Darkness on Umbara " Darkness on Umbara " is the seventh episode of the . It aired on October 28 , 2011 . Official description . " " When Anakin is forced to temporarily turn over command of his clone trooper s to a new commander , the Jedi Pong Krell , tensions begin to run high as the clones are assigned with a very deadly mission to take the capital of Umbara . " " Synopsis . Aboard their command ship , Anakin Skywalker and Obi - Wan Kenobi discuss their attack strategy against the Umbaran capital , which is defended by both Confederate and local Militia forces . Joined by Rex and Fives , Anakin prepares to repel enemy reinforcements striking against Obi - Wan ’ s joint attack with Jedi Masters Saesee Tiin and Pong Krell . Amidst heavy flak fire , the Republic AT - RT forces manage to take out the Umbaran tanks , allowing the Transports to set down their load of clone trooper s . But the assault is hampered by the planet ’ s natural murkiness and foggy conditions , as well as the local wildlife , one of which nearly claims the lives of Fives and Hardcase , though Fives manages to kill it with a thermal detonator . When Anakin Skywalker and his clones take a break before preparing for their mission , the Umbarans execute a sneak attack forcing them to retreat . An airstrike by Master Krell saves them just in time , but Krell then approaches Anakin with a disconcerting message : Anakin is to return to Coruscant and meet with Supreme Chancellor Palpatine , for reasons unknown . Reluctant to leave his troops in the midst of battle , Anakin nevertheless is forced to relinquish command over his troops to Master Krell and make the journey back to the Republic capital . As soon as Skywalker departs , however , Krell adopts a very derisive , condescending stance towards the clone troopers , showing them no respect or consideration . His harsh determination to complete the mission at all costs , no matter what , gradually raises the ire of the clone troopers . When they reach the capital , Krell insists on a change in the attack plan , opting on a full frontal assault on the capital ’ s main road instead of a series of preemptive surprise attacks against the enemy ’ s defenses – a course which would expose them to heavy enemy attack - and , as Rex feared , the battalion begins to run into enemy traps , with mines on the main road and an all - out ambush which claims the lives of many troopers . Rex orders his troopers to retreat in order to attain a more advantageous fighting position . Though frustrated by the clones ' retreat , Krell ordered additional troopers and AT - RT drivers to save Rex and his men . After they manage to repel the natives , Krell immediately begins to criticize Rex for retreating and compromising his plans . Rex and Fives stand up to him , voicing their protests and pointing out that , while they are aware of the importance of their mission , they are also concerned about the welfare of their men . Krell eventually gives Rex a dubious compliment , but before Rex and Fives have the time to contemplate Krell ’ s words more thoroughly , the Umbarans launch a counter - attack , forcing them to refocus on the battle . . . Behind the scenes . In the episode , when General Krell scolds Captain Rex , he refers to him as " CT - 7567 " , even though Rex is a captain and holds the designation CC - 7567 , per many sources . This same mistake is made when Fives stands up to General Krell , in which Krell addresses Fives as " ARC - 5555 , " though Fives holds the designation CT - 27 - 5555 . Rex ' s Encyclopedia entry on StarWars . com later confirmed that his new designation is CT - 7567 .