Time War

The Time War was a conflict between the Time Lords and the Daleks, that supposedly took place across all of time. Not much is known about the war, because all of its combatants died and the entire war was Timelocked by Dialga, meaning no time travel machine can enter it.

Chronologically, from the Galactic point of view, the war began in the late 54th century, soon after the Second Galactic War. However, battles in the war took place across all of time and even in the Time Vortex itself. The War basically annihilated both races, with only a few memebrs of each race surviving on both sides. The Doctor and The Master were the only two Time Lords to survive, and the Cult of Skaro were the only Daleks to survive.

Dalek Caan attempted to break through the Timelock of the War to rescue the Dalek's creator, Davros. However, his Passage of Time was shattered by Dialga whom Caan came face-to-face with. After acknowledging the existence of a being superior to the Daleks, Caan was driven insane and Dialga, sensing a way to stop the Dalek's meddling with Time forever, showed Caan all of Time, which convinced him that the Daleks had commited atrocities that put them beyond redemption. He allowed Caan into the War, in order to execute his plan, which consisted of wiping out the Imperial Daleks using Davros as the catalyst.

The War itself started when the Time Lords decided that the Daleks were getting too violent, since they were butchering innocent people and destroying planets. They declared war on them and fought them wherever they occured. Many people assume that the Time Lords were losing the war from the beginning, but their mastery of Time Travel meant that the Daleks were actually losing.

However, the Daleks were capable of many more atrocious acts. They razed cities, killed millions and destroyed planets just to wipe out the Time Lords. Eventually the war reached the Time Lord's homeworld of Gallifrey. In a desperate move, the Doctor was forced to activate the Time Lord relic called the Eye of Harmony, which destroyed everything in the Time War and slaughtered millions of Daleks and Time Lords. This act weighted heavily on the Doctor's conscience, and fuelled his desire to save as many races from danger as possible.