From Halopedia, the Halo wiki
The 26th Marine Expeditionary Force is a UNSC Marine Corps formation, which was a part of a Rapid Offensive Picket. This unit fought during the Battle of New Jerusalem in the Cygnus system in the summer of 2552, and they suffered tremendous casualties during the battle. Several members of the unit were Orbital Drop Shock Troopers.
The Rookie was the only known member of this unit, and among its few survivors.
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