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 primary 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 alternate 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 encroashing in on and threatening Imperial territory.
In 2155, the NX-class starship 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 a Terran starship 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 Defiant. Archer tried to use the USS Defiant 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. Typhuss James Halliwell betrayed Archer by poisoning the captain along with Hoshi Sato.
Sato established her imperial residence in the historic Kyoto Imperial Palace. One of her primary goals as ruler of the Terran Empire was to crush the rebel forces arrayed against the Empire. Sato was known as "Hoshi the Great" and "Hoshi the Terrible" to the Terran people. Toward this goal, Sato arranged to be married to General Thy'lek Shran of the Andorian Imperial Guard, thus creating an alliance and cutting off a key source of rebel support. She also ensured that the true nature of the starship Defiant was kept secret, claiming it was from the Empire's future, rather than from a parallel universe where the Empire never existed. She also fabricated several "historical" facts she claimed to have found in the Defiant database, including the birth of a child to herself and Shran. When Shran discovered the Defiant's true nature, he arranged to have Sato's ova taken from her, a child artificially gestated from their joined DNA, and then sent Sato into exile, while he took control of the Empire as regent for their unborn child. Sato was sent to Deneva, but her transport was intercepted by Vulcan rebels.
She was held prisoner, and then put on trial for the crimes of the Empire against its subject worlds. However, Sato was able to strike a deal with one of the rebels, and her former Enterprise crewmate, T'Pol. By helping her escape the rebels and helping reclaim the Empire from Shran's Andorian forces, the people of planet Vulcan were then raised to a status equal to that of humans. Later that day Empress Sato made Typhuss James Halliwell her consort and the Grand Admiral of the Imperial Starfleet.
Empress Sato ordered the Imperial Starfleet to make an example of those who dared to oppose the Terran Empire. Using the nearest blood-cousins of the traitorous Vulcans as example, Sato ordered the fleet to Romulus. There Hoshi wiped out most of the planet's population, including that of its moon Remus. This caused the power to shift back into favor of the Terran Empire: it also caused the extinction of the Romulan race since they all committed suicide after the destruction of their world.
In the fifth year of Empress Sato's reign, the year 2160, Empress Sato had managed to not only hold the Terran Empire together, but to consolidate more power in the position of Emperor than ever before. Under Hoshi Sato, true power returned to the Emperorship. All remnants of the Rebellion were crushed usually quite ruthlessly, but with T'Pol's counsel, Hoshi also made several strategic, if inconsequential, concessions to gain an end to the fighting.
The technological advances gleaned from the future-Defiant gave the Terrans all the advantage they needed to expand their borders, and Hoshi ruled this territory from any one of her many palaces spread about the quadrant. The main one was on Earth in Sato City, the new name for what was once Rio de Janeiro, Brazil. With the threat to the Terran Empire over, Empress Sato ordered a new fleet of ships to be built using the Defiant's design. Six years later in 2166, the new fleet of Defiant-class vessels becomes operational.
In 2201, Klingon space is annexed by the Terrans. Several Klingon Houses, accepting the inevitable, declare their loyalty to Empress Sato.
What remains of the Klingon Naval Forces is commissioned to be the Klingon Defense Forces. Their command officers are replaced with Starfleet officers, and many Klingon officers are placed on Starfleet vessels for "re-education".
In 2209, Empress Hoshi Sato contracts a rare form of Rigelian fever and dies within days of contraction. Fleet Admiral Typhuss James Halliwell ascends to the throne and becomes Emperor of the Terran Empire. Halliwell's regin as Emperor only lasted for sixteen years until he was assassinated in 2255 by his senior military advisor.
However it was not until 2267 that any official contact between the universes is recorded. By this time the Terran Empire was the dominant power in the Alpha Quadrant. In a transporter accident caused by interference from an ion storm a landing party from the USS Enterprise switched places with an identical party from the ISS Enterprise in the alternate mirror universe. The four Starfleet officers; Captain James T. Kirk, Lieutenant Nyota Uhura, Lieutenant Commander Montgomery Scott and Doctor Leonard McCoy found the Terran vessel to be brutal regime, almost dictatorial in its command structure. Advancement through assassination was commonplace.
During the encounter, Captain James T. Kirk convinced the counterpart of his first officer, Spock, that the Empire could not sustain itself. Indeed, Spock predicted that in its current form, the Empire had just over two centuries before total collapse. Spock was assassinated in the late 23rd century before his reforms could be implemented. As a result, the Empire and Starfleet continued to exist into the 24th century and controlled all known space after conquering the Klingons and defeating the Romulans centuries before.
During this time, the forces of the mirror universe began implementing safeguards to prevent another crossover event with the primary universe. Transporter design was altered to prevent interdimensional travel, requiring the creation of devices specifically for that purpose, including the multidimensional transporter. In the event of another crossover, those involved would be killed to prevent further interference.
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. By the 2370s, the Maquis was a huge force to be reckoned with across the Alpha Quadrant as was regarded by some as an empire in its own right. By this time the Maquis were bold enough to actively board and capture Imperial Starfleet vessels and repatriate them into their own fleet. Their boldest capture came in 2374 when Captain Selok with support from Intelligence operatives engineered the capture of the prototype Prometheus-class warship, the ISS Prometheus. Following their safe return to Maquis territory, Selok had the Prometheus made the Maquis flagship renamed her IRS Enterprise in honor of Spock's old starship command. Over the course of the next five years, the Enterprise led numerous raids and campaigns against Terran Imperial targets, including Cestus III and Sherman's Planet.
In 2383, Starfleet Captain Typhuss James Kira was pulled into this universe while on a mission in the mirror universe. Typhuss meet a Bajoran women named Kira Nerys. Kira who had been fighting the Empire for years wanted to leave with Typhuss and go to his universe. Later, they use the Flyer class shuttle Charger to escape to his universe when the Terran starship ISS Voyager attacks the shuttle until the USS Intrepid-A engaged in battle with Voyager. The Charger returns to the Intrepid and the Intrepid goes back to the primary universe.
In 2384, Kira requested to return to her universe to get Seska, Ezri Tigan and Jadzia Dax, so they can be free from the Terran Empire too, Kira and Typhuss were able to bring them back to the primary universe.