Captain Rex's battle strategy algorithm was developed by Rex, a Clone Captain within the 501st Legion of the Grand Army of the Republic. Its purpose was to scrutinize Republic Military tactics, pinpointing vulnerabilities and devising countermeasures. The only other individuals privy to this algorithm were ARC troopers Fives and Echo. During the Battle of Anaxes, the unexpected success of the Separatist Droid Army in repelling Republic forces across the board prompted Rex to speculate that the Separatists had somehow obtained his algorithm. This was despite the fact that Echo and Fives were both presumed dead, and he believed himself to be the sole keeper of this knowledge. Rex, accompanied by Tech from Clone Force 99, infiltrated the Separatist Cyber Center on Anaxes and discovered the algorithm was being actively broadcast from Skako Minor by Echo, who was still alive and held captive by the Separatist-affiliated Techno Union. The Separatist Admiral Trench had been exploiting the algorithm to gain an advantage over the Republic's military operations.
Clone Captain Rex of the 501st Legion, a highly regarded officer within the Republic Military during the Clone Wars, conceived the algorithm. Its function was to analyze the strategies of the Galactic Republic and formulate responses to any weaknesses that were identified. Apart from Rex, only ARC troopers Fives and Echo possessed knowledge of its existence.
Sometime before the Battle of Lola Sayu, Rex created the algorithm, sharing the knowledge exclusively with Fives and Echo. Following Echo's apparent death in an explosion during the Lola Sayu mission, and Fives' subsequent demise on Coruscant after his alleged descent into madness, Rex appeared to be the only surviving individual aware of the algorithm's existence. However, Echo's death at the Citadel was not what it seemed. Critically injured in the explosion of the Sheathipede-class Type B shuttle he was attempting to recover, Echo was rescued by the Separatists and eventually fell into the hands of the Techno Union. They transformed him into a cyborg and extracted information from his mind, enabling them to utilize his knowledge of the algorithm. Held in stasis within a facility in Purkoll on Skako Minor, he remained unaware of his surroundings or the exploitation he was subjected to.
As the campaign to secure the planet Anaxes unfolded, Admiral [Trench](/article/trench], who was commanding the Separatist forces, arranged for the algorithm to be transmitted from Skako Minor to a Cyber Center constructed on Anaxes' surface, where he could analyze it from his command ship. The Republic's control over the planet progressively weakened over several weeks, with their forces enduring unexpected and significant losses. Captain Rex suspected that the Separatists were somehow using his strategies and the algorithm against them, leading him to believe that Echo was the only possible source, suggesting that he might still be alive. He and Clone Commander Cody presented their theory to Jedi Generals Mace Windu and Anakin Skywalker, explaining that the Separatists had somehow learned to counter Rex's tactics. However, Rex hesitated to voice his belief that Echo was alive, fearing the Jedi would dismiss him as delusional. Rex and Cody were granted permission to lead a covert team behind enemy lines to the Cyber Center to uncover the source of their strategic disadvantage.
Cody enlisted Clone Force 99, an unorthodox unit of clone commandos known informally as the "Bad Batch," to join the mission alongside himself, Rex, ARC trooper Jesse, and clone medic Kix. Cody initially expressed skepticism upon learning about the theory that Echo was still alive. During their journey, the strike team's gunship was shot down, resulting in Cody's severe injury and Rex assuming command of the mission. Kix remained with Cody to await evacuation. Upon infiltrating the Cyber Center through a coordinated two-pronged assault, Tech from the Bad Batch accessed the center's computer systems and discovered that the algorithm was being transmitted live rather than being a stored program. Further investigation by Tech revealed a transmission containing Echo's voice. When Rex requested identification of the voice, the response was Echo's designation number, CT-1409, confirming his survival.
This revelation prompted Rex, Skywalker, and the Bad Batch to initiate an unauthorized mission to Skako Minor to rescue Echo from the Techno Union. Despite encountering significant resistance from battle droids and Trench's warning to Wat Tambor about the potential infiltration, the strike team successfully reached Echo and freed him from the stasis chamber. After Tambor's battle droids failed to recapture Echo, Tambor decided to delay informing the Separatist military of the loss, as he wanted to find a way to recoup the lost profits that the ARC trooper's escape represented.
In the story reel version of "The Bad Batch," Rex stated that he created the algorithm to counteract the tactical tendencies of the Grand Army of the Republic.