(covers information from an alternate timeline)
The alternate mirror universe is one of several parallel realities next to what is commonly termed as the mirror universe where the Terran Empire was not destroyed by the Klingon-Cardassian Alliance in the late 23rd century.
A highly-organized resistance movement called the Maquis strongly-opposed the Terran Empire in the late 23rd century and for much of the 24th century. Made of Terrans, Romulans, Bajorans, Betazoids, Trills, Orions, Vulcans and other species who felt betrayed in some way by the Empire.
It is not clear if the mirror universe diverged from the regular universe at some point or always existed in tandem, mirroring events and peoples. If the two universes did diverge it was at a point many hundreds of years ago. Phlox noted that the "great works" of literature in both universes were roughly the same except that the regular universe characters were "weak and compassionate," with the exception of Shakespeare. The mirror Klingons culture has a basis of celebrating the victory of Molor over Kahless. In the mirror universe, first contact was slightly different. In 2063 first contact between Vulcans and Humans took place much as it did in the regular universe. However once the Vulcans landed, instead of welcoming them with open arms, Zefram Cochrane shot the first Vulcan to step onto "Terran" soil and the "Terrans" proceeded to raid the Vulcan ship. The gun used by Zefram Cochrane later would come into the possession of Jonathan Archer, who wondered what would have happened had Cochrane not "turned the tables on [the Vulcans'] invasion force."
By the 22nd century with advanced Vulcan technology at their disposal and with the alliance of near dozens of equaly brutal alien races at their side the Terran Empire was an unstoppable force that had expanded and conquered other races, including the Vulcans, Andorians, Tellarites, Orions and Denobulans. However by the middle of the century the Empire had unknowinly "bitten off more then it could chew" as it were with the discovery of a powerful new alien species that was somewhat secretly encroaching in on and threatening Imperial territory.
In 2150 Terran NX-class dreadnought Imperietor had made first contact with a seemingly impossibly advanced species of unknown cybernetic origin, the Borg while they were (on face value) another "friendly" and similar minded allied warrior species no different from most other species allied with the Empire humanity began to plot against them never the less.
Humans (as many other more philosophical species believe) were far too fearful and far too tight in their strangling grasp and their jealous greed that they held over their absolute dominion. The Imperial court was increasingly afraid that their new cyborg allies who had near godlike power and control over there conquered foes would soon turn that power upon them. The council was seemingly desperate for a new and super advanced form of weapons and tactical ship technology that would place a superior advantage over the Borg
In 2155, another NX-class dreadnought the ISS Enterprise, under the command of Captain Maximilian Forrest was the flagship of the Terran Empire's Starfleet. In January of that year, Commander Jonathan Archer mutinied against Forrest in order to take the ship into Tholian space to capture and control a massive alien craft of then unknown origin which the Tholians had obtained.
While the mutiny was ultimately unsuccessful, Captain Forrest had no choice but to continue the mission, since the ship's helm had been locked on auto-pilot. The Enterprise was later destroyed by Tholian ships, but not before Archer was able to take control of the advanced alien craft. Archer tried to use the unknown ship in a seeming mad grab for power, intending to overthrow and replace the Emperor himself, but in the end Archer himself was betrayed and killed by Hoshi Sato, his own communications officer who declared herself Empress.
It was later learned the alien craft was a alternate reality hybrid Romulan/Borg vessel from over three hundred years in the future named as the Narada, the craft was commanded by a "worthless Romulan slave" as the Empress Sato herself had called him, named Nero who had accidentally entered their reality via a reality/time warping Wormhole that entered Tholian space.
It was through the research and massive reverse engineering of many of the Narada's vital components that helped to create the new ship class that replaced both the NX and Daedalus classes known as the Constitution-class (sometimes it was nicknamed as Imperial class due to its early command/flagship status in its early years of development and use.) helped to solidify the Terran Empire's grip on power and dominance over the quadrant, and mainly (as they hoped) over the Borg.