Defense of Mars

The Battle of Mare Erythraeum is a battle where UNSC Forces led by Colonel James Ackerson that fought the Covenant Loyalist Forces on the surface of Mars. It is presumed that the UNSC forces were defeated as Colonel Ackerson was taken prisoner after the engagement.

Military Forces
Considering that James Ackerson is an UNSC Army Colonel, the UNSC more than likely used Army infantry instead of Marines during this battle.