- For the primary universe counterparts, see United Earth Starfleet and Federation Starfleet.
- "Terror Must Be Maintained." -- Imperial Starfleet Prime Directive
In the mirror universe, the Imperial Starfleet was the main military arm of the Terran Empire during the 22nd and 23rd centuries. Like its primary universe counterpart, the Imperial Starfleet was controlled by a group of admirals, but was overseen by a grand admiral who served as Commander-in-Chief of the Imperial Starfleet. The commander in chief was in turn answerable to the Emperor of the Terran Empire.
- References from numerous sources establish that the various depictions of the mirror universe may not take place in concurrent timelines, meaning there could be a number of divergent alternate reality versions of this topic that do not share the same continuity. The references below this note are separated into subsections to minimize confusion regarding the divergences.
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During the mid-22nd century, the main type of starships operated by Starfleet were NX-class battle cruisers, of which the prototype, the ISS Enterprise, served as the flagship. In the 2150s, Starfleet was involved in a long drawn out conflict against rebels, in which it became possible that the Empire could collapse. However, the capture of the USS Defiant from a Tholian base, gave Starfleet a tactical and psychological advantage, when Empress Hoshi Sato proclaimed that the Defiant was a Imperial Starfleet vessel from the future, instead of coming from the primary universe. (ENT episode: "In a Mirror, Darkly"; ENT novel: Age of the Empress)
By the latter part of that century, Starfleet was comparable in size and function to its primary universe counterpart. Following the assassination of Captain James T. Kirk in 2267, Spock rose to the captaincy of the ISS Enterprise, and soon became a very powerful force in Starfleet through the use of the Tantalus field. Recognizing the danger that Spock could cause, Empress Hoshi Sato III ordered Grand Admiral Garth of Izar, and later Matthew Decker to try and assassinate Spock and remove the threat. Following the third failed attempt in 2277, Sato III was forced to appoint Spock as the new grand admiral of Starfleet.
By the 2290s, the Imperial Starfleet had been starved of proper resources and was but a shadow of its former self, due to the radical reforms of now Emperor, Spock. Because of this, the fleet was unable to survive the Klingon-Cardassian Alliance invasion of the Terran Republic in 2295. (TOS novel: The Sorrows of Empire)
In this version of reality, Spock was assassinated in the late 23rd century before his reforms could be implemented. As a result, the Empire and Starfleet continued to exist into the 24th century and controlled all known space after conquering the Klingons and defeating the Romulans centuries before. (TNG novel: Dark Mirror)