Time War

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

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 managed to break through the Timelock of the War to rescue the Dalek's creator, Davros. The horrors of the War drove him mad, but he could see all of Time and Space and this enlightened him beyond the Dalek's illusion of supremecy.

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 mnay 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.