The Human-Hath War was a war on the planet Messaline that lasted seven days, from 17 July 6012 when the two species — humans and Hath — arrived on the planet, to 24 July, when the Tenth Doctor arrived and intervened.
It began when the terraforming colony lost its mission commander. In the power vacuum, the humans and Hath, having now divided into factions, became greedy and waged war on each other. They recalibrated progenation chambers designed to quickly create settlers to breed generation after generation of soldiers. Due to high casualties, they went through about twenty generations a day. This high casualty rate meant that the story of the origins of the war was severely distorted as it was passed on to the new generation as a myth.
By day seven, the original mission profile had passed completely into legend. All the survivors knew was that they had to search for a now-mythical weapon called the Source, in reality the device needed to jump-start the terraforming process.