"They grow up loyal to the Republic or they don't grow up at all."
Two months into the Clone Wars, and after failing to destroy all Republic clone troopers with a nanovirus, the Confederation directly attacked the planet of Kamino in an attempt to destroy the cloning facilities there. As droid forces circled in on outnumbered clone trooper trainees, both child and adult, three ARCs drop down from above and begin to take care of business. A clone child says, "You saved us." An ARC trooper replies, "You could have saved yourselves, you're not defenseless."
Image credit: Dark Horse Comics
Hastily taken out of hibernation, these clones are loyal to Jango Fett first, the Republic second; an arrangement that is null and void once the death of Fett at Geonosis. Still, their distaste of Jedi provide difficulties throughout the war.
Following the defense and repulse of Confederacy troops at Kamino, ARCs quickly go into service at the Battles of Ohma-D'un and Pengalan, performing beyond expectations.