Imperial Starfleet

The Imperial Starfleet was the primary space force for the Terran Empire, and operated with the MACOs in the 22nd century to form the Terran Empire's military arm. It continued in its service through at least the 23rd century, where it would seemingly collapse, along with the rest of the Empire, thanks to the reforms that had been instituted by Spock. (ENT: "In a Mirror, Darkly", "In a Mirror, Darkly, Part II"; TOS: "Mirror, Mirror"; DS9: "Crossover")

Starfleet acted as the iron fist to the Terran Empire, and often formed assault fleets to combat the enemy, such as in 2155 when the Terran Empire was fighting an ongoing war with a Rebellion. Officers within Starfleet would proclaim themselves as soldiers, and would serve long careers spanning most, if not all of their lives. (ENT: "In a Mirror, Darkly", "In a Mirror, Darkly, Part II")

Commander Archer once claimed that he had been a soldier all of his life. This could either mean that he started service at a young age, perhaps in his teens, or that he simply meant his career covered most of his adult life.

Alien species that had been subjugated by the Empire, were able to serve in Starfleet, and were able to advance through the ranks, such as Lieutenant Commanders T'Pol and Spock had done. Others would serve in the enlisted ranks, or even serve as bodyguards to certain officers who had trusted them in such a position. (ENT: "In a Mirror, Darkly", "In a Mirror, Darkly, Part II"; TOS: "Mirror, Mirror")

The ISS Enterprise (NX-01) served as the Terran flagship during it's operation, until it had been destroyed by Tholian forces in 2155. Enterprise had also been under the jurisdiction of Admiral Black, who also held similar power over the ISS Avenger. Enterprise would see combat many times throughout it's service, and as such, Commander Jonathan Archer was of the opinion that none other had been tested more in combat. (ENT: "In a Mirror, Darkly", "In a Mirror, Darkly, Part II")

Starfleet had been fighting a war with a rebellion by the beginning of 2155, and despite the propaganda that said otherwise, was not faring well. In a battle at Tau Ceti, Starfleet lost a total of twelve starships, (ENT: "In a Mirror, Darkly", "In a Mirror, Darkly, Part II")

Starfleet would later avoid an uprising through the Gorlan after Captain James T. Kirk, through his command of the ISS Enterprise (NCC-1701), destroyed the rebel homeworld.

Starfleet was later defeated, destroyed or simply collapsed due to the reforms of Spock, and therefore was not able to stand up to the Klingon-Cardassian Alliance. (DS9: "Crossover")