Thal assault on Skaro

Prior to the Spiridon campaign, the Thals had launched a successful assault on their home planet of Skaro.

Background
Whilst the Thals prospered after taking the Dalek City, the Daleks who had been trapped since the end of the Thousand Year War had perfected underground mining systems, which automatically delved for the metals that their automated production lines needed. Left alone with the vast embryo banks, the Daleks built up their numbers and honed their warfare techniques, studying the computer records that had been entombed with them. Filled with a hatred for all other life forms, the Daleks emerged to invade Skaro, bringing death to the bewildered Thals after five hundred years of peace. This attack was led by the Dalek Prime's warlord, the Black Dalek, reducing Skaro to a burning cinder in a matter of hours.

With the initial attack, the Thal starships still close to the world returned. There, they collected what survivors they could find, and whatever could be salvaged before the Daleks destroyed their society. They then fled for space, leaving the Daleks as the masters of Skaro. After all the Thals' efforts, Skaro was was uninhabitable again. The overcrowded evacuation fleet staggered across space to one of the colony worlds that the Thals had been experimenting with. Now they had no option but to trust the planet for their survival. With the arrival of the ragged fleet, the Thals vowed that never again would the Daleks take them by surprise. Putting aside their stance of pacifism, the Thals determined that they would somehow, someday, utterly destroy the Daleks. (PROSE: The History of the Daleks)

The Assault
In the centuries following the failure of Project Degravitate during the 2150s Dalek invasion of Earth, the Daleks had spread to Skaro's neighboring star systems, mining the planets for materials. In doing so, they committed a grave tactical error by neglecting defense of Skaro, having underestimated the Thals; since their homeworld was worn down and they had no particular attachment to it, they assumed this would be true of others. The Thals, however, were grimly determined to regain their homeworld, and they managed to launch an assault from their colony planets back at Skaro. As a result of Dalek laxity, the Thals managed to seize their planet back again, destroying all Daleks based there at the time. This assault severely crippled the Thal war efforts, for they were still a peaceful people. The Thals, however, understood that they would never know peace until the Daleks were destroyed and felt a deep guilt that part of the blame for the creation of the Daleks rested upon them, vowing to sweep the galaxy free of their enemies. Accordingly, they began to develop deep-spaceships of their own, ready to strike and cripple the Dalek forces whenever they could. In the captured computer files, they discovered details of the Dalek communications system, which they modified so they could monitor their foes. It was from this that they learnt the Daleks were maintaining secret base on the planet Spiridon. (PROSE: The History of the Daleks) The Thals embarked on the Spiridon campaign where they discovered the Daleks had assembled a ten thousand-strong army and encountered the Third Doctor who helped them entomb the army in ice. (TV: Planet of the Daleks)

Aftermath
Whilst considering the development of time technology, the Daleks' immediate concern was the removal of the Thals from Skaro. Although they had no sentimental attachment to their home planet, the Daleks were unwilling to allow the Thals to remain established there, knowing that they would form a second front to attack them. The Dalek Prime deemed that the simplest solution was the mass bombing of the planet, rendering it lethal. As before, the Thals learned of this plan by monitoring Dalek communications. Knowing they had no chance at all of stopping the Dalek attack, the Thals resolved to abandon Skaro again. To prevent the Daleks from realising that they had escaped, a small number of Thal troops remained behind to stage a fairly convincing defense of Skaro in what was effectively a suicide mission, buying time for their people to retreat and consolidate before fighting the Daleks another day. The small band of volunteers fought long and hard, but ultimately fell to the massed Dalek firepower. The Daleks dropped several neutron bombs on the Thal settlements, not realising they were already abandoned. (PROSE: The History of the Daleks)

Behind the scenes

 * The Terrestrial Index stated that the Thals were "exiled" from Skaro before eventually "reconquer[ing]" the planet during the Second Dalek War in the 25th century, forcing the Daleks to adopt a replacement homeworld. Ultimately, following the Third Dalek War, the Daleks launched a new attack on Skaro, before the end of the 26th century, in which they exterminated most of the Thals, leaving the planet a bombed-out ruin which would be not be of use to the Daleks until the Movellan Incident.