Thal assault on Skaro

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

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)

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)