Supreme Commander: Forged Alliance is a stand-alone real-time strategy video game released at the end of October, 2007 as the expansion to Supreme Commander. The second title in the franchise, it was similarly developed by Gas Powered Games and published by THQ.

As 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. 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.

Gameplay in Forged Alliance is largely the same as in the original Supreme Commander. 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. Forged Alliance expands on the original gameplay mechanics, adding new units, modifying old ones, and introducing other improvements to the user interface. The most notable addition to this expansion is the addition of a new faction, the Seraphim. In addition, the three original factions — the United Earth Federation (UEF), Cybran Nation, and Aeon Illuminate — also receive a number of new units. Other new content includes new maps. 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.

The original Supreme Commander allowed the player to employ all four traditional battlespaces — air, information, land and sea. Forged Alliance adds the ability for the UEF faction to construct orbital weaponry which the player can control. These satellites are impossible to harm directly, forcing opponents to find and destroy the control centers maintaining them.

The game supreme commander is set in the 39th century 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. Tunneling let humankind establish many colonies, which were governed by a centralized Earth Empire. As the number of human worlds grew, however, its control eventually weakened, and the Empire collapsed. The Empire's remnants formed the United Earth Federation; a race of cybernetically-enhanced humans seeking independence formed the Cybran Nation; and colonists who befriended and embraced the religion of the dying alien seraphim formed the Aeon Illuminate. The three factions came into conflict, starting the Infinite War. One thousand years later, the events of Supreme Commander take place, ending the war.

All three endings (one for each faction) for the Supreme Commander campaigns feature teaser endings after the credits as a reference to the (then unannounced) upcoming expansion. The Cybran ending shows the Quantum Artificial Intelligence (QAI) supercomputer calling in unidentified enemies, and then to Dr. Brackman's horror, saying "They are coming". The Aeon ending simply depicts Princess Rhianne, the leader of the faction, opening her eyes in shock and exclaiming "No!". The UEF ending shows many unidentified radar signatures appearing near the location of Black Sun on Earth.

The new playable faction, the Seraphim, are a race of aliens long thought to have gone extinct. The colonists who befriended and embraced the religion of the dying alien seraphim formed the Aeon Illuminate.

The game takes place shortly after the first game, which ended with the firing of the Black Sun, a powerful weapon capable of firing a beam through the Quantum Gates to destroy enemy planets. The Seraphim, an alien race long thought to have gone extinct, have exploited the firing of Black Sun to create a rift from the Quantum Realm they inhabit to our universe, on Earth. Once through the rift, they wreak a path of destruction through the civilized galaxy, leaving Earth and most of the core worlds of the UEF in ruins. With most of the UEF's key political and military leaders lost, including General Samantha Clarke who died while fighting on Capella, General Hall and Brigadier General Fletcher lead the last of the UEF Military. QAI, the once-loyal Cybran supercomputer built by Dr. Brackman, has been compromised and is waging war on behalf of the Seraphim. It cripples the Cybran Nation's military, forcing Dr. Brackman and Dostya to retreat to the further reaches of space. The Aeon Illuminate are divided over the arrival of the murderous Seraphim from whom they once learned what seemed the peaceful Way; many take sides with the Seraphim as the Order of the Illuminate, led by Evaluator Kael, and brand Princess Rhianne a traitor and heretic. Crusader Rhiza leads a handful of forces loyal to the Princess in a guerrilla-style campaign. Shortly afterward, the UEF and Cybran factions ally and form the Coalition against the Seraphim, with the Aeon Loyalists joining afterward despite Dr. Brackman's reservations.