Defense of Mars

The Battle of Mare Erythraeum was a battle where UNSC Army forces led by Colonel James Ackerson fought the Covenant Loyalist Forces on Mare Erythraeum on the surface of Mars. It was presumed that the UNSC forces were defeated, as Colonel Ackerson was taken prisoner after the engagement. It was possible that other Humans were taken as prisoners, but this is highly unlikely. The fate of Mars after the battle remains unknown.

Ackerson's work in this battle helped buy UNSC ground forces on Earth precious time.

Military Forces
Considering that James Ackerson is a UNSC Army Colonel, the UNSC more than likely used Army infantry instead of Marines during this battle, or Army infantry were simply involved in the battle.