From Halopedia, the Halo wiki
Pure Resolve, along with Reprisal's Fire, managed to catch up to Unwavering Discipline while it was chasing Gray Team's Class-5 Bumblebee. Because of their thinner hulls the two ships were more focused on maintaining their shields during re-entry than firing on Unwavering Discipline. Pure Resolve's shields soon flickered and died, causing debris to flake off until an explosion inside the ship caused it to blow apart.
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