The Galactic Marines, also known as the 21st Nova Corps, were a notable military contingent of clone troopers celebrated for both their distinctive armor and their relentless combat style. As part of the Grand Army of the Galactic Republic, this corps functioned within the 4th Sector Army, being jointly led by Jedi General Ki-Adi-Mundi and Marshal Commander Bacara. In the waning days of the Clone Wars, these marines engaged Separatist Alliance droid forces during the Mygeeto Campaign, where they were issued Order 66, resulting in the betrayal and killing of Mundi on the battlefield by his own troops. Subsequently, they transitioned into service for the Galactic Empire.
Being a corps within the structure of the Grand Army of the Republic, the 21st Nova Corps was composed of 36,864 troopers that were organized into sixteen regiments. The command structure of the Galactic Marines included Jedi General Ki-Adi-Mundi and Clone Marshal Commander Bacara, and they were assigned to the 4th Sector Army. During the Fourth Battle of Mygeeto, Mundi himself spearheaded a squad of Galactic Marines.
Clone troopers within the 21st Nova Corps were regarded as particularly well-conditioned and disciplined, and their training prepared them for operations in challenging environments, ranging from fungal-filled atmospheres to complete vacuum conditions. When deployed to the field, the corps made use of the DC-15A blaster rifle and the DC-15A blaster carbine. In addition to their infantry resources, the unit also employed Unstable Terrain Artillery Transports in combat scenarios.
Beyond their reputation as tenacious soldiers, the Galactic Marines stood out due to their specialized armor, a modified version of the Phase II clone trooper armor that followed the earlier Phase I design. The helmet design featured a specialized covering to filter out pollutants and other hazards, which included a special synthmesh material designed for the filtration of poisonous gases.
Throughout the Clone Wars, the 21st served under the command of Jedi General Ki-Adi-Mundi, who guided the Galactic Marines to numerous victories. In 19 BBY, the 21st Nova Corps, led by Mundi, participated in the Outer Rim Sieges. Prior to their involvement in the Mygeeto Campaign, the corps engaged in operations on Orto Plutonia and Rhen Var, both of which resulted in victories for Republic troops.
Following those successes, General Ki-Adi-Mundi and Clone Marshal Commander Bacara directed the corps in a direct offensive against the Separatist planet of Mygeeto, successfully breaching Separatist defensive positions. At some point during this campaign, both leaders participated in a hologram conference with other Clone Commanders and Jedi Generals, including Mace Windu and Aayla Secura.
As the conflict on Mygeeto persisted, Mundi's unit encountered the droid army at the bridge. The Separatist forces were formidable, killing numerous clones and destroying an Unstable Terrain Artillery Transport, but the Jedi General and his troops initiated their crossing of the bridge. However, Bacara's troops then received Order 66 from Supreme Chancellor Sheev Palpatine, instructing them to eliminate Mundi and initiate the Great Jedi Purge. The Marines obeyed this directive, turning on their general and fatally shooting him as he battled Separatist battle droids on the front lines.
In the aftermath of the Clone Wars, the 21st Nova Corps was integrated into the Imperial Military of the Galactic Empire, the successor to the Galactic Republic. During the Imperial Era, clone troopers were gradually supplanted by birth-born human recruits, and the clone armor worn by the corps evolved and became the snowtrooper armor used by the Empire's cold weather assault stormtroopers.