Battle of Leonis Minoris
In 2537, the Covenant discovered the human presence in the Leonis Minoris system, and proceeded to wipe out two of the three colonies there. The UNSC rushed to the system's defense with a large force of starships, and won the battle, but at a cost: ten ships were lost in battle against the technologically-superior Covenant, and Admiral Cole was forced to allow the Covenant to destroy two colonies in order to protect the third one.
Although the UNSC Navy considered the loss of ten vessels to be relatively minor, the Covenant succeeded in eradicating two of the three colonies in the system, and the loss weighed heavily on Cole's conscience.
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