Rutan-Sontaran War

The Rutan-Sontaran War was fought between the Rutan Host and the Sontarans. It lasted for thousands, perhaps millions, of years, and was waged across the universe.

The Doctor had considered the war a pointless waste of time, as the Sontarans never fight for anything other than the joy of battle. (TV: The Poison Sky)

Origins of the war
The Sontarans, not yet a species of clones, were already, in the Seventh Doctor's words, a "fierce, martial race". They began to sweep over Mutter's Spiral. Only the Rutans opposed them. Reacting to this, the Sontarans stopped reproducing naturally and turned themselves into a clone species in order to constantly supply their frontlines with fresh soldiers. (COMIC: Pureblood) One account implied that the Rutans began the war with the Sontarans out of boredom, and because it gave them a sense of superiority. (PROSE: The Taking of Chelsea 426)

A definitive date of the start of the war remains unknown. The Tenth Doctor seperately described it as having begun fifty thousand years before 2009 AD and the early 26th century respectively, clearly a rough estimate. (TV: The Poison Sky, PROSE: The Taking of Chelsea 426) Field Major Kaarsh gave the same figure in 1605. However, the Rutan known as Lady Winters gave the contradictory figure of ten thousand years, which would mean that the war would have begun circa 8395 BC. (GAME: The Gunpowder Plot)

The Seventh Doctor described this event as "ten centuries" before the 25th or 26th century, meaning that the conflict would have begun in the 15th or 16th centuries. (COMIC: Pureblood) This contradicted evidence given elsewhere, such as a Sontaran speaking of war with the Rutans in the 12th century (TV: The Time Warrior) and the Rutan plan to destroy the Sontarans in the 13th century. (GAME: The Gunpowder Plot) Kygon Brox described the Earth as a "primitive world" during the Fifty-first Star Fall Campaign. (COMIC: The Instruments of War)

The Rutans and Sontarans both involved themselves in the Millennium War of 150 million years BC. (PROSE: The Quantum Archangel)

Events of the war
During the dying days of the Fifty-first Star Fall Campaign, the Sontarans suffered heavy losses against the Rutans. Sontaran scientists created the world engine weapon the Warsong to change the course of the war. The Sontarans were unable to deploy the weapon, and the "brave warrior" Surnat Gaq escaped the Rutans, and while mortally wounded, he took the Warsong to primitive Earth, where it lay dormant and undetected. (COMIC: The Instruments of War)

During the 13th century, (TV: The Sontaran Experiment) the Sontaran Linx claimed Earth for the Sontaran Empire when he crashed there after being shot down by a squadron of Rutan fighters. He time travelled into the future to kidnap scientists to repair his ship after discovering how crude the materials of the period were.

Linx formed an alliance with a robber baron named Irongron, promising to supply him with weapons Earth would not see for centuries, if the human provided him with shelter so he could repair his ship. He was unable to hold the planet, and was killed as he tried to leave Earth by the archer Hal, the Third Doctor having helped return the scientists. (TV: The Time Warrior)

Around the same time, a Rutan ship containing two doomsday weapons collided with the Doctor's TARDIS, causing it to crash-land on Earth. The Rutans on board were placed in suspended animation for four hundred years and lost one of the weapons. When they awoke in 1605, they sent out a distress beacon, which both the Sontaran Empire and the Rutan Host discovered. Sontarans and the Rutans raced to retrieve the weapon and use it on the other species. The Eleventh Doctor modified the weapon, as well as a second weapon Charlie stole. He gave one to each race, telling both that if one attempted to use it, they faced a possibility of killing their own species instead of the enemy. (GAME: The Gunpowder Plot)

By the early 20th century, the Sontarans were winning the war. The Rutans, who had once controlled all of Mutter's Spiral, had been driven to the far fringes of the galaxy. In the 1900s, a Rutan scout attempted to establish a beachhead for a Rutan invasion of Earth in order to launch a "final assault". The Fourth Doctor prevented it from doing so, knowing the Sontarans would simply bombard the planet with missiles, destroying humanity in the process. The Doctor destroyed the Rutan mothership, finally ending the Rutans' plans for Earth. (TV: Horror of Fang Rock)

By 1941, when humans developed destructive enough technology to cause the Warsong to begin its activation, both the Sontarans and Rutans were able to detect the Warsong stirring on Earth after its "overture" reached out across the galaxy. A Rutan spy disguised himself as a decorated Nazi veteran called Heinz Bruckner. As Bruckner, he ordered artillery strikes in the Sahara Desert to pinpoint the location of the Warsong so he could use the Earth as a weapon against the Sontarans. After activating the weapon, the Twelfth Doctor stopped Bruckner's plans by using a Sontaran osmic projector to send Bruckner's trigger mechanism through time. He then blew up the weapon with his sonic screwdriver so that neither the Sontarans nor the Rutans could claim the weapon for themselves to use against the other side. (COMIC: The Instruments of War)

In the 1980s, the Sontarans investigated the possibility of using time ships to win the war. They attempted to gain the secrets of TARDISes from the Second Doctor, but were betrayed by Chessene of the Franzine Grig. At the same time, the Sontarans were involved in a battle with the Rutans in the Madillon Cluster. (TV: The Two Doctors)

In 2001, when the Kulan attempted to invade Earth, Sa'Motta noted that the Rutan-Sontaran conflict often spilled into the Sol System. (PROSE: Escape Velocity)

In the early 21st century, the Sontarans were apparently losing the war (based on speculation by the Tenth Doctor), though naturally they refused to concede such a possibility. Apparently without authorisation from High Command, the Tenth Sontaran Battle Fleet set about invading Earth, hoping to turn it into a cloning world to produce more soldiers. The Doctor stopped their plan, and the Sontaran fleet was destroyed by Luke Rattigan, with Kaagh as the only known survivor. (TV: The Sontaran Stratagem/The Poison Sky)

Circa the late 21st century, the war saw conflict near the solar system again, leading to the Rutans attempting to capture planet Earth. A Rutan agent was sent to Earth and attempted to cause a war by infiltrating an international school of athletes. Sontarans were sent to the school, however the Tenth Doctor was able to cause the deaths of nearly all of them. The Sontaran Stenx survived and killed the Rutan, though died in the process. (PROSE: The Sontaran Games)

During either the 25th or 26th century, the Rutans destroyed the Sontaran homeworld, Sontar, with photonic bombs, though some escaped with the Sontaran racepool. (COMIC: Pureblood)

In the 26th century the Rutans took over the colony of Chelsea 426 above Saturn by using spores from flowers to take over the people. However the Fourth Sontaran Intelligence Division opposed the Rutans and attacked the colony. Due to the Tenth Doctor's intervention, the Sontarans were victorious, although later there was evidence that one Sontaran had been taken over by the Rutans. (PROSE: The Taking of Chelsea 426)

Involvement of Gallifrey
Gallifrey was involved with the war several times. During the Dark Times, the Gallifreyan hero Prydonius was sent to act as an observer in the war. (PROSE: Cat's Cradle: Time's Crucible)

Alternate timeline
In an alternate timeline, the Cybermen wiped out the Rutans before turning on the Sontarans. (COMIC: Supremacy of the Cybermen)

Other universes
In an alternate universe, the Warburg, led by General Sontar, the Warburg's Supreme Commander and the Rutan Host convened on Gallifrey to meet face to face without violence. They were both trapped in the TARDIS, and they witnessed a future time where their war had destroyed everything, and they eventually reached an agreement thanks to a parallel version of the Doctor. (PROSE: The Infinity Doctors)