The Battle of Cassius took place near the planet of Cassius beyond Pluto's orbit. It occurred after the First Doctor's defeat of the Daleks' plot to turn occupied Earth into a mobile warship, which resulted in the occupation force being crippled. (TV: The Dalek Invasion of Earth)
The decisive moment came when Commander David Jarvis, aboard the Colonial warship Dauntless, disregarded orders and fired a critical volley. (PROSE: The Crystal Bucephalus)
At the conclusion of the battle, the Daleks' solar system blockade was shattered, forcing them to leave Earth. (PROSE: Love and War)