he stopgap battlecruiser to end all stopgap Battlecruisers, the Enterprise class was a visually stunning and mechanically highly questionable ship if ever there was one. Brought down by their awkward engineering due to being designed as a refit to a ship 25 years older the ships were beloved by everyone except their engineers. Ultimately this created a less than optimal ship, but a decent stopgap.
The Enterprise class stems from a need Starfleet encountered rather alarmingly in the 2260's. The Constitution class, excellent ships among the heavy cruisers, had a rather nasty habit of being outgunned by either multiple vessels or some mysterious giant amoeba or something. This had proven remarkably concerning given at least 10 of them were lost, 3 without a trace. Something needed to be done, a Total Modernisation of the class. And so, when the USS Enterprise pulled into dock at the end of her 5 year mission and a rather alarming encounter with a swarm of mining drones over Altamid, the engineers got to work.
The designers at San Francisco spent at least 12 months pouring over the Enterprise, ultimately settling on the decision to refit her based on technology from the Miranda class project. Sharing the Miranda's Chesapeake M-1 nacelles the ship jumped from 289 to 305 metres in length. Her FH6 Phaser banks would be swapped out for 7 dual mounted FH11 banks, with the ship retaining her stern torpedo launcher but having the bow launcher moved to the neck. The neck was also the site of the ship's most controversial change, the installation of a vertical SSWR-XVII Warp Core. This spanned from the keel of the ship to the impulse decks, with her antimatter bottles consequently moved beneath the Arboretum. The deflector dish was replaced with a concave deflector mount, and the secondary deflector in the saucer was removed. This was all clad in an objectively beautiful hull, widely regarded as the finest looking ship Starfleet ever constructed.
Unfortunately, looks aren't everything. The Constitution frame was never designed to sport a Vertical Warp Core, and thus uncomfortable compromises had to be made, namely the placement of it within the neck. This was an extremely awkward arrangement as it not only required the movement of the ship's cargo bay but left the core extremely vulnerable to attack. Additionally the plasma conduits had to be partially routed through the inhabited area, partitioned off by a transparent aluminium panel. This area was extremely radioactive and would've killed anyone short of a Horta if they entered the chamber when active, but Starfleet reasoned that if a ship was so damaged as to require that then the ship was already lost. This engineering decision is entirely to blame for the death of Captain Spock during the Genesis Incident, and his resurrection didn't change the showing of this grievous engineering failure.
Regardless of the ship's grievous flaws, the Enterprise performed up to expectations, and her form would go on to form the basis of the Total Modernisation Program, which saw similar refits for the Pioneer and Shepard classes. Other Constitution Class ships would also be refitted and new examples to replace lost vessels would be constructed. Unfortunately their weaknesses would truly come to pass. Following the collapse of the Treaty of Organia in 2278 Klingon and Federation skirmishes would reignite, culminating in the loss of the USS Constitution over Khitomer in 2287. The ship was lost due to a warning shot which struck the torpedo launcher, igniting the Torpedoes which spread to the Warp Core and atomised the ship. No trace of the ship larger than a single bacteria was found. This incident not only almost started a war but highlighted exactly how deficient the Enterprise Class was, though suspicions had been there for a while.
In 2283 Starfleet had launched the USS Constellation as a reaction to the problems. Fraught with problems at the start the ships quickly found their niche as fast scientific and logistics ships, remaining as such well into the 2360's. However it would be the Excelsior that ultimately doomed the Enterprises to an early retirement, as from the day Enterprise pulled into dock the original memo was "fast battlecruiser to hold over until the Transwarp battlecruiser comes online." In this regard it's hard to call the Enterprise a failure, as it did what was asked of it and very little more. The class saw the beginning of the end in 2290, and by 2295 not a single Enterprise class ship was in active service, bringing the storied history of the Constitution class to an end.
Or so it would seem.
One ship, the USS Republic, was retained by Starfleet Academy for crew training, principally for the Miranda class as the ships are similar in their technical aspects. In this role Republic quietly cruised around the Sol System, occasionally heading out to Vulcan or Andoria on long term cadet cruises, though these escapades ended in 2318. Then, in 2368, she was one of 40 ships to respond to Admiral Hansen's request for a fleet to combat the Borg at Wolf 359. Arriving alongside Hansen's own ship, the Victorious, she stuck close to the Galaxy class vessel as she stood no chance against the Borg on her own. Unfortunately this is what doomed her as, when Victorious ejected her warp core, it flew straight into the path of the Republic. Her saucer was blown clean off the secondary hull, with 2/3rds of her 300 strong crew and all 2500 aboard the Victorious being killed.
However, that was still not the end for the Enterprises. The USS Enterprise A is known to have been used as a low profile admiralty transport during the Dominion War after being reactivated, alongside the USS Excelsior. Whilst most of her brief second career is classified, what isn't indicates no less than 3 separate Admirals commanded their fleets from the ship over the course of the war. With the war's end both her and Excelsior were quietly returned to the museum at Athan Prime, where they can be found today.