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2553 was the first year that saw relative peace for humanity as the Covenant ceased to exist following its collapse during the Great Schism and its defeat at the end of the Human-Covenant War. The war formally came to an end in March 2553 with a treaty agreed upon between the Unified Earth Government and the Swords of Sanghelios. Despite the end of large-scale hostilities and the absence of humanity being threatened with extinction, new factions sprung up in the power vacuum left behind by the ancient empire and new battles followed. 2553 was a year that saw the United Nations Space Command make major advances in terms of its technological prowess and military capability, despite a lesser reach over the galaxy incurred by the loss of so many colonies and assets during the decades of war. Ex-Covenant societies were faced with the challenge of redefining their purpose in a galaxy that no longer provided them with the rigid structure the hegemony had. Some sought revenge on humans and some on others from the Covenant. Some sought to be left alone and others made grabs for power. And some, such as Thel 'Vadam's Swords of Sanghelios, made in-roads with their former enemies for the mutual benefit of all parties.
- January 1: After days of no indications that they were soon to be rescued following the destruction of Glyke and the UNSC Vacuna, Gray Team enters cryostasis.
- At some point before January 7, Jun-A266 visits Sarah Palmer aboard Lockhart medical station to recruit her into the nascent SPARTAN-IV program.
- January 7: The first class of the SPARTAN-IV program, consisting of 145 volunteers, begin their augmentation procedures.
- January 8: The Unified Earth Government formally regains its power back from the United Nations Space Command. The newly-elected president, Dr. Ruth Charet gives her inaugural address.
- January 15: An ONI agent known as Oberon 5 interrogates the San'Shyuum Alo Sebukah, former Vice Minister of Abnegation for the Covenant, at the Midnight Facility.
- January 26: The Office of Naval Intelligence black operations cell, Kilo-Five, is formed by CINCONI Admiral Margaret Parangosky.
- January 27: Kilo-Five departs Earth aboard the UNSC Port Stanley.
- January 27–31: The UNSC Security Council takes actions to address the state of human colonies. Lord Terrence Hood plans to journey on a diplomatic mission to surviving worlds in order to discuss plans for reconstruction. Kilo-Five supplies arms to the Servants of the Abiding Truth on New Llanelli in the Brunel system. The unit also retrieves an Office of Naval Intelligence spy, Mike Spenser, from a UNSC listening post on Reynes. Commander Musa Ghanem holds a meeting with several admirals at HIGHCOM Facility Bravo-6 to discuss the recent creation of the SPARTAN-IV program and the independent branch of the UNSC Armed Forces, Spartan Operations.
- January 31: Dr. William Arthur Iqbal sends a message to his colleagues concerning the ongoing study of Forerunner artifacts and the Excession in particular.
- February 1: Jul 'Mdama joins Avu Med 'Telcam and the Servants of the Abiding Truth in their opposition to the Arbiter, Thel 'Vadam.
- February 1–15: Terrence Hood arrives at Sanghelios to meet with Thel 'Vadam and to formalize a treaty of peace between the Unified Earth Government and the Swords of Sanghelios. Hood also invites Thel to attend a memorial ceremony on Earth dedicated to those who died during the war. Lucy-B091 discovers a population of Huragok within Shield World 006 and the human survivors within the shield world are able to establish communications with the ONI ships prowling the debris field of Onyx. The Huragok bring the shield world into normal space from the slipspace bubble it was residing in, allowing ONI to retrieve Dr. Catherine Halsey, SCPO Franklin Mendez, and the Spartan-IIs and SPARTAN-IIIs of Blue Team. Dr. Halsey is arrested by Captain Serin Osman. ONI dispatches science teams to recover Forerunner technologies from the shield world, designating the sphere as ONI Research Facility Trevelyan at Dr. Halsey's request. Jul 'Mdama is captured by Kilo-Five and is transported to ONIRF Trevelyan.
- February 15: Jun-A266 meets with Edward Buck on Sundown, where the latter had travelled with Veronica Dare for a vacation while on shore leave. Jun offers Buck the opportunity to join the SPARTAN-IV program. To Jun's surprise, Buck declines the offer, feeling that accepting would mean abandoning his team and Dare.
- February 19: 000 Tragic Solitude begins to research and catalog the myriad Flood forms and reflects on the uncomfortable truth the parasite exemplifies.
- February 28: The Shadow of Intent is stationed at Earth.
- Andrew Del Rio is chosen as the commanding officer of the UNSC Infinity.
- March 3: In a ceremony presided over by Lord Hood, a monument is erected in the Kenyan savannah near Mount Kilimanjaro to officially mark the end of the Human-Covenant War and to remember those who gave their lives in it. After the ceremony, the Shadow of Intent departs for Sanghelios. Admiral I. Zhou submits a full report of the URNA Southwest Campaign, labeled Sequence Log 03-0997.
- Dr. Catherine Halsey is interrogated by Admiral Margaret Parangosky on Ivanoff Station.
- Dr. Halsey's personal journal is locked up after being analyzed by ONI.
- March 16: Evan Phillips is assigned to an intelligence-gathering mission on Sanghelios.
- Kilo-Five travels to Venezia and meets with Mike Spenser once again.
- An insurgent attack by Jiralhanae servants results in the death of dozens of Sangheili in Ontom on Sanghelios. During the attack, Evan Phillips and a fragment of Black-Box make their way into the Temple of the Abiding Truth. Eventually, Phillips activates a translocation portal within the structure and is transported to Acroli.
- Kilo-Five arrives at Sanghelios to retrieve Phillips. Upon failing to find Phillips in Ontom, it tracks him to Nes'alun Keep, under attack by forces from the nearby Lacalu Keep. Phillips is rescued but the Sangheili in the keep are killed by the invaders.
- March 22: The UNSC Infinity is launched after receiving its final upgrades with Forerunner technology.
- Forces loyal to the Servants of the Abiding Truth launch an assault on Vadam, starting the Blooding Years. This attack is eventually repelled by the arrival of the UNSC Infinity. During the battle, Jul 'Mdama's wife, Raia, is killed.
- Admiral Parangosky approves the deployment of an automated probe delivery craft to Installation 00 using advanced slipspace technology.
- April 1: Jul 'Mdama escapes captivity on Trevelyan via a slipspace portal. He is transported to Hesduros, where he is welcomed by the local Sangheili as a messenger of the gods. Jul uses the locals' religious devotion to his own advantage and the result is the start of a new faction devoted to Covenant ideals.
- April 5: Kilo-Five returns to Venezia in order to track down the Kig-Yar pirate, Sav Fel, and the insurrectionist, Staffan Sentzke. Serin Osman is promoted to Rear Admiral and a small party is held aboard Port Stanley in her honor by Kilo-Five.
- April 14: Sav Fel sells the CCS-class battlecruiser Pious Inquisitor to Staffan Sentzke, who intends to use the ship to force UEG authorities to provide answers about the disappearance of his daughter many years earlier. Fel demonstrates the ship's capabilities by using its energy projector to glass Forerunner ruins on Shaps III. This causes Line Installation 444-447 to send an automated distress signal to Covert Support Base 4276, located on planet Gao in the nearby Cordoba system. The distress call awakens the support base's archeon-class ancilla, Intrepid Eye, from stasis. After her requests for a status report, Intrepid Eye attempts to launch a reconnaissance drone, triggering a cave-in which cripples Support Base 4276 almost entirely.
- Vasily Beloi and Malcolm Geffen infiltrate the Venezian militia in order to gather information about Sentzke and Pious Inquisitor. Following a series of suspicious incidents, Sentzke begins to doubt their cover story and has them detained and tortured. Naomi-010 and Lian Devereaux eventually arrive and successfully extract Mal and Vaz. During the rescue, Sentzke is also taken captive aboard Port Stanley.
- A Kig-Yar crew led by Shipmistress Chol Von tracks down and boards Pious Inquisitor with the intent of seizing the ship for itself. Kilo-Five boards Pious Inquisitor and a skirmish occurs between it and the Kig-Yar. Chol Von eventually has her crew destroy the ship. Staffan Sentzke is presumed dead in the ensuing explosion, but he actually had managed to escape.
- Several tourists exploring Gao's Montero Cave System find themselves miraculously cured of various illnesses and the caves quickly see an influx of sick people hoping for similar miracle cures. The cures are actually the work of the Lifeworker Huragok Roams Alone, attached to Gao's Support Base 4276.
- Commander Murtag Nelson, a field analyst at an ONI listening post, takes note of strange transmissions emanating from the Montero Cave System on Gao. Nelson correctly hypothesizes that a Forerunner ancilla is responding to the bombardment on Shaps III. Admiral Parangosky subsequently gives Nelson command of the 717th Xeno-Materials Exploitation Battalion and tasks them with retrieving the Forerunner AI.
- ONI's Materials Group forms the Damascus Ordnance Commission, headed by major defense companies and sponsored by ONI. The commission places several conditions on the use of MJOLNIR GEN2 variants.
- May 6: ONI learns the Banished had raided an air base formerly belonging to the Covenant, and that its leader, Atriox, had been captured and taken aboard the assault carrier Enduring Conviction. Twelve hours later, the Enduring Conviction broke formation and contact with the rest of its fleet. The Conviction was hailed, warned, and then fired upon when it ignored orders. The ship performed evasive maneuvers, returned fire and escaped. Shortly after this, the ship and its crew, led by Let 'Volir, swore fealty to the Banished.
- May 7: The research and development vessel Argent Moon is commissioned by ONI.
- May 14 (approximately): A UNSC task force enters the Cordoba system and requests permission to conduct research in Gao's Montero Cave System. While Gao's planetary government initially denies the requests, President Aponte eventually allows a small research team to land. The UNSC subsequently deploys the 717th Xeno-Materials Exploitation Battalion, along with the attached Blue Team, to the Montero region and declares the caverns off-limits to anyone else.
- After the UNSC battalion's arrival, several people are found brutally murdered in the Montero Cave System. The murders are the work of the Forerunner ancilla Intrepid Eye, who seeks to generate conflict among the humans in order to force the UNSC to cease its efforts to capture her.
- A man named W. B. Eton becomes the handler of Catherine Halsey's personal journal and later sends it to someone who was involved in Operation: WHITE GLOVE.
- July 2: The Gao Ministry of Protection's Special Inspector Veta Lopis and her team investigates the recent murders in the Montero Cave System. The Huragok, Roams Alone, is spotted and attacked by the UNSC marine, Private Hayes. Intrepid Eye kills he and Major Ira Halal and discovers a fragment of Wendell, an AI created specifically for Operation: JOVIAN WHISTLE. Veta Lopis learns that the UNSC's investigation on the planet concerns a Forerunner ancilla. Gao Minister of Protection Arlo Casille meets with the dokab Castor of the Keepers of the One Freedom and Reza Linberk of the Venezian militia to plan a coup against President Tejo Aponte.
- July 3: Members of Blue Team accompany Lopis and her partner, Cirilo in a search for the missing Halal and Hayes. They arrive at their location and discover their corpses. Fred recovers the fragment of Wendell from Halal's damaged TACPAD and places him in his helmet's data chip. While investigating the scene, Cirilo finds a trail of blood from Roams Alone and the team resolves to follow it.
- July 4: Intrepid Eye directs Aggressor Sentinels to attack the humans and though they destroy the Sentinels, Cirilo is killed and Olivia-G291's legs are badly wounded. Veta Lopis learns of Gamma Company's unique augmentations and places them, Mark-G313 in particular, at the top of her list of suspects for the murders. The humans are met by Roams Alone, who they agree to take with them back to the surface. The Keepers launch an attack on Wendosa, believing that Intrepid Eye is in the town. The group also dispatches forces into the caverns which come into conflict with Veta and the Spartans.
- July 5: Fred-104 captures Intrepid Eye with a scramble grenade. The ancilla escapes and is pursued by the UNSC and Keepers alike and is eventually recaptured by he, Olivia, and Ash. Fred boards a UH-144 Falcon with the ancilla but is shot down. Veta Lopis attacks Castor and wounds him before the Jiralhanae can harm Fred. The 717th Xeno-Materials Exploitation Battalion's Alpha Company arrives at their location to fend off the Keepers.
- July 6: Blue Team destroys Covert Support Base 4276 with a HAVOK tactical nuclear weapon, having successfully captured the Forerunner ancilla Intrepid Eye. With the UNSC forces withdrawing and the majority of the Keepers of the One Freedom task force killed, the situation on Gao is stabilized; however, the colony is left under a staunchly anti-UEG administration headed by President Arlo Casille. Spartans Ash-G099, Olivia-G291, and Mark-G313 are reassigned to an ONI Ferret unit led by Veta Lopis, Admiral Parangosky having deemed public knowledge of Gamma Company's brain mutations too dangerous.
- The official status of Ash-G099, Olivia-G291, and Mark-G313 is changed to killed in action during Operation: JOVIAN WHISTLE as part of the ONI cover-up of Gamma Company.
- The San'Shyuum prelate, Tem'Bhetek, and the Minister of Preparation, Boru'a'Neem, seek revenge against the Sangheili for the losses they incurred during the Great Schism. They hatch a plot to use a prototype Halo installation to commit genocide on Sanghelios. Blaming Rtas 'Vadum personally for the loss of his wife, the prelate attempts to draw him in so he can capture his ship by attacking two Sangheili colonies, Rahnelo and Duraan. The efforts of the San'shyuum are thwarted by 'Vadum and the crew of the Shadow of Intent and Tem realizes that he has been lied to and used by the Minister of Preparation. He ultimately gives his life to destroy the Halo ring.
- October 14: Veta Lopis and her Ferret Team are stationed at UNSC Recreational Facility 6055-NA-A on Neos Atlantis to participate in a training exercise that turns into a fight for their lives as they fall under attack by Dark Moon Enterprises. However, with Spencer Hume and Ota Gallo dead, ONI is able to avert a crippling public relations crisis.
- November 23: Around 1419 hours, Damon, a fifth generation "dumb" Class-L military artificial intelligence, is created.
- November 28: Luther Mann, while exploring Trevelyan, discovers the location of Installation 07.
- A pioneer group arrives at Reach to prepare for the planet's eventual reterraforming.
- The ONI probe delivery drone dispatched in March arrives at Installation 00 and deploys its probes. After initial transmissions confirm the Ark's survival, all of the probes go offline.
- December 6: Vice Admiral Graselyn Tuwa is discovered killed aboard the UNSC Donoma, with her family having been abducted.
- December 8: The Office of Naval Intelligence sends Veta Lopis' Ferret Team to find her killer and rescue her family.
- December 12: Office of Naval Intelligence operative Veta Lopis and her team of Spartan-III Ferrets are present at Trattoria Georgi in New Tyne, Venezia, as part of a mission.
- December 21: Two ONI agents, 'D' and 'S', speculate which are Atriox's motivations after the demise of the Covenant empire.
- The UNSC commissions the UNSC Rubicon, a ship equipped with state-of-the-art Forerunner slipspace drive technology, remote contact teams and survey drones, for a manned mission to the Ark.
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- Gabriel Thorne's grandfather dies.
- Big M Studio, a company that deals with collectible items and replicas, is established.
- Holly Tanaka and two other survivors from Minab's glassing are rescued by the UNSC.
- The UNSC Navy takes inventory of its remaining fleet assets; all surviving warships are recalled to the Sol system. The UNSC Navy has been reduced to a fleet of several hundred vessels as plans for reconstruction are immediately enacted.
- Before May 2553, a iteration of the ENFORCER-class Mjolnir passed tests per the Ankara Field Stricture BL58-0.
- At some point before the ship's official launch, a New Colonial Alliance strike force led by Ilsa Zane infiltrates the UNSC Infinity and briefly commandeers the flagship, but is defeated by the five Spartan-IVs aboard the vessel at the time.
- The Smart AI Leonidas is created around this year.
- The M510 Mammoth, the AV-49 Wasp, and the STINGER-class Mjolnir are introduced.
- Gudam Keschun is born.
- Halo Wars 2 GAMEPLAY - E3 2016 Interview (Interview with Kevin Grace) - "The Covenant are dead"
- Halo: Envoy, Chapter 12
- Halo: Initiation, Issue #1
- Halo Waypoint - Spartans
- Halo Waypoint: Catalog Interaction (Query Answer: Human records indicate transition to noncombatant administrative control [ref: UEG] [January 8, 2553]. No notable adjustment in military [bureaucratic tactical] behavior and scope of authority.)
- Halo Waypoint - Halo: Glasslands excerpt, Chapter 2
- Jessica Shea Twitter Today in Halo - 1/8/2553
- Halo Legendary Crate, Data Drop #8
- Halo: Glasslands, page 79
- Halo: Initiation, Issue #3
- Halo: Evolutions - Essential Tales of the Halo Universe, From the Office of Dr. William Arthur Iqbal
- Jessica Shea Twitter Today in Halo - 2/1/2553
- Halo: New Blood, pages 49-55 (Google Play edition)
- Twitter: Today in Halo 2/15/2553
- Halo Wars 2, Phoenix Logs — An Uncomfortable Truth Part 1
- Halo Waypoint: Catalog Interaction - Page 14
- Halo: The Thursday War
- Halo 3, campaign level Halo
- Halo Legendary Crate, Data Drop #2
- Jessica Shea's Twitter - Today in Halo: 3/16/2553
- Halo: The Thursday War, page 28
- Halo: Hunters in the Dark, Chapter 2
- Twitter: Today in Halo 4/1/2553
- Halo: Last Light, pages 22-23 (Google Play edition)
- Halo: Last Light, pages 36-39 (Google Play edition)
- Halo: Last Light, pages 8-10 (Google Play edition)
- Halo Mythos, page 128
- Halo 4: The Essential Visual Guide
- Halo Wars 2, Phoenix Logs — Rise of Atriox III
- Halo Waypoint: Argent Moon
- Halo: Last Light, pages 5-6 (Google Play edition)
- Halo: Last Light, Chapter 28
- Halo: Last Light, Chapter 28, pages 330-331 (Google Play edition)
- Halo Waypoint: Catalog Interaction - Page 38
- Halo: Shadow of Intent, pages 79-80 (Google Play edition)
- Halo: Fractures, "A Necessary Truth"
- Halo: Retribution, chapter 4
- https://twitter.com/bsangel/status/670658429149540352, Today in Halo
- Halo Waypoint - Catalog Interaction (Page 43)
- Halo: Retribution Chapter 1
- Halo Wars 2 - Phoenix Logs: Rise of Atriox IV
- Halo: Escalation - Issue #7
- Halo: Reach Legendary Edition - Noble team Statue Instructions
- Halo: Escalation - Issue #17
- Halo Mythos, p. 132
- Halo 4: The Essential Visual Guide - Page 152
- Halo: Bad Blood
- Halo: Official Spartan Field Manual, page 202
- Halo: Official Spartan Field Manual, page 204
- Halo Waypoint: Canon Fodder - Tales & Stingers