Spock was the Terran/Vulcan hybrid first officer of the ISS Enterprise in 2267. He was a logical being who was loyal to his captain, James T. Kirk, and chose to warn him when he was ordered by the Terran Empire to kill him and take command if he did not launch an attack against the Halkans in retribution for their refusal to negotiate with the Empire.
Based on his captain's apparently erratic behavior, Spock correctly deduced that Kirk had actually been accidentally replaced by his counterpart from a mirror universe, and devised a means of returning Kirk, Scotty, McCoy, and Uhura to their original ship.
Before Kirk left, believing that Spock would one day become captain of the Enterprise, he planted a seed of doubt about the inevitability of the Empire's collapse, asking Spock if violence was the only logical answer. Spock, as logical as his counterpart, had already calculated that the Empire's subject peoples would revolt in approximately 240 years, and the Empire' defeat was a foregone conclusion. Kirk submitted that it was illogical of Spock to continue to serve an Empire destined to collapse, but Spock said the odds of one man bringing about peaceful change to the Empire were virtually insurmountable. Kirk revealed the existence of the Tantalus field, an assassination device that the mirror universe Kirk had used to eliminate his enemies, and urged Spock to ally the device's power with his own formidable intellect. Spock appeared intrigued, and promised to consider Kirk's words.
As Kirk predicted, Spock later on became the captain of the Enterprise. Spock used the ship as a power base to accumulate influence, and eventually rose to become leader of the Terran Empire. He instituted major reforms, turning the Empire into a more peaceful and less aggressive power known as the Terran Republic.
Unfortunately, Spock's reforms left the Empire unprepared to fight the united Klingon-Cardassian Alliance, who conquered the entire Terran Empire, enslaving the Terrans themselves as well as the Vulcans. Spock was killed by the mirror universe version of Gorkon when the alliance conquered the Republic.
In the Dark Mirror permutation of the Mirror Universe, Spock continued on as first officer of the ISS Enterprise after the incursion of the primary universe Enterprise crew members. Some time after the incursion, Spock transferred off the Enterprise, taking a position at Starfleet Command. He began working to reform the Empire into a more peaceful and less aggressive institution.
What Spock unfortunately did not realize was that Kirk had become resentful of Spock as he had advanced in his career while Kirk was still a starship Captain. About 12 years after the incursion, Kirk trumped up charges of treason against Spock, with "evidence" found in his quarters at Starfleet Command. Spock was put on trial and subsequently executed. Not long afterwards his father was killed by another Vulcan who wanted his position.
With Spock out of the picture, the Empire continued on its aggressive course. In this version of the Mirror Universe the Terran Empire survived in to the mid 24th century, but by then was running out of new territory to conquer.