The Citadel are a fascist star empire that came to dominate twenty-one of the twenty-two known worlds in the Vega system. The dominant race of the empire were the Citadelians themselves, who were all clones of the Citadelian progenitor, himself a hybrid of an Okaaran and a Branx Warrior. This progenitor later merged his essence with a massive computer network and became known as Complex-Complex. Over the span of centuries, the ruthlessly aggressive, though intellectually stilted Citadelians took control of world after world beginning with Okaara. Because of a ban on the Vega system imposed by the Guardians of the Universe, the Citadel was allowed a free hand in its expansive goals without interference by the Green Lantern Corps. Various dictators arose to power during the Citadel's reign, but Complex-Complex was always the true power. Such rulers included Lord Damyn, Komand'r and the opportunistic Earth human Harry Hokum. The reign ended in the Citadel War when the revolutionary Omega Men destroyed the Fortress Ring that served as the defensive grid for the Citadel Homeworld. The Omega Men's interim leader Tigorr detonated an explosive charge that destroyed Complex-Complex, severing all communications between the various Citadelian factions. With this single action, the Citadel War came to an end, though many fringe elements of the Empire continued to exist.
The surviving Citadel supreme commander and his aide Harry Hokum succeeded in capturing several members of the Omega Men in the weeks following the war. Those who were still free aligned themselves with the bounty hunter Lobo in an effort to free their comrades. The Citadel commander and Harry Hokum intercepted the Omega Men during the breakout, at which time the Citadel supreme commander was killed by Lobo. In order to quickly solidify his position, Harry Hokum assumed control of the fringe elements of the Citadel and entered into an agreement with Omega Men leader Primus. The pact stipulated that a New Citadel Empire would be allowed to thrive unobstructed so long as the worlds under its charge did so willingly. Those who refused to become part of the new empire were invited to join the New Vega coalition. The prison planet upon which this accord was reached was renamed New Alliance.
The Branx Warriors, Spider Guild, the Changralynians and the Gordanians resided under Hokum's command in that star system.
The Citadel also became part of a galactic Alien Alliance bent on the invasion and subjugation of the planet Earth led by the Dominators during Invasion!.
One year after the Infinite Crisis, the Citadel made a surprise raid on Thanagar, seeking to disrupt the reconstruction efforts from the Rann-Thanagar War for unknown reasons. Vath Sarn and Isamot Kol, the Green Lanterns of Rann and Thanagar (respectively) were able to drive them off, but were only able to pursue them to the Vega system. Recently, the Guardians repealed the Vega ban as part of the ongoing War of Light, allowing the Green Lantern Corps to begin operating within the system. It remains to be seen how this will affect the Citadel's expansive intentions.