The Mirror Universe was first introduced in the original Star Trek episode "Mirror, Mirror". It was established that in the Mirror Universe the United Federation of Planets counterpart was the brutal Terran Empire, run by humans and their Vulcan allies. In the Mirror Universe, the term "Terran" is predominantly used for humans. The Mirror-Captain Kirk of the Imperial Starship Enterprise was a mass-murderer, who had been promoted to Captain by assassinating Captain Christopher Pike.
In the Terran Empire, officers were promoted for assassinating their superiors and order was kept by use of the "Agonizer" pain-giving devices. In some serious cases, the "Agony Booth" could also be used. Aesthetic differences included: the Mirror-crew's uniforms were flamboyant and somewhat robe-like, with ceremonial daggers for the officers; Mirror-Sulu was a Gestapo-like political officer with a disfiguring facial scar; Mirror-Spock had a goatee (which has led to a number of pop culture references of people from "evil alternate universes" having goatees), whereas the regular character does not; the "United Nations" emblem of the Federation was replaced in the Terran Empire symbol, which was a vertical sword of conquest savagely thrust through the Earth.
In this original encounter, Captain Kirk, Dr. McCoy, Lieutenant Uhura, and Chief Engineer Scott had been turned down by the peaceful Halkans, who did not want to trade for dilithium. They feared the use of the material for war. An ion storm causes a beam-up to go awry, switching the crew with their alternates.
Kirk tries to save the alternate-Halkans from destruction; the common tactic used by the Terran Empire when meeting resistance. He suffers several assassination attempts.
Both Spocks find out the dual nature of the affected officers and work on a way to get them home in time; there is a limit.
Before leaving the Mirror Universe, Kirk gave an impassioned speech in which he told Mirror-Spock that because the Empire would die in a few hundred years, that it was illogical, and Mirror-Spock was illogical for supporting the Empire. He asks Mirror-Spock to work for change, and Mirror-Spock reDuring the encounter, Captain 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 240 years before total collapse. (TOS: "Mirror, Mirror")
Shortly thereafter, Spock rose to become leader of the Terran Empire, proposing a series of reforms designed to make the Empire more secure and less dictatorial in nature. These included a significant disarmament program. Unfortunately, once these reforms were complete, the Empire was unable to defend itself against the equally aggressive and powerful forces surrounding it. The Klingon-Cardassian Alliance overran the Empire, conquering Earth and leaving Terrans and Vulcans enslaved, and freeing several worlds that still remained under Terran occupation, including Bajor. (DS9: "Crossover")
The story begins here...at the fall of Spocks Homeworld Vulcan by the hands of the Klingon-Cardassian Alliance:http://goanimate.com/movie/0eCDNfqBlT1Q?utm_source=linkshare