2525 marks the beginning of the Human-Covenant War, a twenty-seven year long engagement between the Covenant empire and the United Nations Space Command. Major events include the Skirmish on Minor Transgression, First Battle of Harvest, the Assault on Eridanus Secundus, and the Battle of Chi Ceti.
- Staff Sergeants Avery Johnson and Nolan Byrne, along with Captain Ponder and Petty Officer First Class Healy, begin training Harvest's Colonial Militia.
- The militia goes on another march, this time with 25kg rucksacks. In the aftermath, Johnson and Ponder attend Harvest's Solstice Celebration. Johnson is briefed by Jilan al-Cygni as to why he and Byrne are here; to prevent Insurrectionist attacks on UNSC freighters.
- The Skirmish on Minor Transgression: The Covenant ship Minor Transgression attacks another human ship (the Bulk Discount), said human ship having the Walk of Shame in support and hosting not only Johnson and Byrne but also an arsenal of weapons. The aliens are repelled (though Johnson receives a fractured skull) and the ship destroyed, though Dadab and Lighter Than Some are able to escape in a escape pod after making a report on the relics they've found.
- Colonial Militia trainees Wallace Jenkins, Forsell, Wick and Andersen begin target practice.
- The Ministry of Fortitude holds a statistic-based meeting. The Minister of Fortitude takes stimulants to help him through the ordeal.
- Fortitude wakes up, the hangover effect of the stimulants taking hold. The Vice Minister of Tranquility alerts him to the Luminary detections made by Minor Transgression. They resolve to use them to usher in an Age of Reclamation and secure their position as the next trio of Hierarchs.
- Walk of Shame is dispatched to Reach to request that FLEETCOM send a battle group in the event of first contact escalating into bloodshed.
- Johnson regains consciousness and is debriefed by Nils Thune and al-Cygni. Johnson resolves to accelerate the militia's training.
- Harvest's Tiara makes long-range radar contact with the Covenant cruiser Rapid Conversion. Contact is lost with Harvest soon after.
- First formal contact between the Covenant and humans on Harvest at the Harvest Botanical Gardens. Communications go awry and the Human-Covenant War begins. The Covenant forces, led by Jiralhanae Chieftain Maccabeus, retreat to Rapid Conversion in high orbit over Harvest.
- Jilan al-Cygni deploys a RQ-XII drone.
- After a week of remaining still, Rapid Conversion attacks the small, remote town of Gladsheim, while Type-25 Spirits, Jiralhanae, and Yanme'e slaughter its residents. The Harvest militia evacuates the town's few survivors, resulting in the injury of Maccabeus and the death of Ritul.
- Evacuation of Harvest. About 22,000-23,000 civilians are killed. Tartarus kills Maccabeus and takes control of the Covenant forces. Rapid Conversion is disabled by Harvest's mass driver while colonists and militiamen escape to the Tiara.
- Large firefights at the Tiara and Harvest's reactor complex ensure. Notable deaths include Ponder, Vorenus, Dadab, and Lighter Than Some. Loki destroys the Tiara.
- The Office of Naval Intelligence declares Onyx classified after the discovery of Forerunner artifacts.
- SPARTAN-IIs undergo rigorous chemical, physical and biological augmentations. Thirty-three Spartans successfully pass the alterations. Thirty subjects die, and twelve are permanently disabled.
- Funeral service for those who died undergoing augmentations in the SPARTAN-II program aboard UNSC Atlas in the Lambda Serpentis system.
- Doctor Catherine Halsey has the bodies of the deceased SPARTAN-IIs cryogenically preserved in hopes of reviving them in the future. The augmentation washouts are given desk jobs to allow them to continue to support the UNSC.
- CMA Argo arrives in the Epsilon Indi system and makes a confirmation transmission. No further reports are received.
- Mack detects the CMA Argo enter Harvest's orbit. He attempts to warn Argo of the Covenant, but the scout ship is destroyed by Rapid Conversion.
- John-117 kills two Marines of the 105th Shock Troops Division and severely wounds another aboard Atlas in an incident secretly engineered by ONI to test the efficiency of the SPARTAN-II augmentations.
- Preston Cole's liver fails from cirrhosis, and is subsequently replaced - as are his damaged heart and endocrine system - with flash-cloned transplants.
- Dr. Halsey begins to plan therapies, experimental surgeries, and rehabilitations for eighty percent of the SPARTAN-II augmentation washouts and creates to protocols. Artificial intelligences predict a fifty percent success rate, though Halsey believes the Spartans' will to serve the UNSC will boost the success rate.
- Dr. Halsey moves her research from Camp Hathcock to CASTLE Base, declining an invitation of working in a corner suite in Olympic Tower.
- A test case file was unintentionally "attached" to an official response to Catherine Halsey. The file showed a volunteer attempting to use a prototype of the MJOLNIR Powered Assault Armor, resulting in his death due to his lack of training and proper physiology.
- Dr. Halsey works on improving the MJOLNIR Powered Assault Armor prototypes. Due to its secrecy, she relocates the Materials Group division working on the armor to Damascus Testing Facility in Vhalkem on Chi Ceti IV.
- SPARTAN-IIs train with the Mark I armor in the Reach UNSC Military Complex. Dr. Halsey determines the soldiers are ready for their first mission.
- SPARTAN-IIs are briefed on their first mission aboard the destroyer UNSC Pioneer, capturing United Rebel Front leader Colonel Robert Watts.
- John-117 is promoted to Petty Officer Third Class.
- The Assault on Eridanus Secundus. Blue Team—consisting of John-117, Samuel-034, Linda-058, Kelly-087, and Fred-104—infiltrate URF military base Eridanus Secundus in the Eridanus system's asteroid belt. Robert Watts is captured by the SPARTAN-IIs and returned to the UNSC. The asteroid's hangar bay is damaged by Sam, causing it to vent atmosphere.
- Battlegroup 4, composed of the CMA Heracles, the CMA Arabia and the CMA Vostok enters the Epsilon Indi system and make contact with a Covenant ship. The Covenant ship destroys the Vostok and the Arabia with only the badly damaged Heracles escaping.
- The Covenant ship broadcasts the following message to the battelgroup: "Your destruction is the will of the Gods...and we are their instrument" in complete English, showing that the Covenant have been studying humanity for some time.
- UNSC is put on full alert for the Covenant. The training of the SPARTAN-IIs is accelerated.
- Office of Naval Intelligence agents Lieutenant Commander Jack Hopper and Lieutenant Demos convince Vice Admiral Preston Cole to come out of retirement and lead the fleet to retake Harvest.
- The SPARTAN-IIs are informed of hostile contact with the Covenant and the fate of Harvest. They are introduced to Project MJOLNIR. 
- Dr. Halsey begins to analyze the Covenant.
- Many high-ranking personnel within the UNSC begin to show fear of the Covenant. Dr. Halsey and Franklin Mendez discuss the SPARTAN-IIs and Covenant threat.
- The Battle of Chi Ceti. The SPARTAN-IIs are delivered to the Damascus Testing Facility on Chi Ceti IV by the UNSC Commonwealth. While en route to the planet, Commonwealth is attacked by the Covenant's Unrelenting. The Commonwealth manages to damage Unrelenting, allowing the Spartans to escape to Chi Ceti IV.
- Within the UNSC Damascus Materials Testing Facility, the SPARTAN-IIs are introduced and fitted with the MJOLNIR Powered Assault Armor/Mark IV.
- While the Spartans are returning to the Commonwealth, Unrelenting returns and attacks the frigate. The SPARTAN-IIs board the Covenant ship. They plant Anvil-II air-to-surface missiles at the ship's reactor and detonate them, destroying the ship—resulting in the death of Samuel-034.
- Ninth Age of Reclamation begins with the ascension of the High Prophets of Truth, Mercy, and Regret. They declare that the human race destroyed a vast Forerunner reliquary and are to be exterminated, starting the Human-Covenant War.
- Some time before April 22, then-Second Lieutenant Antonio Silva is assigned to command an Orbital Drop Shock Trooper platoon onboard UNSC Atlas.
- Biko is glassed by the Covenant sometime during this year.
- The Assembly holds a tribunal for one of its members, who is believed to have attempted communication with the Covenant. Later in the year, they discuss the SPARTAN-II program and how it managed to reach operational capacity a year before the Insurrection expanded beyond control.
- ^ Halo: Contact Harvest, page 105
- ^ Halo: Contact Harvest, pages 105-123
- ^ Halo: Contact Harvest, pages 124-136
- ^ Halo: Contact Harvest, page 137
- ^ Halo: Contact Harvest, page 143
- ^ Halo: Contact Harvest, pages 143-158
- ^ Halo: Contact Harvest, page 166
- ^ Halo: Contact Harvest, pages 159-166
- ^ Halo: Contact Harvest, pages 256-259
- ^ Halo: Contact Harvest, page 391
- ^ Halo: Contact Harvest, pages 279-286
- ^ Halo: Contact Harvest, pages 335-336
- ^ Halo: Contact Harvest, pages 368-385
- ^ Halo: Reach Limited Edition, Intersystem News, LCDR Miranda Keyes Awarded Silver Star
- ^ Halo: Ghosts of Onyx, page 127
- ^ Halo: The Fall of Reach, pages 55-59
- ^ Halo: The Fall of Reach, pages 60-63
- ^ a b c d e f g Halo: Reach, Dr. Halsey's personal journal
- ^ Halo: The Fall of Reach, page 118 (2010 edition)
- ^ Halo: Contact Harvest, page 393
- ^ Halo: The Fall of Reach, pages 63-69
- ^ Halo: Evolutions - Essential Tales of the Halo Universe, "The Impossible Life and the Possible Death of Preston J. Cole", page 459
- ^ Halo: Contact Harvest, page 395
- ^ Halo: The Fall of Reach, pages 69-74
- ^ Halo: The Fall of Reach, pages 75-81
- ^ Halo: The Fall of Reach, page 77
- ^ Halo: The Fall of Reach, pages 81-94
- ^ Halo Wars: Genesis
- ^ a b Halo: The Fall of Reach, page 98
- ^ Halo: Evolutions - Essential Tales of the Halo Universe, "The Impossible Life and the Possible Death of Preston J. Cole", page 460
- ^ Halo: The Fall of Reach, pages 94-101
- ^ Halo: The Fall of Reach, pages 102-113
- ^ Halo: The Fall of Reach, pages 113-119
- ^ Halo: The Fall of Reach, pages 119-130
- ^ Halo: Contact Harvest, pages 379-384
- ^ Halo: The Flood, page 134 (2003 edition)
- ^ Queen's Diary Entry.
- ^ Halo: Reach, Data pad
- ^ Halo: Reach, Data pad 9