The United Nations Space Command (UNSC) in Halo Wars is the main faction in the game, and it will be who the player controls in campaign mode. The UNSC has the Spartan soldiers on their side and they control familiar vehicles like Warthogs. Their main enemy in the game is the Covenant.Halo Wiki: UNSC
Other Factions
Besides the UNSC, there is one other major faction in the game called the Covenant. They're the enemies of the UNSC and they're who you'll fight through campaign mode. In multiplayer modes, players can control the Covenant.
In addition to the two major factions there are also two lesser factions that are considered neutral, these two faction are the Forerunners and the Flood. The Forerunners are the ancient race who build important structures like the Halos. The Flood are considered a parasite that feed on other life forms. There was once a major war between the Forerunners and Flood, and now the Flood attack both sides of the current war.Halo Wiki: UNSC