Mars

Mars is a planet in the Sol System. It is fourth planet from Sol, after Earth but before Jupiter. Since it is the closest planet to Earth, it was presumably the first planet terraformed.

Topology
Its populated areas include New Legaspi, New Manila, Katagalgun and Argyre Planitia. It is also home to a large number of harsh military training facilities, including the Chiron Testing Station. Mars is home to the Reyes-McLees Naval Shipyards which built the Halcyon-class Cruiser Pillar of Autumn.

Interplanetary War
Mars was a site of a decisive battle during a period where the United Nations organization transitioned into UNSC. The UNSC Marines routed the neo-Communist Koslovics on the planet during a series of brutal conflicts in the 2100s before moving on to destroy the Koslovic and Frieden movements throughout the solar system.

Human-Covenant War
Once again, Mars was the site of a massive engagement called the Battle of Mare Erythraeum, set on the fields of Mare Erythraeum. There, Colonel James Ackerson and his UNSC Defense Forces fought against the Covenant Loyalist Army, commanded by Jiralhanae Chieftain Lepidus.

It is speculated that the UNSC forces lost the battle against the Covenant Loyalists. James Ackerson was also taken prisoner on the Covenant Ship, Triumphant Declaration, a ship in the Fleet of Furious Redemption.

Trivia

 * It is the location of Misriah Armory, which makes the M6 Series and the SRS99C-S2 AM sniper rifle.
 * Nicole, SPARTAN-458, of Dead or Alive fame, is a Martian.