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. 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.