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In the mirror universe, the Tholians were an extremely xenophobic, non-humanoid civilization native to the planet Tholia in the Alpha Quadrant. The Tholian interstellar state was the Tholian Assembly, which was bordered the Terran Empire and the Klingon Empire during the 23rd century. (VAN - Mirror Universe short story: "The Black Flag"; TOS - Mirror Universe novel: The Sorrows of Empire)
In 2155, the Tholian Assembly detonated a tri-cobalt warhead inside the gravity well of a dead star. The explosion created an interphasic rift which acted as a doorway to a parallel universe. Because of its instability, the Tholians reasoned that it was too dangerous to send one of their own ships through, so they transmitted a distress call into the opening, hoping to lure a ship from the other side. (ENT episode: "In a Mirror, Darkly")
In the primary universe in 2268, the Federation starship USS Defiant would enter this region of space, annexed by the Tholians, in response to the mirror universe distress call. Three weeks later, the starship USS Enterprise would discover the Defiant trapped in a spatial interphase, only to observe the starship, and consequently landing party leader Captain James T. Kirk, crossover into the mirror universe. (TOS episode: "The Tholian Web")
Unknown at the time, the Defiant had travelled back in time to the mirror universe, where, when fully clear of the interphase, it was towed by the Tholians of that universe to a small moon drydock in the Vintaak system, deep in Tholian space. There, they employed several humanoids to gut the ship. Commander Jonathan Archer of Starfleet learned of the vessel's presence in his universe. He commandeered the ISS Enterprise and flew it deep into Tholian space while cloaked to steal the Defiant.
After sending a boarding party on board the Defiant, the Tholians spun a tractor web around the Enterprise and destroyed it. The boarding party managed to activate the Defiant's systems, but not before the Tholians spun a tractor web over the entrance of the drydock. The Defiant then destroyed three Tholian ships and the moon's drydock, before escaping the system. (ENT episode: "In a Mirror, Darkly")
Although the Tholian Assembly and the Klingon Empire also laid claim to the Taurus Reach, it was the Terran Empire which had done the most to substantiate its claim through establishing several colonies in the region, as well as the construction of Vanguard Station or Starbase 47. In fact, Vanguard Station was the center for the empire's activity in the Taurus Reach. Under its commanding officer, Commodore Diego Reyes, the personnel of Vanguard undertook extensive archaeological digs of planets in the Reach all in an effort to uncover lost technology once used by the Shedai, the former dominant species in the region. Going so far as to capture a female Shedai of unimaginable power, Reyes believed he could have the region secede from the empire and turn it into his own little fiefdom.
In 2277, Reyes' efforts were thwarted by Emperor Spock's operative, T'Prynn, and the region fell under the control of the Tholians. However, war soon broke out with the Klingons over the region. (VAN - Mirror Universe short story: "The Black Flag"; TOS - Mirror Universe novel: The Sorrows of Empire)
In the late 2270s, the Tholian Assembly was engaged in numerous border conflicts with the Cardassian Union. At Spock's order, the Terran Empire provided ships and small arms to the Cardassians in order to aid them in their wars against the Tholians. (TOS - Mirror Universe novel: The Sorrows of Empire)
In 2377, in the wake of the weakening of the Klingon-Cardassian Alliance and the rise of the Terran Rebellion, the Tholians formed an alliance with the Gorn Hegemony and the Breen Confederacy known as the Taurus Pact. During the war between the Terran Rebellion and the Klingon-Cardassian Alliance, the Taurus Pact did not side with the combatants. The weakening of the Alliance made the Pact bolder, however. The Tholians took ten systems between Rudellia and Vanden while the Breen captured Lazon II from the Cardassian Union. Conflict with the Pact was a looming menace for the Klingons and the Cardassians, who tried to broker a truce in order not to face a two- or three-front war.
Following the victory of the Terran Rebellion and the dissolution of the Alliance in 2380, the Terran Protectorate, the forerunner of the Galactic Commonwealth, was recognized by the Pact as a nascent state. In addition, nonaggression treaties were signed with the Klingon Empire and Cardassia. (ST - Mirror Universe novel: Rise Like Lions)