Mars

Mars is the 4th planet from Sol. The closest planet to Earth, it was presumably the first planet terraformed. It was the site of a major battle in the Interplanetary War. Its cites include New Legaspi. 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.