Tunneling let humankind establish many colonies, which were governed by a centralized Earth Empire. The setting of Supreme Commander is a future in which humanity is able to travel through the galaxy quickly using a quantum tunnel, a portal opened in the fabric of space leading to a designated location potentially light-years away. These satellites are impossible to harm directly, forcing opponents to find and destroy the control centers maintaining them. Forged Alliance adds the ability for the UEF faction to construct orbital weaponry which the player can control. The original Supreme Commander allowed the player to employ all four traditional battlespaces - air, information, land and sea.

The interface has been revamped, as well as improved pathfinding and aircraft flight dynamics. Several improvements to the controls and behind the scenes mechanics have been made as well. In addition, the three original factions - the United Earth Federation (UEF), Cybran Nation, and Aeon Illuminate - also receive a number of new units. The most notable addition to this expansion is the addition of a new faction, the Seraphim. Forged Alliance expands on the original gameplay mechanics, adding new units, modifying old ones, and introducing other improvements to the user interface. The player uses a giant bipedal mech called an Armored Command Unit (or ACU) to build an army with which they can conquer their opponents. Gameplay in Forged Alliance is largely the same as in the original Supreme Commander. Besides the new playable faction, Forged Alliance adds new gameplay features to the game, several new units for the three preexisting factions, and has optimizations for increased performance in response to issues with the original game. The second title in the franchise, it was similarly developed by Gas Powered Games and published by THQ.ДЄs a standalone expansion, it is possible to play Forged Alliance without owning the original game, although without a valid CD key for Supreme Commander, online play is limited to the new faction, the Seraphim. Supreme Commander: Forged Alliance is a stand-alone real-time strategy video game released in November 2007 as the expansion to Supreme Commander.

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