Harvest was the furthest colony from Earth, but quickly became the breadbasket of the outer colonies due to its climate that supported high crop yields. Though important for food production, the planet was relatively unpopulated, with most of the farming work falling to automated equipment controlled by a planet-wide AI. When the Covenant stumbled upon humans at Harvest in 2525, there wasn't much in the way of a human presence outside of a single city and a few smaller towns, and even less in the way of planetary defense. Though the citizens of Harvest tried to make peace with the newly-discovered aliens, their offer was greeted with an attack on their city and entire planet. Without any real means of fighting back, the planet was quickly lost.
Humanity wouldn't give up the colony so easily though, and a year later Vice Admiral Preston Cole led his fleet into battle over Harvest, securing a victory, though at the cost of thirteen UNSC ships, versus one Covenant super-destroyer. This sparked the Harvest Campaign, a five-year conflict for control of Harvest, leading to a UNSC victory, but at the cost of many ships and the planet's habitability. By 2531 the Covenant had given up trying to retake Harvest, but the planet was too damaged for humanity to do anything other than abandon it.