History:
Origin: Gamma Quadrant. A mighty planetary alliance or trading consortium in the Gamma Quadrant, at first unknown to most beyond the wormhole.
Composed of "hundreds" of dominated races, the major political, economic and military power in the Gamma Quadrant is led by the Changeling (shapeshifting) Founders, administered by the cloned Vorta, and defended by the Jem'Hadar storm troops. Its age is unknown, but a female shapeshifter says her people began "many years ago" to impose order on their chaotic surroundings. The Founders created the Dominion and command the genetically-bred Jem'Hadar armies; those who resist membership face elimination. A toehold was gained in the Alpha Quadrant with Cardassia's joining, prompting a full-scale invasion in 2373-74 as the greatest threat to the Alpha Quadrant since the appearance of the Borg. The threat has since been met and subdued by the Federation.
DS9 "Rules of Acquisition" ; Star Trek Encyclopedia II
The Jem'Hadar was a genetically engineered warrior species from the Gamma Quadrant. The Jem'Hadar functioned as the army of the Dominion, controlled by the Founders through the Vorta.
DS9 "The Jem'Hadar" ; Star Trek Encyclopedia II
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Dominion