Though the Pillar of Autumn is best known for its actions during the Fall of Reach and the Battle of Installation 04, It was actually put into service fifteen years prior to first contact with the Covenant. Its structural design was considered over-engineered, with a series of cross-bracings and honeycomb structures that made the Halcyon-class light cruisers exceedingly strong, but also overly massive and costly to build. Due to this design deficiency, only a total of eleven Halcyon-class ships were ever built, and all were mothballed before being brought back into service as the UNSC tried to make up for the continual losses it faced against the Covenant. It was the same over-engineered design. however, that led to the Pillar of Autumn being chosen for a refit and selection for Operation: RED FLAG, a last-ditch effort by the UNSC to force a cease-fire with the Covenant by capturing a hierarch. Though Operation: RED FLAG was never implemented due to the Covenant invasion of Reach, the Autumn's refit was likely responsible for its survival during the Battle of Installation 04.