The Terran Empire which Captain Kirk discovered in 2267 was undoubtedly a despicable organisation, and despite Starfleet's general policy of non-interference he can hardly be blamed for urging the Mirror Spock to try and reform it.  Although well intentioned, Spock succeded in weakening the Empire to such an extent that it fell to the Klingon/Cardassian/Bajoran alliance, and Terrans became an enslaved people. The Terok Nor station used Terran slaves in the ore processing plant, with Intendant Kira as a ruthless task-mistress.  Eventually the Terrans rose up against their oppressors, and despite the great odds against them they ultimately succeede3d in seizing control of Terok Nor. Whilst capturing the station was a notable achievement, holding it in the face of a determined counter-attack looked next to impossible. The rebels had only one hope  - on a previous visit to our own universe the mirror O'Brien had stolen the plans for the starship Defiant.  The rebels hastily constructed their own version of the ship in an attempt to provide a defence force for the station. Unfortunately the same problems which had plagued the original Defiant now hampered the rebels. Faced with no alternative, the rebels had to gain the co-operation of the man who had intimate knowledge of the Defiant - Captain Sisko. 
The rebels used the Mirror Jennifer Sisko to entice Jake into going to the Mirror universe, knowing that his father would surely follow. The Captain agreed to repair the Defiant even as the Regent's fleet approached. 
When the attack came, Sisko decided to captain the Defiant. The Regent's fleet was comprised of his massive flagship along with a small force of Klingon and Cardassian vessels; the battle opened with a direct attack on the station by this screen. Defiant successfully harrased the attackers, destroying or driving several of them off. It then closed the Regency herself and trade fire. 
Although the Mirror Defiant performed well, the ship did not have the firepower to trade blows with a vessel the size of the Regency. With his shields gone Sisko assumed the helm station and used the Defiant's agility, closing to point blank range and quite literally dodging betwween the enemy shots to inflict a series of blows on the flagship. He was assisted by the timely arrival of the Mirror Bashir and Dax, who had returned for a delaying action they had fought to give Sisko more time to prepare the Defiant. Together the two ships unleashed a devastating barrage of fire on the Regency, crippling the ship to the point where it could no longer remain in the fight. The Regent chose to retreat rather than face destruction, leaving Terok Nor in the hands of the jubilant rebels. 
Although a success, the battle was not without its losses. Intendant Kira had been a prisoner on the station during the battle, and had used the distraction it afforded to escape with the assistance of the Mirror Nog. She murdered Nog and headed for a ship to leave the station, but on the way she encountered the Mirror Jennifer and Jake. Kira attempted to kill Jake to avenge his father's previous dealings with her, but Jennifer threw herself in the way and took the shot instead. Kira left Jake alive, informing him that his father now owed her a debt for her act of mercy.