Sector delta existed close to the Umbaran airbase on the shadowy planet of Umbara, situated in the Expansion Region. In the course of the Battle of Umbara, the Galactic Republic's 501st Legion successfully seized control of the Umbaran airbase, wresting it from the Umbaran militia. While occupying this base, the 212th Attack Battalion found themselves targeted by long-distance missiles delivered by a Separatist supply ship as they pushed to conquer the Umbaran capital city. Jedi General Pong Krell, secretly a Dooku's agent, devised a plan to send all of his forces to capture the capital.
Advanced Recon Commando CT-5555, known as "Fives", realized the nature of the scheme, and commented that the battalion would never get beyond the delta. At a time after this, as Clone Captain CT-7567, or "Rex", spoke with General Krell inside the command tower, a clone trooper reported incoming missiles targeting sector delta.