Dalek timeline

"We foresee a time when they will have destroyed all other lifeforms and become the dominant creature in the universe."

- A Time Lord messenger.

The Time Lords once foresaw a time stream (PROSE: Doctor Who and the Genesis of the Daleks) or timeline (PROSE: Dalek Combat Training Manual) in which the Daleks would succeed in destroying all other life and so become the dominant species in the universe. The Fourth Doctor was sent to Skaro to interfere with the creation of the Daleks by Davros' hand, averting this time by creating the Genesis timeline by setting the Daleks back by 1000 years. (TV: Genesis of the Daleks) While some accounts indicate this averted timeline was the Dalek history which the Doctor had heretofore known, (PROSE: A Device of Death, Lungbarrow) others indicated it was an aberrance from the unchanging Dalek timeline which had been reinstated by the 1000-year delay. (PROSE: The Dalek Problem, A History of the Universe)

One account indicated that the supposed future in which the Daleks had destroyed all other life was just been a claim from Narvin to save Romana, sending Valyes back to give the Fourth Doctor his mission from a Gallifrey in the Doctor's future. (AUDIO: Ascension) Other accounts treated it as a legitimate future the Time Lords had foreseen. (PROSE: The Whoniverse, Meet the Doctor, et al.)

Regardless, numerous observers including the Doctor themself saw the mission as the "first shot" (AUDIO: The Dalek Conquests, The Innocent, The Eternity Cage, PROSE: Engines of War, The Astounding Untold History of the Greatest Enemies of the Universe, COMIC: Hunters of the Burning Stone) of what ultimately became the Last Great Time War, a conflict which had far-reaching consequences across the history of the universe. (PROSE: The Whoniverse, The Astounding Untold History of the Greatest Enemies of the Universe, Dalek Combat Training Manual) Indeed, the Daleks did at some point become aware of the Time Lords' attempt to subvert their development, which they henceforth viewed as the Gallifreyans having launched a pre-emptive strike and act of aggression. (PROSE: The Slyther of Shoreditch, WC: , AUDIO: The Dalek Conquests, The Innocent, The Eternity Cage) The Tenth Doctor later admitted that, for the Daleks, their history was "confusing enough" before the Time War. (PROSE: Prisoner of the Daleks) In a parallel universe in which he chose to destroy the Daleks, the Fourth Doctor was embroiled in the Time War as it started prematurely, regenerating into the Warrior. (AUDIO: Dust Devil)

By the Time War, Ironically, there were some Gallifreyan scholars who believed that, had the Time Lords allowed the Dalek timeline to evolve without interference, and so let Davros refine his creations at that early crucial stage, then they might have become the less aggressive creatures that had sought to create. (PROSE: Dalek Combat Training Manual)

Nature
When Davros interrogated the Fourth Doctor on future Dalek conflicts, the Doctor mentioned that the Dalek invasion of Earth occurred in 2000. (TV: Genesis of the Daleks) By one account, this was evidence of a crucial divergence in history which allowed Daleks to come to power earlier, allowing them to one day destroy all life. (PROSE: A History of the Universe)

Extermination of all other life
Indeed, the first Daleks created by Davros acknowledged that their programming did "not permit to acknowledge that any creature [was] superior to the Daleks" and that "all inferior creatures were to be considered the enemy of the Daleks and destroyed". (TV: Genesis of the Daleks)

In the post-Time War universe, The Daleks came especially close to fulfilling this goal during the 21st century Dalek invasion when, with the support of Davros, they attempted to use the reality bomb to destroy all life across not only their universe but all universes save themselves, with the Supreme Dalek proclaiming that they would "become the only life forms in existence". Ultimately, however, this plan was thwarted by the DoctorDonna with the support of the Children of Time, with the Tenth Doctor crediting Donna Noble with the saving of the universe. (TV: Journey's End) In one universe hinging on a change to history resulting in Donna not being present for such an event, the stars began to go out; (TV: Turn Left) as well as people, the reality bomb was capable of destroying planets and stars. (TV: Journey's End) Siblings Different and Same visited timelines in which Donna herself had never existed. (PROSE: The Paradox Moon)

After acquiring the Eye of Time, the New Dalek Paradigm established themselves on their home planet Skaro and began to blatantly change history, invading Earth and exterminating humanity in the year 1963. Having discovered this, the Eleventh Doctor went to Skaro and confronted the Dalek Emperor, who vowed that "soon, throughout all time and space there [would] only be Daleks". However, the Doctor thwarted these ambitions by going back in time to the ruined Skaro and setting the Eye free, thus negating the Daleks' changes to history. (GAME: City of the Daleks)

Behind the scenes

 * The Dalek Handbook suggested that the Time Lords had ignored the Daleks' original wave of conquests and their development of time travel technology before the combination of the Daleks' large-scale efforts to alter history along with the Master's alliance with them and the Doctor's subsequent appeal for help motivated the Time Lords to notice the Daleks, foreseeing the potential future which led them to enacting the Genesis Incident.