Another trooper nicknamed Fives discovered the existence of the chips and allowed Rex and two other clones to remove them surgically, allowing them to ignore Order 66 and survive until the days of the Empire. Near the end of the war, one of his men's biochips activated prematurely, prompting him to kill a Jedi Knight in battle.
During the war, Rex commanded the 501st Legion alongside Jedi Knight Anakin Skywalker, earning his division a high reputation. After his training, Rex fought in the Clone Wars against the Separatists as a clone captain, nicknaming himself 'Rex'. Like many other clone troopers, CT-7567 was bred from the DNA of Jango Fett and trained from birth to serve the Republic.