The UNSC Marine Corps is the branch of the UNSCDF responsible for planetary combat on the UNSC's colonies or on worlds belonging to enemy entities.
The UNSC Marine Corps was formed in the wake of the Interplanetary War of the early 22nd century, and is the direct descendant of the United Nations Marine Corps. Marines were among the defenders on Harvest during the first battle of the Human-Covenant War, and fought in every major battle during the rest of the war.
After the war ended, the military of the UNSC was vastly depleted. After a number of studies were conducted, it was definitively proven to the UNSC High Command that, with the current amount of soldiers and their training, the UNSC would find it impossible to keep peace if another Insurrection sparked. In the face of this data, UNSC High Command implemented a modernization plan that promoted cross-service training, use of commando teams, and surgical operations- in short, they planned to make every soldier able to do as much as possible while still retaining specializations. This was a tall order, and the plan took twenty years to implement, but the end results were good. One Navy officer humorously remarked, "they've taught these Marines everything except how to pilot a cruiser- and that might be next." One result of this attempt to make the Marines as effective as possible was Project AEOLIA, a mobile-suit program that arose shortly before the Saulosian Campaign began.