Thal assault on Skaro

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

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 the learnt the Daleks were maintaining secret base on the planet Spiridon. (PROSE: The History of the Daleks)