Halo Wars is a prequel to the the Halo trilogy, set 21 years before the events of Halo: Combat Evolved. In this real-time strategy take on Halo, the player leads the UNSC troops from the ship "Spirit of Fire", as they fight war against the Covenant on the partially destroyed human colony planet Harvest.

Most of the familiar units from the Halo games are featured - on UNSC side, one can find anything from marines to the near-legendary SPARTAN-II super-soldiers, from Warthogs to Scorpion tanks. The ranks of the Covenant are well-represented from lowly Grunts all the way to the most feared Elites, Brutes and even the Prophets, with all the vehicles from Banshees to Wraiths and Scarabs.

The game offers both a single-player campaign and an ability to play through the campaign in co-operative mode. The player-versus-player Skirmish mode, which supports up to 6 players, also lets the players to take the Covenant's side.