The classic Mech Commander Series was released for free by Microsoft
You play the commander of the 'Federation Suns' charged with ridding a planet of the forces of the evil 'Smoke Jaguars'. The game can be played against the computer or human players on-line (either in battles or co-operative games). The player assumes the role of the commander of Zulu company from the Federated Commonwealth's First Davion Guards, and the planet Port Arthur must be taken back from the Clan Smoke Jaguar. The plan is to systematically establish a beachhead on the planet, then cut off supply lines to the Jaguars, and finally retake cities and territories until the final mission at the Port Arthur starbase. The campaign mode is 30 missions long, divided into five operations, each six missions long.