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In the mirror universe, the Klingons were a humanoid species native to the planet Qo'noS in the Beta Quadrant. They forged and ruled the Klingon Empire and subsequently the Klingon-Cardassian Alliance, two of the most prominent interstellar states over many centuries.
By the early 23rd century, the Klingon Empire was engaged in a cold war with the Terran Empire. The conflict flared up from time to time with the Klingons winning a major victory in the 2220s. However, the Klingons respected the Terrans as worthy foes.
In 2268, in a effort to bring peace between Elasians and Troyians, Elaan, the Dohlman of Elas, was arranged to marry Hakil, the Caliph of Troyius. The Terran Empire assigned the ISS Enterprise to transport Elaan to Troyius in order to prevent the Klingon Empire from conquering their worlds and gaining access to their rich supplies of dilithium crystals. Kryton, the leader of the Dohlman's guards, hated the arrangement, as he loved Elaan and wished to marry her. Consequently, he conspired with the Klingons to sabotage the mission. However, the Klingons' plan failed as Captain Spock, via the Tantalus field, observed Kryton sabotaging the Enterprise. Kryton was then taken into custody and executed on Spock's orders while the captain used the Tantalus field to disintegrate the entire crew of Klingon battle cruiser which was poised to attack the ship. The ship itself was undamaged. A peace treaty was signed between Elas and Troyius shortly afterwards, scuppering the Klingons' plan.
In 2289, the Klingon Empire conquered Korvat, an unarmed farming colony less than a lightyear from Klingon space which had previously been located in inviolable Terran territory. The political and military elite of the Klingon Empire met on Korvat to celebrate their victory. During a speech by Kang during which he stated that the Terrans were no longer the able workers which they had once been, the Terran Empire's ambassador Curzon Dax spat at Kang's feet in disgust and decried the manner in which the Klingons were celebrating the "least audacious victory in [their] history" and accused them of having a "miserable empire" before storming out, which earned him Kang's wrath.
By this time, Regent Gorkon's senior military adviser General Chang had successfully overseen the development of a prototype bird-of-prey which had the ability to fire its weapons while cloaked. Based on the military capacity of the Terran Empire at that time, he informed the Regent that it would take nine years to build a fleet of such vessels large enough to conquer the Empire. The construction of this fleet was carried out on Praxis, one of the moons of Qo'noS. To power the production, and the cloaking field that kept it hidden from Terran spy arrays the energy production on Praxis was tripled In 2293, Praxis exploded, causing massive ecological damage to Qo'noS: toxic elements from Praxis' crust began to break down the atmosphere and contaminate water supplies, threatening the end to higher-order lifeforms within fifty years. Additionally seventy percent of the population were inflicted with deadly xenocerium poisoning. This led to the staging of the Khitomer Conference, a diplomatic conference held by the major Alpha Quadrant powers on the Klingon-held world Khitomer. At the conference, Gorkon offered Spock the opportunity to forge an alliance between their empires but the latter declined.
Following the failure of the Khitomer Conference, Spock postulated that the Klingons and the Cardassians would meet to discuss a grand alliance between the Klingon Empire and Cardassian Union, which would allow them to defeat the Terran Empire. This prediction was proven entirely correct. In 2295, the Klingon-Cardassian Alliance was formed. In a relatively short period of time, the Alliance was able to completely defeat the Terran Republic, the short-lived successor state of the Terran Empire, using the advanced fleet of Klingon birds-of-prey which had the ability to fire their weapons while cloaked. The Terrans, the Vulcans and other former subject species of the Terran Empire were massacred while the survivors were enslaved. (TOS - Mirror Universe novel: The Sorrows of Empire)
In spite of the fact that the Klingons were generally not comfortable with sentient species shaped differently than themselves, they respected the reptilian Pahkwa-thanh's predatory nature. The Klingon Empire had lost thousands of warriors to their failed attempt to conquer the Pahkwa-thanh homeworld. Granting such honourable and songworthy deaths to so many warriors was the Klingon idea of a generous diplomatic overture so the two species became stalwart allies. (TTN - Mirror Universe short story: "Empathy")
The Alliance remained the undisputed major power in the Alpha Quadrant until the early 2370s, at which time its stability was threatened by the success of the Terran Rebellion. (DS9 episodes: "Crossover", "Through the Looking Glass", "Shattered Mirror", "The Emperor's New Cloak")
In January 2379, the Terran Rebellion, with the assistance of Memory Omega and M'k'n'zy of Calhoun, was victorious in its war against the Alliance. The Terrans, the Vulcans and the other enslaved species were freed and the Galactic Commonwealth was founded. A neutral zone was subsequently established between Alliance and Commonwealth space. This effectively ended the unprecedented power and influence over galactic affairs which the Klingon people had enjoyed for the previous 84 years as one of the two principle races in the Alliance. In 2380, the Alliance was officially dissolved. (ST - Mirror Universe novel: Rise Like Lions)