The USS Defiant was a warship constructed by the Terran Rebellion in the mirror universe, based on specifications stolen from Starfleet at Deep Space 9. The Human-designed warship proved of crucial importance in fighting the Klingon-Cardassian Alliance at the Battle of Terok Nor, and represented an important boost to the rebels' fighting power. (DS9: "Shattered Mirror", "The Emperor's New Cloak")
Shortly after the rebels captured Terok Nor in 2372, the Alliance launched a major assault fleet to take back the station, a fleet personally commanded by Regent Worf. Recognizing the danger in light of the fact that they had only a handful of raiders remaining, "Smiley" O'Brien and the other rebels began constructing a replica of the Starfleet warship based on a set of schematics he had "acquired" while visiting DS9 the year before. Construction proceeded relatively smoothly and rapidly until the final testing phase. The Defiant was equipped with a massively powerful warp core, and the ship nearly shook itself apart when the reactor was tested at full power.
Desperate for help, O'Brien recruited Benjamin Sisko, who had overseen the construction of the original Defiant several years previously. Sisko recommended a complete overhaul of the ship's inertial dampers to enhance structural integrity. Although the repairs were rushed due to the incoming Alliance fleet, they were successfully completed before the battle began.
Like her counterpart, the mirror-Defiant proved herself as one of the most powerful warships in the quadrant. Her heavy phaser cannons and advanced quantum torpedoes (the latter an unknown technology in the mirror universe) made quick work of the smaller Klingon Birds-of-Prey and inflicted heavy damage on the larger Galor-class cruisers. Although the small Defiant was heavily outgunned by the Regent's flagship, under the piloting of Captain Sisko the Human ship inflicted major damage on the massive warship while avoiding most return fire. Under the Defiant's heavy assault, the Regent was forced to abandon his goal of taking Terok Nor and retreat from the battlefield.