From Halopedia, the Halo wiki
During the Fall of Reach, Chapman shared command of Charlie Company with First Lieutenant Buckman, commanding the sub-unit Gamma 1. As the Covenant assaulted the Orbital Defense Generators in force, a large portion of Charlie Company was killed near ODG A-331 by a poorly targeted carpet bombing. Gamma 1 and Gamma 5 then separated, with Buckman moving out to defend ODG A-412 and Chapman contacting a recon team observing a large Covenant ground force coming their way. After the recon team panicked and opened fire on the Covenant, Chapman called in Beta Red along with the Third Mechanized. Soon, Buckman and the remnants of Gamma 5 retreated from Facility A-412, now also overrun by the Covenant, to regroup with Chapman and Gamma 1. Beta Red, engaging the incoming Covenant force 30,000 strong by themselves, bought some time for Chapman and Buckman to retreat. Chapman then contacted Iron Fist, who advised them to head for a temporary evac station some 50 kilometers away. Chapman and his unit were still in the splash zone when the UNSC Majestic opened fire on several Covenant cruisers over Beta Red's position, but at least Chapman and Buckman survived the blast.
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