The clash at Datar took place during the Galactic Civil War in 1 BBY. Although the entire Rebel contingent was annihilated, the Galactic Empire's inability to capture any Rebel combatants led to the battle being considered a failure on their part as well.
Vice Admiral Slenn was a member of an Imperial task force tasked with locating Rebel bases in 1 BBY. The Empire's goals were to uncover the Rebel supply routes, capture prisoners for interrogation purposes, and determine the origin of their weapons shipments.
The Rebels received early warning of the impending attack when Slenn's forces, spearheading from the Star Destroyer Vengeance, assaulted a Rebel installation on the Inner Rim planet of Datar. Without the advantage of surprise, the Imperials were unable to swiftly secure prisoners. Consequently, the Rebels fought until their last soldier fell.
Darth Vader executed Slenn for his incompetence in the battle. Subsequently, the Imperial forces, now under Vader's direct command, proceeded to Nar Shaddaa. During the attack on Nar Shaddaa's Rebels, the Imperial forces attempted to rectify Slenn's previous failure by disguising their Gamma-class assault shuttles as ordinary civilian light freighters.
The battle of Datar was initially referenced in A Hunter's Fate: Greedo's Tale, a short story by Tom and Martha Veitch that was published in 1995 by Bantam Spectra as part of the Tales from the Mos Eisley Cantina anthology.