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A "ghost battalion" is a UNSC slang term used to describe units representing colonies that no longer exist.
Due to the UNSC's mobile nature, the armies of each colony could be found across human space. During the Human-Covenant war, this nature allowed entire battalions to escape the glassing of their planets and continue to represent their colony in the conflict. An example of a ghost battalion is 1st Battalion, 7th Marines, a unit from Eridanus II. This battalion functioned until at least 2552, twenty-two years after Eridanus II's glassing, and consisted of Marines who lived on Eridanus II at the time of its glassing.