In the mirror universe, the ISS Defiant was a warship constructed by the Terran Rebellion, and was based on Defiant-class specifications stolen from Deep Space 9 in 2371. Once operational, the Defiant helped turn the tide in the war against the Klingon-Cardassian Alliance, especially at the Battle of Terok Nor, and represented a major boost to the rebels' morale.
The specifications for the Defiant were stolen by Miles O'Brien during a visit to the primary universe. "Smiley" O'Brien was able to download the schematics from Deep Space 9's computers and brought them back to the mirror universe, where construction began at Terok Nor, now under rebel control. While construction proceeded well with materials stolen from the Alliance, the powerful warp core on the Defiant caused the ship to almost shake itself apart during test flights, much like had occurred with the counterpart version.
With an Alliance fleet on its way to retake the station, O'Brien enlisted the help of Professor Jennifer Sisko to bring Captain Benjamin Sisko into the mirror universe, so that he could solve the problems on the Defiant. Sisko was able to iron out the problems and even commanded the Defiant in the fight against Regent Worf's fleet. (DS9 episode: "Shattered Mirror"; WizKids module: Tactics)
A few months later, Rebellion Generals Zek and Julian Bashir gained the resources to build twenty-four more Defiant-class warships at their new shipyard at Empok Nor. Twelve of these were made, including the Capital Gain, Jadzia and Terra Victor. However, eleven were destroyed in the explosion that destroyed Empok Nor, and the Capital Gain was disabled.
The remaining twelve warships were built some time later by O'Brien as part of his plan for a Alpha Quadrant-wide "revolution" against the Alliance. That made a total of thirteen Defiant class warships including the Defiant herself. (DS9 - Obsidian Alliances novella: Saturn's Children)
- While the vessel was never actually named onscreen as the ISS Defiant , the ship's dedication plaque - barely visible onscreen - referred to it as such.