A planet located in the Periphery, Kalgan was a world of no particular resource or strategic value which arose to prominence during the reign of the Galactic Empire as a pleasure-planet. Imperial nobles would visit Kalgan as a means to indulge themselves, making the planet and its leadership immensely prosperous. Because of its ability to stay neutral from conflict and to provide tourism as its main amenity, Kalgan survived the decline of the Empire with ease and eventually came under the control of a warlord.
In 310 F.E., the Mule, as chronicled in Foundation and Empire, took over Kalgan by Converting its Warlord into his mind-slave. For a brief time, over a third of the Galaxy was ruled from Kalgan through the Mule's Union of Worlds, but after his death, the Lords of Kalgan were unable to maintain this level of control. The Union disintegrated to a mere 27 worlds, and was almost completely encapsulated by the economic and political control of the Foundation.
In 376 F.E., Lord Stettin, urged by his own egomania, decided to invade the Foundation. For a brief time, the power of Kalgan was extended, before the morale boost of the Seldon Plan caught up to Kalganians fighting on the front. Demoralized, they were easily overcome by the brilliant technical maneuvers of the Foundationer Navy. The peace deals following the Stettinian War made the subject worlds of Kalgan autonomous and, through popular vote, they were permitted to become independent or to join the Foundation Federation. After this crushing defeat, Kalgan ceased to play a major role in galactic history.