The enemy ship destroyed...and 13 of ours obliterated. It did not bode well.
The Second Battle of Harvest was fought by the UNSC and the Covenant during the Human-Covenant war. It was the direct follow on to the First Battle of Harvest which resulted in the destruction of the Harvest colony and seizure of the planet by the Covenant. The UNSC carried the day with superior numbers and a last-minute tactical maneuver, but suffered heavy casualties.
In early 2525, Harvest was attacked by the Covenant. The colony was partially glassed with many colonists fleeing on freighters. A UNSC scout ship and two Frigates were destroyed, and a Destroyer heavily damaged, in an unsuccessful attempt to dislodge the single Covenant Battleship holding the planet.
Thirteen months later on March 1, the UNSC-wide mobilization was under way, with Vice AdmiralPreston Cole assembling a fleet to retake Harvest. UNSC Battle Group X-Ray is conceived as the largest ever assembled in human history at that time, it ultimately consisted of 40 ships with the admiral commanding from the cruiser UNSC Everest.
— VADM Cole during the final charge quoting Col. Joshua Chamberlain from the American Civil War.
The UNSC fleet arrived at Harvest on March 1, 2526, and engaged the lone Covenant ship through a slingshot maneuver around the planet. Multiple ships, including the UNSC Sacramento and the UNSC Lance Held High, were quickly destroyed. UNSC weapons, including nuclear ordnance, had very little effect on the enemy. Cole realized the enemy was using energy shields, like the one previously encountered at the Battle of Chi Ceti, and ordered the fleet to fire all of its weapons simultaneously at the target. This overwhelmed the shields and destroyed the Covenant vessel.
The UNSC victory came at the cost of thirteen warships. Cole realized the disparity between UNSC and enemy losses "did not bode well". However, ONI Section II propaganda exaggerated the victory to encourage recruitment; initially the foe was claimed to be an Insurrectionist weapon platform but this was later recanted. For their part, the Covenant celebrated the "Victory of Epsilon Indi" through fasting and prayer.
Harvest continued to be contested. The Covenant continued searching for Forerunnerartifacts, while the UNSC refused to withdraw. The UNSC finally prevailed in 2531, but the planet lost its remaining strategic value as the Covenant began its advance into the Inner colonies.
The presence of 40 UNSC ships-of-the-line against one overwhelmingly powerful enemy ship strikes a remarkable resemblance to the Star Trek-Universe's Battle of Wolf 359. At Wolf-359 the Federation's (or Humanity's) numerically superior fleet of 40 starships took on a single BorgCube. The results, however, were completely opposite to the Halo-Universe. The Federation lost 39 of its 40 vessels, and the Borg Cube left the battle without so much as a scratch on its hull and moved on towards Earth. (See also: List of Pop Culture References in Halo).
In numerous books, it is hinted that the Covenant had numerous vessels in the battle, and in the Cole Protocol novel, Cole is said to have "thrown three of his ships against the Covenant for every one they lost". In fact, until the publication of Halo Wars; Genesis, every Halo novel that mentioned the battle stated, in one form or another, that the Covenant had several ships at Harvest during the battle (between 7 and 10). In Halo; the Fall of Reach, Lovell notes that Cole only won because he outnumbered the Covenant 3 to 1; and even then lost 2/3 of his fleet.