Third Dalek War

The Third Dalek War was fought between the empires of Earth and the Daleks, some time after the Human-Draconian War and the Second Dalek War which both took place during the 26th century. There was a tremendous number of human casualties that added to the losses of the previous conflicts, bringing the Earth Empire's vast population growth to an end and concluding their expansionist phase. Planets such as Mechanus were left uncolonised and the Mechonoids, Earth's servant robots, completely faded from memory. (PROSE: The Chase)

The Lucifer system was a strategic region of space during the war. However, it was completely inaccessible due to a concentration of singularities. To understand why this was, IMC hired Ace and sent her back in time to 2157 to meet with their agent Legion to try and salvage Lucifer so it could be used in the conflict. Along with the Seventh Doctor and Benny, she interfered with their plans and the Angels of Lucifer still obtained their singularities. (PROSE: Lucifer Rising)

Behind the scenes

 * The only in-universe mentions of the Third Dalek War are in John Peel's 1989 novelisation of The Chase and the New Adventure Lucifer Rising. Future stories, however, would concern themselves with the Second Dalek War, such as Love and War, Return of the Living Dad, Deceit, Abslom Daak... Dalek Killer and Prisoner of the Daleks, focusing heavily on that conflict.
 * In the televised version of The Chase from 1965, Steven merely makes reference to "interplanetary wars". John Peel replaced this with specific examples when novelising the serial, since the mythos was much larger.
 * A Dalek Wars source from Doctor Who: Battles in Time #55 gives an account of the final battle of the Third Dalek War, although it is fought by the Second Great and Bountiful Human Empire rather than the Earth Empire, which would mean it wouldn't fit into the same time period as the Human-Draconian War. After being all but defeated by the humans, the Supreme Dalek sent the remaining forces to attack the planet JT227, a planet rich with the element Promethium 62 which the humans were using to break Dalek forcefields. However, the humans' Spindroids mining the element fought back and proved resistant to Dalek fire and the Daleks were pushed back from human space.