412th Engineer Command
From Halopedia, the Halo wiki
The 412th Engineer Command[note 1] is an engineering unit of the UNSC Army Corps of Engineers established sometime before 2556. The command conducts civil and combat engineering roles in support of United Nations Space Command operations in the conflicts following the end of the Human-Covenant War. Holly Tanaka served with the 412th as a sergeant in 2556 before transferring to Spartan Operations.
The 412th Engineer Command exists in the modern day United States Army as a theatre-level reserve component. It's likely that the 412th Engineer Command in United Nations Space Command service carries over the original American unit's name and lineage as with many other units.
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