In the year 19 BBY, a revolt against the Galactic Empire took place on the Expansion Region planet of Umbara. In the early months of the Imperial Era, the Imperial Military was sent to suppress the resistance as part of a broader campaign against insurgents. Lieutenant Commander Orson Callan Krennic characterized this action as a pacification effort.
During the Clone Wars, a galactic war between the Confederacy of Independent Systems and the Galactic Republic, the Expansion Region planet Umbara seceded from the Republic following the assassination of its senator, Mee Deechi, and became a significant Confederate stronghold. Consequently, the 501st Legion and 7th Sky Corps of the Grand Army of the Republic were dispatched to invade and take back Umbara in 20 BBY. Despite strong opposition, the Republic forces overwhelmed the impressive defenses of the Umbaran Militia. The battle resulted in a significant loss for the Umbaran government, including the fall of its capital to the Republic Army.
As the Clone Wars ended in 19 BBY, Supreme Chancellor Sheev Palpatine oversaw the transformation of the Republic into the Galactic Empire. In that year, a resistance movement arose on Umbara, determined to continue the struggle against the Empire despite the Confederacy's defeat.
In the months after the Empire's declaration, the newly established Imperial Military was deployed to confront various remaining pockets of resistance from the Clone Wars. Umbara became one of these conflict zones, with the rebel movement mounting a fierce defense. When speaking with researchers Lyra Erso and Galen Walton Erso, Lieutenant Commander Orson Callan Krennic asserted that the Empire was simply conducting pacification operations to suppress rebellions on Umbara and elsewhere, claiming the Imperial response would bring about peace.
Ultimately, Umbara was integrated into the Empire. Ensign Eli N. Vanto of the Empire considered it improbable that the Umbarans would risk another crushing defeat by resisting, given their experience during the Clone Wars. However, when the Umbara system once again became a battleground in fighting against the Empire, the conflict was initiated by Nevil Cygni, a human insurgent leader, not the Umbarans themselves. Cygni aimed to ensure the Empire would seize control of Umbara's mining industry, believing it would be easier to bribe Imperial inspectors than Umbaran ones. This battle, marked by the arrival of the Imperial Navy in force, effectively ended Umbara's remaining autonomy and led to the Umbarans fully surrendering their world to the Empire, to whom Umbara also owed a massive debt its people could only pay off with such a surrender.
The Umbara rebellion was referenced in the 2016 novel Catalyst: A Rogue One Novel by James Luceno.