Supreme Commander is focused on using a giant bipedal mech called an Armored Command Unit (or ACU) to build a base, then upgrading units to reach higher technology tiers, and conquering opponents. Forged Alliance uses the original game mechanics, and expands them with new possibilities, strategies, and improvements. The most notable addition to this expansion is the addition of a new faction, the Seraphim. All of the original factions, the United Earth Federation, the Cybran Nation and Aeon Illuminate, return and receive a number of new units. Other new content includes new maps.

The original Supreme Commander allowed the player to employ all four traditional battlespaces - air, information, land and sea. The fifth and last battlespace, space, could only be used indirectly. Forged Alliance expands upon space warfare, by adding orbital weaponry to the player's arsenal. Several improvements to the controls and behind the scenes mechanics have been made as well. The interface has been revamped, as well as improved pathfinding and aircraft flight dynamics.