In The Daleks, Thal records - a mixture of oral legend and historical texts - are said to go back about half a million years. The Thals were a warrior race, while the ancestors of the Daleks (Kaleds) were teachers and philosophers. About 500 years before the events of The Daleks a thousand-year-old degenerative war between the two races came to a conclusion (Genesis of the Daleks). Davros, the leading Kaled scientist, developed the Daleks as a 'housing' for mutated Kaleds. He was killed by his creatures after slaughtering the majority of his people. As a term of contempt, the Thals termed the (remaining) Kaleds 'Dalek people' (or - on one occasion in The Daleks - Dals).
Despite these events, the war did not end until the Thals exploded a single neutron bomb. The destruction was so great that the Thals themselves were affected by the radiation. They committed themselves to pacifism as a result.
A group of advanced Daleks survived the explosion by leaving the planet in a hastily constructed spacecraft. Those Daleks left behind were early products of Davros's experimental programme and, although many survived the neutron bomb, they remained trapped in the bunkers beneath the Kaled city for centuries. These Daleks were dependent on both static electricity and high levels of radiation, and their weapons were comparatively weak. The primitive Daleks were destroyed by the Doctor in this story.
(The Doctor never establishes the date of this story, and his comment in The Dalek Invasion of Earth - that The Daleks is set a million years in the future - is pure (inaccurate) speculation. By the time of Planet of the Daleks (the 26th-century ) the tale of the Thal’s penetration of the Dalek city has become a legend.)
The more advanced Daleks developed their own technology, which enabled non-Kaleds to become Dalek mutants. A large number then returned to Skaro and began working on time travel technology in conjunction with Theodore Maxtible (The Evil of the Daleks). (The date is difficult to ascertain, but would have to be somewhere between the 19th and mid-22nd centuries.) Civil war broke out on Skaro, and for a long time the Thals were once more able to live there in peace.
One of the ships that escaped the destruction on Skaro crashed on Vulcan in the 21st or early 22nd century. (Therefore, they recognise the second Doctor, and the Earth colonists have not heard of the Daleks.) Mankind's first major encounter with the Daleks came a little later in 2164. When the Daleks were defeated (The Dalek Invasion of Earth) they completed their time travel programme ('The Daleks discovered the secret of time travel') and attempted to turn this defeat into victory by going back in time and invading the Earth in the late 21st century. ('We have invaded Earth again. We have changed the pattern of history.')
The Doctor's intervention a century later (Day of the Daleks) was so successful that this alternative time-line did not happen. (The Daleks knew of the Doctor, but did not recognise his third incarnation, using a mind analysis machine to confirm his identity.)
Stung by their twin defeats, the Daleks pursued the first incarnation of the Doctor (The Chase), reasoning that if his future intervention could be neutralised their plans would succeed.
Between the 23rd and 25th centuries the Daleks encountered the third Doctor in an untelevised adventure, and then developed a deadly space plague (Death to the Daleks). (The Doctor is recognised in Death to the Daleks, which would seem to take place before Planet of the Daleks.)
On Skaro meanwhile, the Thals ended a period of pacifistic isolation by developing space craft. Their first lengthy mission was to Spiridon, where, with the Doctor's help, the Daleks were defeated again. (The Daleks in that story identify the Doctor.)
Mission to the Unknown states that for over a thousand years the Dalek campaign ignored the Milky Way completely. From approximately 3500 they waged campaigns in the Ninth Galactic System and in the constellation of Mir, conquering 70 and 40 planets in the two regions respectively. The Daleks also returned to Skaro, wiping out the Thals who lived there. They had hoped to receive a new direction from their creator, but Davros had not survived.
In the year 4000 the Daleks returned their attention to the solar system, forging an alliance with other races (The Daleks' Master Plan). Once again, the Doctor interceded.
Despite all these defeats the Daleks were never entirely wiped out and the Time Lords predicted a time when the Daleks could become the dominant life-form in the cosmos. As a result of the Doctor's intervention Dalek 'history' was massively changed. (See The second history of the Daleks).