- "I've seen the status reports from the War. The list of casualties. I can read between the lines. We're losing, aren't we?"
- —Washington to the Counselor in Washed Hands.
The Great War was an intergalactic conflict between the UNSC and the Covenant, a large alliance of aliens that included the Sangheili. The war began at least 23 years before the events of Blood Gulch and ended sometime around the Reds and Blues reassignments at Rats Nest. Throughout the course of the war, the UNSC attempted to find alternative ways to gain the upper hand. One of these attempts was Project Freelancer, a program run by Dr. Leonard Church with the intent of creating super soldiers. The victor of the war is unknown, but humans and aliens have been seen working together since then.
The Great War is heavily based on the similar war taking place between Halo's 1-3 in the Halo series, referred to as The Halo Event. Unlike The Halo Event however, The Great War seems to have only been between Humans and Sangheili, with no mention of other races or the The Flood, although the Halo rings have been referenced a few times in Red vs Blue. Because of this, it's unknown how similar The Great War is to its Halo counterpart.
Several known characters took part in the war. Almost all UNSC personnel were involved in some capacity, including the simulation troopers before they were given to Project Freelancer for the simulation outposts.
- Felix & Locus: Before they found themselves in the employ of Malcolm Hargrove, and before they had even become mercenaries, Felix and Locus were a part of the same squad in the UNSC during the war. Their squad had seen some of the worst fighting and the two banded together to survive.
- Reds and Blues: All of the simulation troopers of Blood Gulch were first members of the UNSC before they were deemed unfit and were given to Project Freelancer.
- Lavernius Tucker: During his time in the UNSC, he impersonated a military physician named Dr. Cloitus and was caught.
- Michael J. Caboose: Before he was in Blood Gulch, Caboose was stationed on a remote moon. He received multiple demerits because of his misunderstanding on how oxygen works in space. He was transferred to the Blue Team simulation army following these incidents.
- Sarge: During his time in the UNSC, Sarge jumped from high orbit as an Orbital Drop Shock Trooper (ODST). He was removed, however, after causing extensive damage to his unit's ship by using parts from it to build a robot.
- Dick Simmons: Several of Simmons' officers in the UNSC complained that his constant need for attention and approval was disruptive, so he was transferred to Project Freelancer.
- Dexter Grif: After enlisting, Grif was stationed on a doomed colony that was wiped out in a battle during the Great War. Grif survived it because he had fallen asleep at his post, causing aliens and humans alike to mistake him for a corpse. He was transferred to Project Freelancer soon after.
- Franklin Delano Donut: Donut violated strict UNSC laws and had wine and cheese hours in the middle of field exercises.
- Leonard Church: Underwent UNSC basic training where he met his wife Allison but never went on to become a soldier.
- Allison Church: After marrying Leonard and giving birth to Carolina, Allison Church joined the war effort and was killed in battle many years later.
- Washington: Before joining Project Freelancer, Wash was stationed in the Leonis Minoris system, where he assaulted a commanding officer because he disagreed with an order that would get everyone killed.
- Carolina: After her supposed death after the events of the Freelancer break-in, Carolina used forged documentation to enlist in the UNSC to fight in the Great War, still feeling a sense of duty to make sure humanity did not lose. During this time she went under the alias 'McCallister'.
- Dylan Andrews: While not an active combatant during the war, Dylan Andrews of Interstellar Daily reported from the frontlines and witnessed the fighting firsthand.
- Malcolm Hargrove: Not an active combatant either, Hargrove's political position and influence within the UNSC allowed to him to achieve the rank of Oversight Sub-Committee Chairman, as well as remaining the CEO of Charon Industries.
- The Sangheili's home planet was mentioned in the Ultimate Fan Guide on page 181, where it was confirmed that Tucker and Junior reunited and visited the planet.
- Tucker references the Halo Rings at one point in Called Up, implying that similar to its Halo counterpart. A major aspect of The Great War was about fighting for control over them.
- The outcome of The Great War is a truce between the Humans and Sangheili, similarly to it's Halo counterpart, although it's never specified if the circumstances surrounding the truce are the same in Red vs Blue as Halo.
- The planet of Chorus was inhabited by the UNSC during the war, but after the war was finished they abandoned it and left the people behind.