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 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 workers. Now they bred 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 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.

At that point the Doctor arrived, found the Source and used it to demonstrate to the combatants their true mission and end the war. (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".
 * Doctor Who: The Time Traveller's Almanac states that fighting broke out after the mission commander died of "Byzantine Fever".