From Halopedia, the Halo wiki
The Agincourt was a part of Battlegroup India, under the command of Admiral Preston Cole, which took part in the Battle of Psi Serpentis. During the battle the Agincourt charged headlong into concentrated streams of incoming plasma lines, sacrificing itself to save her sister ships. It was destroyed during the battle along with 70 other ships from the battlegroup.
The vessel is likely named for the Battle of Agincourt, a battle between the English and French during the Hundred Years' war.
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