The Battle of Sullust unfolded during the Clone Wars era, marked by Sullust's decision to join the Confederacy of Independent Systems. The Separatist victory over the Republic forces was facilitated by a turncoat within their ranks.
Sullust declared its secession at the dawn of the Clone Wars. In response, within a month, the Galactic Republic launched an invasion to reclaim the planet. This task force, originating from Eriadu, consisted of clone troopers combined with local planetary defense forces, and was under the command of Jedi Knight Rees Alrix. At some point during the conflict, a Republican betrayed his comrades, causing a shift in the battle's momentum against the Republic.
The Republic suffered significant losses and were defeated in the battle for Sullust due to the betrayal from within. However, despite the defeat, the Republic managed to regain control over a large portion of the Sluis sector. Although the Separatists experienced a loss in a space battle above Sullust in 21 BBY, the initial Separatist triumph at Sullust enabled the planet to remain a highly valued asset of the Confederacy until the war's conclusion.
Concerning the Battle of Sullust, The Clone Wars Campaign Guide indicates that Sullust's secession to the Confederacy occurred at the beginning of the war, with the battle following less than a month later. This account potentially contradicts The New Essential Chronology, which suggests that Sullust remained a loyal member of the Galactic Republic for the first year of the war, serving as a fortified world along the CIS border, before eventually seceding to the Confederacy, prompting the Republic to deploy a task force to attempt to reclaim the planet.