Human-Hath War

The Human-Hath War also known as the Seven-Day War was a war on the planet Messaline that lasted seven days and began when the terraforming colony lost its mission commander. In the power vacuum, the Human and Hath factions turned greedy and raged war on each other. They recalibrated progenation chambers originally designed to quickly create workers to breed generation after generation of soldiers, and with due to high casualties went through about 20 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.

By day seven the original mission profile had completely passed into legend, and 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 thus end the war. (DW: The Doctor's Daughter)

Trivia

 * Contrary to the episode giving a power vacuum as the cause of the conflict, the official 2009 annual says that "it grew from an argument over a game of cards".