Deep Space 9 is the third live-action Star Trek series, set aboard the eponymous spaec-station, a former Cardassian mining station that was originally located in orbit above Bajor, a world occupied by the Cardassians. After the Cardies withdrew (largely because they'd strip-mined the planet's mineral resources and were getting their butts kicked by resistance fighters), Starfleet and the Federation moved in to help stabilise Bajor's government and keep the peace. In the process a stable wormhole was found leading to the Gamma Quadrant, a distant area of space. the sixth and seventh seasons of DS9 revolved around a galaxy-spanning war between the Feds and their allies the Klingons and the Romulans against the Cardies and their allies from the Gamma Quadrant called the Dominion.
Sisko is the station commander. Admiral William Ross was Sisko's superior during the war.
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