2551

January

 * January 17: UNSC "smart" artificial intelligence Iona begins service.

February

 * February 19: As part of Project CHRYSANTHEMUM, the SPARTAN-IIIs of Gamma Company begin their week-long augmentation process on the in Sector K-009 folllowing its departure from Reach.
 * February 27: The Hopeful returns to Reach following the Spartans' augmentation procedures.

Before May

 * Before May, the Enduring Conviction, led by Let 'Volir, faces against the UNSC. During the battle, the Enduring Conviction performed impressive evasive maneuvers.

May

 * May 8: Dr. Catherine Halsey undergoes the Hemley & Aubourn Behavior Analysis; the results make ONI increasingly wary about her reported unreliability.

November

 * November 24: Mark V[B], the initial production model of the Mark V MJOLNIR armor, enters service.

December

 * December 22: The Car & Pilot Magazine releases its December 2551 Edition. This includes a review of the new 2553 edition of the Civilian "Hog".

Unknown

 * Pawel Czernek and John Tippett enter active service with the UNSC Marine Corps.
 * Concord is attacked by the Covenant; the colony survives largely intact. The Venator MJOLNIR [GEN1] variant gains prominence in the attack.
 * Gray Team is deployed on a mission against a Covenant facility within humanity's former territory.
 * The Battle of Meridian takes place. Meridian is glassed and Chalybs Defense Solutions facilities on the moon are abandoned. The Covenant discovers a Luminary on Meridian that leads to Earth.
 * The begins construction.
 * Veta Lopis' mother dies.