Imperial Starfleet

In the Mirror universe 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.

History
Amongst the people to serve in the role of grand admiral were Garth of Izar, Matthew Decker, and Spock. 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 (NX-01), 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.

By the late 23rd 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.

Alternate Mirror Universes
In the alternate mirror universe the Imperial Starfleet still exists in the 24th Century due to the central fact that the Terran Empire was not destroyed by the Klingon-Cardassian Alliance in 2295.

In another mirror universe the Imperial Starfleet overthrew Spock because of the influence the Halkans carried in his government, but was later defeated by the Klingon-Cardassian Alliance after an attempt to secure an Iconian Gateway backfired. (The Gift)