Thousand Year War

The Thousand Year War was fought on the planet Skaro between the Thals and Kaleds. The war led to the creation of the Daleks by the Kaled scientist Davros.

Causes
The exact reason or purpose of the war remains unknown.

By the time that the Fourth Doctor visited Skaro on a mission to prevent the Daleks' creation, the Thals and Kaleds lived in two domed cities with a wasteland between them. Inside the Kaled Dome, the Scientific Elite, headed by Davros and his assistant Nyder, and a Military Elite made decisions. The use of nuclear and chemical-biological weapons had led to the creation of the Mutos. The Mutos were mutated outcasts from both races who scavenged the wastelands.

By this time, so many of the Kaleds and Thals had died that they resorted to commissioning boys in their teens and twenties as military officers. (TV: Genesis of the Daleks)

Weapons and Technology
An anachronistic mixture of technologies on the battlefield showed a society which had devolved rather than evolved, with combatants using energy weapons alongside primitive rifles and machine guns and engaging in trench warfare as in Earth's early 20th century. Their uniforms were frequently mismatched - soldiers could be seen wearing modern combat jackets made of synthetic fibre with other articles of uniform made of leather.

In the long-range, as well as the biological and nuclear weapons already mentioned, both sides used chemical rockets. Towards the end of the war, the Thals intended to use a neutronic missile against the Kaleds. (TV: Genesis of the Daleks)

Conclusion
The war did not conclude so much as peter out. Davros gave the Thals information to allow them to breach the Kaled Dome, causing massive casualties amongst Davros' own people.

This provided a rationale for the Kaleds in turn to set the newly created Daleks against the Thals. At last, a Thal rocket destroyed the Kaled Dome. Thals, led by Bettan, and Mutos, led by Sevrin, joined to fight against the Daleks emerging from the ruined Kaled city. (TV: Genesis of the Daleks)

A considerable discrepancy exists between future accounts describing the war as just a quick neutronic war (TV: The Daleks). This may be a memory of the start of the war or indicate that legend replaced accurate history. It could be that the neutronic war followed the thousand year war. It is also possible that the Fourth Doctor's alteration of the timeline averted the neutronic war altogether.

The war led to the near extermination of the Thals and traumatised them to a point that they embraced strict pacifism and simple living, something that they would not change for many hundreds of years. (TV: The Daleks)

Behind the scenes
According to the non-narrative source REF: Doctor Who: The Dalek Handbook, the war lasted from 450 AD to 1450 AD.