Later in the war, the planet Umbara seceded from the Galactic Republic and joined the Separatist cause.^[36] As a result, the Republic launched a massive campaign to retake the planet throughmilitary force. When the Republic invasion army arrived on Umbara, the clone troopers of the 501st Legion spearheaded the assault under the command of General Anakin Skywalker and Captain Rex. The initial objective was to capture the planet's capital, and the success of the campaign ultimately depended on the 501st Legion. Upon landing on the planet's surface, however, the 501st immediately encountered heavy resistance from the soldiers of the Umbaran Militia. Although the 501st suffered some casualties in the first wave of the invasion, Skywalker and his troopers ultimately succeeded in repelling the Umbarans.^[7]


When Skywalker allowed the 501st time to rest before continuing the march to the capital, the Umbarans ambushed the unsuspecting Jedi and clones. Their position was resecured when General Pong Krell asquadron of BTL-B Y-wing starfighters cleared the area of hostiles with a bombing run. Afterward, Krell informed Skywalker that he was ordered to return to Coruscant by command of the Jedi High Council and at the request of Chancellor Palpatine. With Skywalker removed from the campaign, General Krell became the temporary commanding officer of the 501st Legion for the duration of Skywalker's absence.^[7]


Some of the clones in the 501st, such as Captain Rex and Arc trooper Fives, had grown accustomed to Anakin Skywalker's personal approach to leadership. Thus, they were unprepared for Pong Krell's different command style, which centered on discipline and military protocol. While the rest of the clones obeyed their Jedi officer without question, Rex and Fives came to privately and openly question the general's strategy. Prior to the Umbara campaign, Krell built a reputation as a successful general who's unwavering focus on military objectives won many key victories in the Clone Wars. This track record for success also included high casualty rates among the clones who served under him. A stern and unyielding Jedi Master, Krell had no tolerance for failure or insubordination.^[37][7]