Neutronic war

A neutronic war was a war fought with neutron bombs, just as a nuclear war was a war fought with nuclear weapons. A neutronic war was fought between the Thals and the Humanoid Daleks, which led to the mutation of both races and the creation of the non-humanoid "machine Daleks", and the ravaging of their homeworld, Skaro. (TV: The Daleks, COMIC: Genesis of Evil)

Accounts differ as to the reasons for the war. According to one history recorded a long time afterwards, the war was deliberate. (TV: The Daleks) According to another, the detonations of the neutron bombs were an accident caused by a meteorite strike. (COMIC: Genesis of Evil)

Human historians, ones who gave little credence to the history of the Humanoid Daleks, came to believe that the Neutronic War was a short conflict waged in the immediate aftermath of the Thousand Year War where the Thals, though weakened by the attack on the Thal Dome, attempted to destroy the burgeoning Dalek race who responded by detonating a neutron bomb. (PROSE: Dalek: The Astounding Untold History of the Greatest Enemies of the Universe) Time Lords who scrutinised Dalek history during the Last Great Time War believed that a neutronic war came "generations" after the Thousand Year War, while also referring to multiple "neutronic wars" as well as the "great Neutronic War" in which the Twelfth Doctor met Davros as a child. (PROSE: Dalek Combat Training Manual)

According to one account, the purported two origins of the Daleks were actually distinct events within a single history. After Davros's Daleks were buried for a thousand years by the Fourth Doctor's efforts, the Kaleds evolved into the humanoid Daleks, with the Thousand Year War having come to an end. However, after the neutronic exchange that wiped out the "humanoid Dalek", or Dal, civilisation, the Dalek Prime created by Davros reemerged from underground, and, passing itself off as a new mutation, directed the new, "natural" mutant Daleks to climb into Dalek War Machine casings, seeing it as an easy way to expand its fledgling race. It then set itself up as the new race's leader. The Dal-descended Daleks remained subservient to the Dalek Prime and the other few Daleks made directly by Davros, as programming within the casings directed them to act so. (PROSE: The History of the Daleks)

Behind the scenes
The Discontinuity Guide claims that the neutronic war and the Thousand Year War were one and the same. In the timeline before the Fourth Doctor interfered with the Daleks' creation, Davros saw to the slaughter of the majority of the Kaleds, though some remained following his death at the hands of the Daleks he had created and were contemptuously referred to by the Thals as "Dalek people" or Dals. Ultimately, the war ended when the Thals exploded a single neutron bomb, causing destruction so great that the Thals themselves were affected by the radiation, committing themselves to pacifism as a result. A group of advanced Daleks survived the explosion by leaving the planet in a hastily constructed spacecraft whilst more primitive Daleks, early products of Davros' experimental program, remained on Skaro before they were destroyed in a failed attempt to wipe out the Thals 500 years later.

According to the non-narrative reference book Doctor Who: The Dalek Handbook, the Kaled-Thal war lasted from circa 450 AD to 1450 AD when the Daleks were created and was followed by a brief neutron war falling roughly 500 years before the Thal-Dalek battle in 1963.