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The 405th Marine Infantry Division is a UNSC Marine Corps unit. Stationed at Diego Garcia, it was among the first UNSC forces to respond to the Covenant's invasion of Mombasa, Kenya during the Battle of Earth.
The 405th assisted John-117 in the destruction of a Scarab that was moving through the city. They also assisted the 17th Marine Regiment, after most of their company started to dwindle. Several surviving members of this unit then traveled to Installation 05 along with John-117 aboard UNSC In Amber Clad while the rest continued to fight in New Mombasa. It is assumed those who went to Delta Halo were killed in either the Battle of the Quarantine Zone or the First Battle of High Charity.
- If Staff Sergeant Banks was in command of A Company, as Corporal Perez says, that means that all of the Lieutenants in the 405th have been killed or none were in the immediate area.