Mars

Mars is the 4th planet from Sol. The closest planet to Earth, it was presumably the first planet terraformed. Its populated areas New Legaspi and Argyre Planitia. It is also home to a large number of harsh military training facilities, including the Chiron Testing Station.

Interplanetary War
Mars was a site of a decisive battle during a period where the United Nations organization transitioned into UNSC. The UN's 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.