Starfleet and the MACOs were the military arm of the Terran Empire in the mirror universe in the 22nd and 23rd centuries.
In 2155, the flagship of Starfleet was the ISS Enterprise (NX-01), which was assigned to an assault fleet in a protracted war against rebels. During this war, the Enterprise had been in combat many times, and Commander Jonathan Archer was of the opinion that no other ship had been tested more in battle. The Enterprise operated under the jurisdiction of Admiral Black. (ENT: "In a Mirror, Darkly")
A century later, the ISS Enterprise (NCC-1701), under the command of Captain James T. Kirk was responsible for the suppression of the Gorlan uprising through the destruction of the rebel home planet. (TOS: "Mirror, Mirror")
Starfleet later apparently collapsed, along with the Terran Empire, after the reforms that Spock implemented left the Empire weak and unable to stand against the Klingon-Cardassian Alliance. (DS9: "Crossover")