The backbone of the military.
The naval battles fought during the Human-Covenant War were massive, with each side losing hundreds, if not thousands of ships, and millions of lives. By the start of the war with the Covenant, the UNSC had a sizable, though underpowered, fleet of warships. While unable to put up much of a fight head-to-head, the UNSC was able to put up a stiff defense against the Covenant using superior tactics and sheer numbers to slow their advance down considerably. Despite this, the might of the Covenant fleets was too much for the UNSC to hold off indefinitely. The exact size of the entire Covenant fleet is unknown, but it likely numbered in the thousands of ships. The High Charity defense fleet, for example, consisted of five hundred ships alone, and the total Covenant ships that attacked Reach was over three hundred.