The Terran Empire was the repressive interstellar government dominated by the Terrans from Earth in the mirror universe. The Empire ruled by terror, with its Imperial Starfleet acting as its iron fist. In Starfleet, officers promoted themselves by killing superiors that did not follow the rules of the Empire. Torture was a common form of interrogation. The Empire was the counterpart of the prime universe United Earth and the United Federation of Planets governments.
After the defeat of the Alliance and gaining back they're freedom; Terran rise back to power, but with Smiley O'Brien as Emperor of the Terran Federation. The goal was keep the peace in the galaxy and refrain from returning to their old, brutal days of spreading terror. His son James O'Brien and a group terrans including; Captain Mercury, Vice Admiral Solarmetric still believed that terror is still way to keep order in the galaxy. James O'Brian assassinated Smiley and took his place as Emperor and brought back the Terran Empire as a super power in the quadrant.
In early 2415 during the final events of the War of the Universes. Desiree Mendroza took over as Empress of the Terran Empire and issued major reforms. Unlike before when the Empire now has the Interspecies Republic providing protection and support from the United Federation of Planets. The Reformed Terran Empire works more like the Federation. Every planet and species of the old Terran Empire are now liberated and are receiving relief supplies and rebuilding service.