From Halopedia, the Halo wiki
Combat engineers are personnel who are tasked with construction of allied fortifications and demolition of enemy emplacements.
During the Battle of Draetheus V in 2555, UNSC combat engineers assisted Spartan Edward Davis in destroying a Forerunner structure on the moon of X50. Several engineers would be deployed on Draetheus V itself.
UNSC combat engineers may be structured within the UNSC Marine Corps of Engineers.
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