local Army, Navy, and Marine forces. The UNSC organize a small strike force led by John-117 to attack the Covenant Loyalist-held Tsavo Highway, and thus have a platform for a strike at Voi.
- The UNSC Marine Corps is able to retake the Tsavo Highway, and it serves for a staging ground for the battalion that would retake Voi. The UNSC forces stationed already at Voi have suffered severe casualties, with only pockets of Marines drawn in a scattered last defense against numerically superior and vehicle-supported Loyalist forces.
- Fleet Admiral Sir Terrence Hood declares that it is imperative that John-117's attack force retake Voi. Lord Hood's plan is that he will launch a low-level aerial strike on Truth's fleet above the Forerunner artifact, and prevent him from activating the presumed Ark. However, a Loyalist anti-aircraft emplacement stationed at Voi is preventing Hood's fleet from advancing; John-117 is to retake Voi and destroy the anti-aircraft emplacement to allow Hood to attack Truth's fleet, and prevent him from activating the Ark.
- They later defeat the AA guns, opening up a pocket in the Loyalist Defenses for the Home Fleet to come in and engage Truth's forces. They are unsuccessful in damaging the Dreadnought, and it escapes through the Portal. At the same moment, the Flood-infested Indulgence of Conviction slips in, crashing in Voi.
- Lieutenant Commander Miranda Keyes orders her troops to fight to the core of the Flood battlecruiser and overload its reactors, destroying the ship and the town and any Flood along with it. The Fleet of Retribution, having followed the Flood infected ship from their blockade at Installation 05 to its arrival at Earth, begins to drop forces to push towards the ship, seeking something that lay within it. It isn't until they come back into Traxus Facility that the Sangheili tell the UNSC that Cortana, the AI formally attached to the Spartan-IIs, the UNSC Pillar of Autumn, and companion of John-117, is supposedly on the downed ship.
- The joint UNSC/Separatist force press on to the crash site and eventually break through, fighting past hordes of Flood. With the remaining forces giving cover to John, he enters its Flood created carcass and, with surprise assistance from 343 Guilty Spark, the former Monitor of Installation 04, retrieves what is believed to be Cortana. After the ground forces are evacuated safely the Sangheili begin to glass the immediate area, annihilating the Flood and eliminating any chance of further Flood infection, effectively ending the Battle of Tsavo.
The Prophet of Truth was killed in the later months of 2552 along with a sizable Covenant armada, leaving the remaining Covenant forces crippled and leaderless as well as the full destruction of the Gravemind and its Flood forces.
Battle of Installation 00
- Main article: Battle of Installation 00
- The Prophet of Truth, having successfully activated the artifact in Kenya, left Earth through a Slipspace portal to the Ark, a Forerunner structure that could be used to fire all of the Halo installations at once.
- After returning to the Sangheili cruiser Shadow of Intent with a construct believed to be Cortana, John-117 discovers that it is not Cortana after all, but a looped message from her. It contains vital information nonetheless: the message states that the Gravemind has firmly entrenched itself and the Flood on the Ark, but that there is also a way to destroy it without lighting the Halo rings.
- Despite Lord Hood's urging, Commander Miranda Keyes, along with John-117, Arbiter Thel 'Vadam, Shipmaster Rtas 'Vadum, and 343 Guilty Spark, took ships from the Home Fleet with a small fleet of Separatist cruisers led by the carrier Shadow of Intent and proceeded through the Slipspace portal and arrived at the Ark.
- The Separatists on arrival immediately engaged the Loyalist fleet stationed to protect the Prophet while the UNSC Forward Unto Dawn deployed Marine Shock Troopers to locate Truth. Despite being outnumbered 3-to-1 in space, the Separatist fleet made short work of the Loyalist space forces, likely due to the Sangheili superior tactical skills. The Covenant Loyalist Fleet had been destroyed and the only Loyalists left alive were Truth and his forces on the Ark. 343 Guilty Spark, now working with the UNSC-Separatist Alliance, led John-117 and the Arbiter to the Ark's Cartographer where they pinpointed the Prophet's location. By then Truth had already holed himself in a secure facility near the core of the Ark and had activated a large barrier around the entire core to ward off the attacking forces.
- The UNSC ground forces, along with Separatist reinforcements, assaulted three emitter pylons that powered a small section of the barrier. When the defending forces were eliminated and the pylons shut down, the Shadow of Intent began its attack on Truth. At the same time the former Covenant city ship High Charity arrived from slipspace and crashed on the Ark. Debris, spread from the city-ship, heavily damaged the Shadow of Intent and helped spread a new Flood infestation on the Ark as a means to stop Truth and to be safe from the Halo activation array. Without any heavy support, John-117 and the Arbiter personally attacked the facility where Truth was to activate the Halo installations.
- Sergeant Major Johnson, who had disappeared during the assault on the towers, was in fact captured by Truth's forces and was being tortured into pressing the final switch to activate the other Halos, as fail-safes within the structure prevented anyone other than a Forerunner or a Reclaimer to do so. With John-117 and the Arbiter too far away to stop Truth, Commander Keyes grabbed a Pelican dropship and crashed into the facility to rescue Johnson, but was killed soon thereafter by Truth himself. The Gravemind dispatched Flood forces to assist John-117 and the Arbiter in order to prevent the activation of the Halo Array system; their support enabled John-117 and the Arbiter to reach Truth in time. The Arbiter personally killed Truth with his sword. However, the Gravemind betrayed the two of them after knocking them out of Keyes' Pelican,(then piloted by Johnson and also carrying the Commander's body), thwarting their plan of escape and forcing John-117 and the Arbiter to fight their way outside.
Raid on High Charity
- 343 Guilty Spark, John-117 and the Arbiter learned that the Ark's Forge had created a replacement for the destroyed Installation 04. Realizing that they could use it to fire a tactical pulse which would eliminate the Flood infestation, they decided to light it. However, this new Halo lacked an Activation Index. The only way they could get another Index was to rescue Cortana from High Charity since she still possessed the Index from Installation 04. John-117 and the Arbiter successfully entered the ruins of High Charity and recovered Cortana, using a partly-damaged Pelican to escape while High Charity was destroyed.
End of Human-Covenant war
- With the Index secure, the Arbiter ordered that the remaining UNSC forces relocate to the Separatist ships and escape back through the Slipspace portal to Earth while he, John-117 and Sergeant Major Avery Junior Johnson moved in on the Control Room of the incomplete Halo, the latter coming in alone aboard the Forward Unto Dawn. The Dawn was left behind as the only means of escape. By now what was left of the Gravemind had relocated to the new Halo. The Arbiter, Johnson, and John-117, along with 343 Guilty Spark made it through waves of Flood to the Control Room. 343 Guilty Spark explained that the new Halo would be ready to fire within a few days, but with Flood moving in all around them Johnson explained the necessity of firing the ring immediately, prompting Guilty Spark's protest that activating Halo now would destroy both the Ark and the installation, and Johnson's subsequent reply of, "deal with it," as he prepared to activate the ring. Unwilling to let "his" ring be destroyed again, Spark went rampant, attacking Johnson, John-117, and the Arbiter. However, with the help of mortally-wounded Johnson and his Spartan Laser, John-117 managed to destroy Guilty Spark and start the firing sequence for the new Halo. Just after Johnson's final remarks to John-117, he died. John-117 and the Arbiter then escaped the control room by taking Johnson's Warthog to the Forward Unto Dawn. Due to the unstable, incomplete nature of the ring as it fired, the Slipspace portal collapsed as the Dawn entered it, causing it to be sheared in two. The forward half with the Arbiter made it to Earth, crashing into an ocean (presumably the Indian Ocean). Cortana and John-117 were in the hangar area at the time of the separation, and were thus left in the aft section of the ship, which fell out of Slipstream space at an unknown location, eventually drifting towards Requiem.
- Codename: SURGEON filed a report to Codename: USUAL SUSPECTS entitled "Historical/Psychological Analysis of Cole, Preston J." The report was filed from UNSC Point of No Return in synchronous orbit on the far side of Luna.
- Battle of Sigma Octanus IV
- Battle of Tribute
- Battle of Verge
- Battle of Line Installation 1-4
- Fall of Reach
- Battle of Installation 04
- Massacre at Eridanus Secundus
- Operation: FIRST STRIKE
- Battle of Earth
- Battle of the Gas Mine
- Battle of Installation 05
- Battle of Onyx
- Battle of Mare Erythraeum
- Battle of Installation 00
The Great Schism
Quarantine of the Flood
- The first and second parts of 2552 equal 2+5=7 and 5+2=7, another one of Bungie's 7 references.
- ↑ Halo 2, level Cairo Station
- ↑ Halo: Ghosts of Onyx
- ↑ Halo 3, level The Storm
- ↑ Halo: Combat Evolved, level 343 Guilty Spark
- ↑ Halo: Combat Evolved, level The Library
- ↑ Halo 2, level Sacred Icon
- ↑ Halo 3, level The Ark
- ↑ Halo 3, level The Covenant
- ↑ Halo: Helljumper Issue 1 page 3 "UNSC Operation 2552-JA-8 Codename: Fireside"
- ↑ Halo: Reach level Noble Actual
- ↑ Halo: Evolutions - Essential Tales of the Halo Universe - Blunt Instruments
- ↑ Halo: Evolutions - Essential Tales of the Halo Universe - Dirt
- ↑ Bungie.net: Halo 3: ODST Field Guide
- ↑ Halo: First Strike - page 104
- ↑ Halo: The Flood, page 1
- ↑ Halo: First Strike
- ↑ Halo Wars: Halo Timeline
- ↑ Halo: Evolutions - Essential Tales of the Halo Universe - The Mona Lisa
- ↑ 19.0 19.1 ELEVENTH HOUR – Transmission 5
- ↑ Halo Legendary Crate Naval Records Archive
- ↑ 21.0 21.1 Bungie.net: Bungie Weekly Update: 01/23/09
- ↑ Halo: Evolutions - Essential Tales of the Halo Universe - The Impossible Life and the Possible Death of Preston J. Cole, pages 413-487