(coexisting on another dimensional plane)
On April 5, 2063, a group of Terrans were filled with fear and suspicion when a Vulcan scout ship landed on Earth to initiate first contact. The Terrans slaughtered the Vulcan crew and used the technology within the scout ship to create the interstellar Terran Empire.
The Terrans conquered many of the races they encountered, including the Vulcans, Orions, and Tellarites. Many species became slaves of the Terrans, but some species, like the Vulcans, were treated better, and could even serve in the Empire's military. (ENT: "In a Mirror, Darkly")
This interstellar empire lasted almost three centuries. In the year 2267, Spock encountered the prime universe James T. Kirk, and became determined to change the empire. Spock eventually became the empire's ruler, and instituted reforms that made the empire much more peaceful. (TOS: "Mirror, Mirror")
Unfortunately, these reforms led to the Terran Empire's downfall; they resulted in the empire's military strength being unable to prevent the equally vicious aspirations of the Alliance of Klingons and Cardassians from conquering the Empire. When their empire fell, the Terrans were forced into slavery by the Alliance.
In the year 2371, the Terrans, as well as the Vulcans, were still slaves of the Alliance. However, after an encounter with several prime universe individuals, Benjamin Sisko, a Terran pirate working for the Alliance, decided that he'd had enough of slavery and began a revolt. He led a number of Terrans and other species in the Terran Rebellion, a revolution against the Alliance. Though Sisko was killed the same year, the rebellion still lived on. (DS9: "Crossover", "Through the Looking Glass")
A year later, in 2372, the rebels captured the space station Terok Nor above the planet Bajor. The Alliance's ruler, Regent Worf, led a fleet to retake the station, but the rebels managed to drive the Alliance fleet off. (DS9: "Shattered Mirror")
Two years later, in 2375, the rebels captured Worf himself. When last seen, the rebels appeared to be well on their way to achieving victory over the Alliance and winning back their freedom. (DS9: "The Emperor's New Cloak")