From Halopedia, the Halo wiki
The Mars Campaign was a pivotal military campaign undertaken by United Nations forces against human rebel forces on the planet Mars in December 2163. The campaign marked the beginnings of the United Nations Space Command (UNSC), and the dedicated usage of large forces of Marines in space-to-ground attacks.
In a conflict that stemmed from the Rainforest Wars, the three primary Earth factions clashed again, this time on Mars. Attempting to bring the human worlds under a communist state, the Koslovic rebels sought to take control of the plentiful corporate production facilities on Mars. In particular, the Koslovics concentrated their occupation to the shipbuilding facilities on the Argyre Planitia. The newly-established United Nations Space Command, created by the UN to combat the increasing threat posed by the rebel factions, responded with a series of lightning strikes on the Koslovic positions, deploying the newly-created UNSC Marines in the first extraterrestrial marine deployment—a tactic which would define military doctrine for centuries. The campaign was an unquestionable success. As a result, future military doctrine favored large contingents of Marines for ground assaults and ship-boarding actions.