Human-Hath War

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 to Byzantine fever. In the resulting power vacuum, the humans and Hath divided into factions, became greedy, and began to wage war on each other. They used recalibrated progenation chambers, originally intended to create new settlers, to breed generation after generation of soldiers who had no names but were trained in combat and the basic history of their colony.

Due to the high casualties of the war, both sides went through about twenty generations a day. This high casualty rate meant that the story of the war's origin became increasingly distorted as it was passed on from one generation to the next. 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 object called the Source. The humans believed the Source to be a weapon, but in reality it was the device needed to jump-start the terraforming process.

At that point the Doctor arrived, found the Source, and used it to demonstrate to the combatants their true mission and end the war. The very next day, human and Hath were working side-by-side to build their new society. (TV: The Doctor's Daughter)

Behind the scenes

 * Doctor Who The Official Annual 2009 states that the war "grew from an argument over a game of cards".
 * The Time Traveller's Almanac states that fighting broke out after the mission commander died of "Byzantine Fever". While the Doctor is looking at the ship's logs, they also state that the captain died of Byzantine Fever.