The clash at Polis Massa represented a notable engagement in 0 BBY during the initial stages of the Galactic Civil War. It involved forces of the Galactic Empire and the Alliance to Restore the Republic. This conflict unfolded on the mining asteroid colony of Polis Massa, a location then utilized by the Rebel Alliance.
Following the seemingly contained Death Star Uprising, the Empire discovered that some rebels had managed to smuggle away crucial data pertaining to project blueprints from the Death Star. Discovering an active rebel installation on Polis Massa, Darth Vader opted to deploy units from his distinguished military unit, the 501st Legion. Their mission: to seize the data before the rebels could decipher it.
Upon the 501st Legion's arrival at the asteroid base, they initiated their operation, swiftly securing the rebel radar facility. Subsequently, they moved to assault an elevated command center believed to house a holodisk containing the stolen schematics. However, as they approached the stairwell leading to the room, a multitude of rebels, including Wookiee combatants, flooded the hallway at the base of the stairs, eliminating nearby stormtroopers. A subsequent Imperial surge quickly suppressed the exposed rebels.
The data was then extracted from the disk and transmitted to Imperial systems while the 501st were forced to defend the radar-communications facility, which was now completely surrounded by Rebel soldiers. Concurrently, Imperial analysts examining the data on the holodisk were surprised to find it was a forgery. Through covert espionage by Bothan agents within the rebel ranks, the actual plans had been transmitted beyond the Empire's reach, replaced by duplicates on the holodisk.
The 501st found themselves isolated in the radar area, fighting a futile battle without further instructions. When questioned about the authenticity of the holodisk, their superiors ordered them to continue fighting. In a surprising turn of events, the 501st executed a daring maneuver, breaking free and dispersing throughout the base.
While reorganizing in various sectors of the base, the 501st Legion received new directives: to destroy a databank, located in an area known as the Cavern, which the Rebels were using to intercept Imperial communications. Driven to fulfill this objective, the stormtroopers of the 501st fought their way into the depths of the base and destroyed the databank.
Now without any further objectives, the 501st Legion resolved to crush the rebels at this base—the insurgents who had tarnished their renowned reputation in an obvious trap—and escape from the asteroid alive. Ultimately, both objectives were achieved. Rebel and Imperial forces operated the same AAC-1 Hovertanks in the outside perimeters of the base until Rebels captured the central hangar. Always prepared, stormtroopers raided the hangar furiously, though, and it was firmly secured.
As the fighting subsided, 501st officers recognized the extent to which they had weakened the rebel forces. After a few more engagements, the base was completely cleared of rebels. The captured stronghold was not occupied for long, however, as the 501st soon deactivated it and were deployed to the Tatooine system.
Shortly thereafter, the Empire traced the CR90 corvette, Tantive IV of Alderaan, using the holodisk acquired at Polis Massa. The ship was suspected to be carrying the real plans. The Empire, determined not to be deceived again, dispatched Darth Vader to lead the next attack personally.
The Polis Massa raid was initially featured in the Pandemic Studios video game Star Wars: Battlefront II, which debuted on November 1, 2005.