(coexisting on another dimensional plane)
In the mirror universe, the Battle of Terok Nor was a crucial battle in the Terran Rebellion in 2372, in which the Terran Resistance Forces managed to hold the captured station Terok Nor against a large Klingon-Cardassian Alliance assault fleet. (DS9: "Shattered Mirror")
Following the capture of Terok Nor, the rebels had only a few ships left – most of them had been destroyed during the initial attack on the station. Fortunately, during his previous visit to the prime universe the year before, Miles O'Brien had copied Starfleet's schematics for the Defiant-class warship. Using the schematics as a template, the rebels hurriedly constructed their own version of the warship, which they also named Defiant. Like her counterpart, the ISS Defiant had the potential to be one of the most powerful warships in the entire Alpha Quadrant.
Meanwhile, Elim Garak, former second-in-command of Terok Nor, had escaped back to the Alliance, where he was brought before Regent Worf. The Regent was preparing to launch a counterattack against the rebels to recapture the station and put an end to the troublesome uprising. Apart from the massive Regent's flagship, the fleet comprised of several Cardassian Galor-class cruisers and a number of Klingon Birds-of-Prey.
The rebels were encountering difficulties in bringing their new warship online, however. The Defiant was overpowered for its size, and nearly shook itself apart every time the warp core was brought to full power. To complete the work in time, O'Brien and Jennifer Sisko recruited Benjamin Sisko, who had overseen the construction of Starfleet's Defiant, to help them overhaul the ISS Defiant's inertial dampers to compensate for the increased stress.
However, when word reached the Regent that the rebels were constructing a new warship, he ordered the fleet to increase speed in an attempt to launch the attack before the rebels completed their work. With the Alliance only eight hours away, Julian Bashir and Jadzia Dax took the rebels' only remaining raider in a desperate attempt to delay the incoming fleet long enough for Sisko and the others to complete work on the Defiant. Based on information provided by the captured Intendant Kira Nerys, Bashir projected a series of false warp shadows to fool the Alliance sensors, making it appear that there was a whole squadron of raiders instead of just one. The warp shadows were successful, drawing away the entire Alliance fleet, even if only temporarily.
By the time the Alliance fleet finally got to Terok Nor, the Defiant was nearly ready. As the station's weapons opened fire, the Defiant jettisoned its docking scaffold and prepared for departure. Sisko volunteered to remain on as the ship's commander, having had more experience with the design than anyone else.
The Defiant's first shots proved that the whole effort had been worth it – a Bird-of-Prey was immediately vaporized by the ship's phaser cannons. As the Defiant began hunting down the other smaller attack ships, the station's batteries continued defending against the larger craft. The agile Starfleet-designed warship quickly destroyed several smaller vessels and inflicted major damage to the larger Galor-class cruisers.
However, the Regent's flagship massively outgunned the little Defiant, and once it entered weapons range began pummeling the smaller ship with disruptor fire. Sisko ordered the Defiant to close to point-blank range (or as Smiley called it, "Pattern Suicide"). The tiny Defiant began circling around the more massive cruiser, avoiding much of the return fire while scoring several hits. However, the Defiant, though equipped with powerful energy reserves, was gradually losing shield integrity. When it appeared that one more direct hit from the cruiser could leave the Human ship crippled, Sisko personally took the helm and piloted the Defiant on a dizzying attack run that left the Klingons reeling.
The flagship called for assistance, and one of the few remaining Birds-of-Prey moved in behind the Defiant, ready to make the kill and destroy the troublesome ship.
However, at the last minute, Bashir's raider triumphantly streaked in, destroying the Bird-of-Prey and leaving the Defiant clear to continue its attack against the flagship. Together, the two Terran ships continued to pummel the massive cruiser, causing several major hull breaches and shaking the ship to pieces.
Recognizing the inevitable defeat, Garak suggested it was time for the Regent to "remove his august presence from the battlefield". Reluctantly accepting defeat, the Regent ordered a full retreat – leaving the station in the hands of the victorious rebels.
With the rebels' victory, the station remained secure as a base of operations for the rebellion for at least the next three years. The Regent, convinced that he had been betrayed, focused all of the blame for the humiliating defeat on Intendant Kira, whom he believed had arranged for the outcome in order to avoid capture. The Regent was in this regard being cleverly manipulated by Garak, in order to create a useful scapegoat so that both he and the Regent could avoid blame for the disaster.
Ultimately, Terok Nor would become a major base of operations for further attacks against the Alliance – attacks that were made possible thanks to their first battlefield victory.