The Empire ruled by terror, with its Imperial Starfleet acting as its iron fist.
It is not clear when the Empire began. Commander Jonathan Archer once stated that the Empire had existed for "centuries" as of 2155. (ENT: "In a Mirror, Darkly") Archer did not mention how many centuries, but by his statement, the Empire can be traced back to at least 1955, suggesting that it was a Terran (Earth) political party or movement before it became an interstellar empire. At some point an astronaut planted the flag of the Terran Empire on Earth's moon, Luna.
Humanity's first contact with an alien species in the mirror universe began exactly as it did in the traditional universe. Upon detecting Zefram Cochrane's warp signature, a Vulcan scout ship landed in Bozeman, Montana to make first contact with humanity. Instead of welcoming the Vulcans in a spirit of friendship and understanding, however, the mirror Zefram Cochrane killed the first Vulcan to set foot on Terran soil, as the Humans boarded and ransacked the Vulcan ship. (This may have been wise, as according to remarks made by the Mirror Archer, the Vulcans were landing not in friendship but as a prelude to invasion, though this may also have been merely Imperial revisionist history)
Instead of the Vulcans gradually releasing technology to Earth over time, the Terran Empire applied the stolen Vulcan technology to a policy of aggressive expansion. Because of this, the Empire was able to engage in technological research and development considerably earlier than its traditional counterpart. Thus, the Starfleet of the mirror universe was somewhat more advanced than its counterpart. (ENT: "In a Mirror, Darkly")
Due to the rapid initial expansion made possible by the captured Vulcan technology, The Empire's hold on its territories was initially weak. In the 2150s, some of the worlds conquered by the Terrans were begining to rebel against Terran rule, leading to a long-running conflict in which the Empire came to the brink of collapse. Propaganda, however, conveyed the message that things were going in the Empire's favor and that the war would be over soon.
By 2155, the Terran Empire had already conquered the Vulcans, Denobulans, Andorians, Orions,Betazoids and Tellarites and had launched attacks against the Klingons and Xindi. However, the flagship of the Empire, the Enterprise, under the command of Captain Maximilian Forrest, had a much more racially-diverse crew than its regular universe counterpart, with numerous Vulcans and Tellarites serving as crewmembers.
In that year, the USS Defiant, a Federation ship launched in the 23rd century of a parallel universe, was reported in Tholian space. The first officer of the ISS Enterprise, Commander Archer, reviewed the reception of this distress call and proposed a bold surgical strike at the asteroid base at which the Tholians were keeping the Defiant. Archer's proposal was quickly rejected by Forrest, causing Archer to mutiny against his Captain and take control of Enterprise to retrieve the Defiant so its technology could be utilized against the rebellion. Enterprise arrived and dispatched a boarding party to gain all information they could about the ship, and destroy it to prevent the Tholians from being able to use it. Unfortunately, during the retrieval operation, the Tholians attacked Enterprise and destroyed it, stranding the boarding party aboard the Defiant. (ENT: "In a Mirror, Darkly")
Following the destruction of the ISS Enterprise and the death of Captain Forrest, Commander Archer and his away team commandeered the USS Defiant. They proceeded to destroy the Tholian hangar in which the ship was being held and rescued a number of former Enterprise crewmembers, including Hoshi Sato, after apparent consideration of leaving their comrades stranded. Archer made a rendezvous with the ISS Avenger, the flagship of Admiral Black. Archer vaporized the admiral and took command of both vessels. Commander T'Pol and Crewman Soval led a mutiny of the non-Human crewmembers on board the Avenger and attacked the Defiant in hopes of destroying it. They were destroyed after Commander Charles Tucker reinitialized the power systems that Phlox had attempted to disable. Commander Archer, acting as Captain, then set a direct course for Earth, where he intended to declare himself Emperor of the Terran Empire. However, Hoshi Sato poisoned him with the assistance of his bodyguard Travis Mayweather. The two then took control of the Defiant, and upon arriving at Earth, Sato declared herself Empress. (ENT: "In a Mirror, Darkly, Part II")
Somewhere between 2155 and 2267, the symbol of the Empire appears to have been altered. The earlier symbol closely resembles that of the United Earth government, depicting all of Earth's continents, though replacing a laurel of peace with an aggressive sword. However, by the mid-23rd century, the symbol, while remaining essentially the same, depicted only the continents of Earth's western hemisphere, suggesting the seeds of the Empire were sown in that hemisphere.
23rd and 24th centuries
In 2267, crewmembers of the USS Enterprise, including Captain James T. Kirk, were accidentally transported to the mirror universe aboard the mirror-version of the Enterprise, the ISS Enterprise. Before Kirk left, believing that the mirror Spock would one day become captain of the ISS Enterprise, he planted a seed of doubt about the inevitability of the Empire, asking Spock if violence was the only logical answer. Spock promised to consider Kirk's words, after realizing the Empire would only last about 240 years before being overthrown. (TOS: "Mirror, Mirror")
As Kirk predicted, the mirror-Spock later became the captain of the ISS Enterprise and eventually rose to become Commander-in-Chief of the Empire. He began instituting major reforms that were very popular, turning the Empire into a more peaceful and less aggressive power. Unfortunately, Spock's reforms left the Empire unprepared to defend itself against the emerging threat of a united Klingon-Cardassian Alliance, which managed to conquer the entire Terran Empire, turning the Terrans themselves into a slave race. The Bajorans, a people conquered by the Empire, came to be a powerful voice in this Alliance. (DS9: "Crossover")