(coexisting on another dimensional plane)
In the mirror universe, the Battle of Vintaak was a conflict between the Tholian Assembly and the Terran Empire, fought when the ISS Enterprise attempted to investigate a captured Federation starship in the Vintaak system. Though Enterprise itself was destroyed, the capture of the USS Defiant proved a crucial victory for the Terrans. (ENT: "In a Mirror, Darkly", "In a Mirror, Darkly, Part II")
In January 2155, the Tholians detonated a tricobalt device inside the gravity well of a dead star, creating an interphasic rift from which they recovered the USS Defiant, a Constitution-class starship from 2268 of a parallel universe. The Defiant was towed to a Tholian asteroid dock orbiting the Vintaak system's gas giant, where they began using humanoid slave labor to strip the ship of any useful technologies. One of these slaves sent a message to Starfleet Commander Jonathan Archer, first officer of the ISS Enterprise.
At the time, the Terran Empire was struggling to put down a massive rebellion and had just lost twelve ships to the rebels at Tau Ceti. When Archer received the slave's message, he was intrigued by the advanced technologies the Defiant might contain and their potential to turn the tide of the war. He attempted to convince Captain Maximilian Forrest to investigate, but Forrest refused to do so, prompting Archer to mutiny and do it himself. Forrest was locked in the ship's brig, and Archer ordered Enterprise' into Tholian space with the aid of a stolen Suliban cloaking device. En route, the warship intercepted a Tholian ship and abducted its pilot to learn the whereabouts of the Defiant.
Forrest later regained command of Enterprise with the help of Commander T'Pol, but was unable to change course as Archer had locked down the helm controls. Furthermore, Archer had transmitted a message to Fleet admiral Gardner explaining his reasoning for seeking the Defiant, and the admiral soon ordered Forrest to continue to Vintaak while retaining Archer as first officer.
Arriving at Vintaak under cloak, the crew confirmed the existence of the Defiant and sent an assault team consisting of Archer, T'Pol, Major Malcolm Reed, Sergeant Travis Mayweather, and Commander Charles Tucker III to board the ship, download its database, and then overload the warp core. Archer objected to destroying the ship, arguing that it could be used as a weapon against the rebels, but was overruled. The team transported aboard the Defiant and made their way to the ship's bridge.
As the assault team began their work, the Tholian captive aboard Enterprise began using its crystalline structure to send a transmission, alerting the Tholian patrols to the Terrans' presence before Phlox was able to kill it. Seven Tholian ships approached Enterprise and tried to hail the ship before generating a Tholian web that enveloped Enterprise.
The Terrans opened fire, first on a Tholian ship and then on the web itself, but none of these attacks were effective and the cloak partially failed due to a lack of power. The Tholian ships then began firing on Enterprise through the web, inflicting heavy damage. Forrest ordered the cloak shut down and all power transferred to engines so they could force their way through, but the cloak was damaged and would not disengage. Within a few minutes the ship's antimatter containment fields began to fail, and the crew began to abandon ship. Forrest himself chose to remain on the bridge, continuing to operate the weapons and buying time for his crew to escape.
Archer and his team became aware of the battle after reactivating the Defiant's main power, and attempted to get the starship underway to assist their colleagues, but Enterprise exploded before they could do so. Many escape pods managed to clear the ship, though a number of them were promptly destroyed by Tholian weapons fire or the web itself. (ENT: "In a Mirror, Darkly")
Without a mothership to return to, the assault team committed to escaping on the Defiant. They managed to raise the starship's deflector shields, and then alternated port and starboard thrusters to free themselves from the station's docking clamps. As they continued trying to activate the engines and weapons, six Tholian ships approached and created a smaller web in front of the facility, trapping the Defiant inside. A tractor beam then locked on to the Defiant from behind, towing the ship back into dock.
However, at this point Tucker and T'Pol managed to activate the weapons, and Reed fired a phaser blast that quickly destroyed the tractor beam. Reed then fired forward, destroying three of the six ships blocking their escape and forcing the other ships to retreat. As the Defiant flew clear of the asteroid, the facility itself began firing on it with a particle weapon; the Terrans retaliated with a spread of photon torpedoes that shattered the base. (ENT: "In a Mirror, Darkly, Part II")
The Defiant recovered several escape pods, containing 42 crew members including Doctor Phlox, Lieutenant Hoshi Sato, and Ensign Kelby. This made for 47 survivors overall, not including the Tholians' humanoid slaves who'd been on board during the battle.
Although the Tholians had given up on pursuing the Defiant, the starship remained stranded in the Vintaak system for a time as the Terrans learned how to repair and operate its warp drive. These efforts were disrupted for a time by Slar, a Gorn slavemaster who sabotaged the ship and murdered several crew members before finally being killed by Archer. The warp drive was soon repaired and the Defiant departed the system in time to save the ISS Avenger from a rebel fleet.
Archer eventually became power-hungry and tried using the Defiant to stage a coup against the Terran emperor, but was opposed by T'Pol and the other aliens aboard the ship, who sabotaged the Defiant and hijacked the Avenger to destroy it. However, Tucker was able to repair the damaged systems and the Avenger was destroyed with all hands lost. Archer himself was soon deposed by Sato, who continued to Earth and declared herself Empress. (ENT: "In a Mirror, Darkly, Part II")
Successfully stealing the Defiant proved to be a crucial victory for the Empire, who used its advanced technology to halt the rebellion's advance and ensure at least another century of Terran supremacy. (DIS: "Despite Yourself", TOS: "Mirror, Mirror")