From Halopedia, the Halo wiki
UNSC Valiant was drafted by the UNSC Navy as a compromise between a battleship and a heavy cruiser. Valiant was commissioned and released from her Martian shipyard docking clamps in 2493, after undergoing years of feasibility studies and redesigns. Valiant showed promise during her initial testing and evaluation, though the high costs and delays in incorporating next-generation fusion drives seemingly doomed the Valiant class.
Less than two years after her commissioning, Valiant and the fellow ships of her class were relegated from the UNSC Navy rolls and partially scrapped. However, in 2497, the United Nations Space Command discovered that insurrectionist sympathizers had infiltrated the Colonial Military Administration, which led to a massive expansion of the UNSC as it absorbed and dismantled the CMA, and assumed their role fighting the Insurrection. With most command ships now engaged in lengthy counterinsurgency operations, Valiant and other members of the class were soon recommissioned as command-and-control flagships, with the tonnage and firepower of the class successfully augmenting battle groups of smaller warships. Valiant and a handful of other super-heavy cruisers, including UNSC Everest, were outfitted with a complete suite of command upgrades to aid the UNSC in the Insurrection.
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