In the mirror universe, Spock was a Vulcan Starfleet officer who served on the ISS Enterprise, rising through the ranks to command that vessel before gaining suzerainty of the Terran Empire. Under his leadership, the Empire enacted many reforms which ultimately left it defenseless to an invasion by the Klingon-Cardassian Alliance. (TOS: "Mirror, Mirror", DS9: "Crossover"; Glass Empires)