From Halopedia, the Halo wiki
During the Siege of New Alexandria the roof had collapsed while Gunnery Sergeant Edward Buck's men were in still in the building, taking fire from the Covenant. Buck took it upon himself to get to his men and get them out. After telling Command this, Kat-B320 tasked Spartan-B312 with escorting Buck's Falcon to the Nomolos Tower.
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