Origins
In the 26th century the Dalek Empire tried to manipulate the rival empires of Earth and Draconia into warring with each other; they were allied with the Master and supported him with Ogrons. They also quietly conquered the planet Spiridon and began experiments to turn their armies invisible; an army of 12,000 Daleks awaited this treatment. However, the Third Doctor exposed their plan and later ensured the Spiridon army was imprisoned in ice. (DW: Frontier in Space, Planet of the Daleks)
The War
The Second Dalek War was a vicious conflict that ravaged many worlds and lasted decades. (NA: Love and War, Return of the Living Dad, Deceit) The situation was grim enough that Earth would give its worst criminals the choice of execution or becoming highly-armed Dalek Killer soldiers. (DWM: Abslom Daak... Dalek Killer)