The Galactic Marines, originally known as the 21st Nova Corps of the 4th Sector Army (Outer Rim Sector Army), was an elite unit that was created as the Grand Army of the Republic diversified during Clone Wars. The Marines were cross-trained to fight on a variety of environments in ground and space, making them extremely versatile. This unit was commanded by High General Ki-Adi-Mundi and Clone Commander Bacara, and were deployed to many planets such as Rhen Var, Aargonar, Boz Pity, and Mygeeto.
The marines were distinguished by their unique, uncommon visor gear: synthmesh designed to keep out various hazards such as snow, sand, airborne fungus, and ash. Bacara demanded the best of his men, and would tolerate nothing less, unilaterally reassigning soldiers who did not meet his high standards.
At the Battle of New Bornalex, the Galactic Marines made use of prototype spacetrooper powersuits. Although the weapon systems on their new suits failed, the Marines continued to fight on. They engaged B-2 battle droids in hand-to-hand combat, using the enhanced strength granted to them by their powersuits. Their actions at New Bornalex earned the Marines a fierce reputation, making them one of the Republic's most feared military units.
Commander Bacara and a squad of Galactic Marines fought alongside Ki-Adi-Mundi during the Battle of Mygeeto. After receiving Order 66 from Supreme Chancellor/Emperor Palpatine, Bacara and his squad turned on the Jedi and fired upon him. Although he was able to defend himself briefly, Ki-Adi-Mundi was overwhelmed by numerous blaster shots and gunned down.