Rex's platoon of the 501st Legion served in the Grand Army of the Republic during the Clone Wars between the Galactic Republic and the Confederacy of Independent Systems. Part of a deployed 501st battalion, the platoon was part of the Grand Army of the Republic's offensive on the planet Umbara, and participated in the initial landing of the battle.
As the battle progressed further towards the planet's capital, Jedi General Anakin Skywalker, the 501st's revered leader, was relieved by Jedi General Pong Krell after he was called back to Coruscant. Krell wasted no time in deploying the platoon on a march down the main road to the capital city, a move protested by Clone Captain Rex, who also led the unit. Nonetheless, they followed orders, which led to a deadly ambush from Umbaran Militia, causing numerous casualties and the forced retreat of the platoon.
As the 501st progressed further into Umbaran territory, Krell ordered the platoon, along with the rest of the 501st, to advance through a narrow gorge towards an airbase, a vital target for the Republic due to its capability to supply starfighters to the joint Confederacy and Umbaran defensive efforts. The decision was met with some resistance, as members of the platoon voiced their concern about Krell's full-frontal attack method, which barely constituted as a tactic at all.
Concerns aside, the platoon, backed by the rest of the legion, advanced into Umbaran territory. They were met with heavy resistance by Impeding Assault Tanks and mobile heavy cannons, but due to the rebellious actions of platoon members Hardcase and Fives, the clone forces were able to repel the attacks using the Umbaran's own technology against them, as the two troopers managed to commandeer and haphazardly perform deadly strafing runs on the Umbaran tanks.
The platoon served as a part of a battalion of the Grand Army of the Republic's 501st Legion and was comprised of clone troopers divided into four squads, some of whom were ARF troopers. They were equipped with Phase II clone trooper armor, and utilized All Terrain Reconnaissance Transports, RPS-6 rocket launchers, and mortars during the Battle of Umbara. Several clones in the platoon wore backpacks.
AT-RTs similiar to those used by the platoon begin the assault on Umbara
During the Galactic Republic's attack on the Confederacy-affiliated Expansion Regions world of Umbara in 20 BBY, during the Clone Wars between the Galactic Republic and the Confederacy of Independent Systems, the platoon saw intense action during battle as a part of the 501st Legion. They were among the first units on the planet, and spearheaded the attack into Umbaran territory. After the legion completed the landing phase of the invasion, they were forced to repel attacks from Umbaran Militia, including a well-placed millicreep droid, with the combined efforts claiming the lives of numerous clone troopers.
Once the 501st gained a foothold on the planet, Jedi General Anakin Skywalker, who had been leading the legion until that point, was relieved and ordered to return to the Republic capital of Coruscant. The 501st's leader was subsequently replaced by Jedi General Pong Krell, a Besalisk known for his high success rate in battle, but at an astonishingly high casualty rate. Shortly after taking command, Krell ordered the platoon, to march down the main road in an attempt to reach the Umbaran capital city as fast as possible, a decision that was looked upon as near suicide due to the likelihood of traps and ambushes from the Umbaran militia, which was highly active in the area.
Clone trooper Hardcase provides covering fire for retreating platoon members
Although Clone Captain Rex protested the order, he resignedly led the platoon into battle. Before the platoon were able to make contact with enemy forces, they lost two soldiers to land mines placed by the insurgency. As expected, the militia attacked in force, resulting in heavy casualties to the platoon. The platoon retreated, taking losing even more troopers in the process. The depleted unit was eventually rescued by the main 501st as they withdrew from contact, angering Krell but saving the survivors of the platoon.
Fives, Captain Rex, and General Pong Krell participate in the fighting on Umbara
As the battle for Umbara continued, Krell ordered the 501st to continue their offensive toward an airbase in an attempt to relieve the pressure off of Jedi General Obi-Wan Kenobi and the 212th Attack Battalion, who were engaged in a separate assault on the planet. Although Rex suggested that the legion should send recon units out to provide intelligence regarding terrain and the enemy defenses, Krell ordered that the entire legion into a frontal attack. This command was met with some resistance from the already beleaguered clone troopers, especially from Advanced Recon Commando trooper Fives. His dissent was backed by other members of the 501st, but Captain Rex curtly told his friend and comrade that, as soldiers, expected to follow orders—even when they were bordering on suicide.
The 501st, led by members of the platoon, marched into tentatively into the Confederacy-occupied territory. As expected, the 501st—including Rex's platoon—was attacked by Umbaran Impeding Assault Tanks, which emerged from underground ambush locations to attack the troopers. Significant casualties were sustained by the legion before troopers eventually lured the tanks into a narrow kill zone, enabling troopers armed with RPS-6 rocket launchers and thermal detonators. The clones continued moving forward, executing Umbaran survivors as they moved. However, their victory was short-lived, as Umbaran reinforcements arrived, this time equipped with spider cannons, which were impervious to blaster or rocket fire. The 501st was pushed back once again and was forced to leave behind dead and injured troopers, much to the chagrin of the platoon medic, Kix.
Hardcase and Fives infiltrate the airbase
Although the 501st was in no position to repel the Umbaran attack, Krell ordered the clones to continue the assault, disregarding the horrendous losses being sustained. In a desperate attempt to destroy the tanks, Rex authorized troopers Hardcase and Fives, both members under his platoon under his direct command, to infiltrate the airbase—a mission that they had been specifically ordered not to attempt. Their mission was successful, and they were able to commandeer two Umbaran starfighters, which they subsequently utilized to strafe the Umbaran tanks, destroying them and enabling the main 501st force to finally capture the base.
Rex's platoon first appeared, albeit unnamed, in the seventh episode of Star Wars: The Clone Wars Season Four, titled "Darkness on Umbara," which was released on October 28 2011. In the episode guide to the later episode "Carnage of Krell," released on November 18 of the same year, another platoon with identical composition is noted. Since both platoons display the same members and leadership, this article assumes that the platoons are one and the same.