Following the Borg incursion of 2366, Starfleet believed that another Borg attack was imminent. The Defiant project, started the year prior but having made little progress since, was quickly reworked under Commander Benjamin Sisko of Utopia Planitia as a test platform for new anti-Borg technologies. Sisko believed that this would give Starfleet a warship to reinforce the existing fleet in the short-term, while also accelerating the development of larger and more advanced projects in the long-term, such as Akira, Prometheus, and Sovereign.

Construction of the prototype began in 2367 and took 13 months. It was outfitted with an overdimensioned warp core to provide adequate energy for any sort of experimental technology, however this led to unforeseen structural stresses and routine power failures. Morale also proved an issue for the test crews, as the small crew quarters and almost complete lack of social spaces were seen as regressions by many officers used to larger, more comfortable posts. After several successful if problematic technology tests, and a lack of Borg activity leading Starfleet to pull back its readiness targets, the prototype was mothballed in late 2369.

In early 2371 Commander Sisko, now CO of Deep Space 9, requested the prototype spaceframe USS Defiant be assigned to the Bajoran sector under his command, as an additional line of defense against the new Dominion threat. The vessel was successful in this new role, and being attached to a starbase removed the impact on crew morale that the small accommodations had created during testing. Following a major refit later that year to address the structural issues, construction of further ships of the class was approved, and by the start of the Dominion War just two years later 11 Defiant-class starships were in service, with 29 more under construction.

USS Defiant NX-74205

The prototype ship for the class proved a versatile, if cramped, vessel after being assigned to Deep Space 9 in 2371. Tasked with defending Federation interests on both sides of the Bajoran wormhole, its impressive offensive capabilities were supplemented by a Romulan cloaking device, provided by the Empire to help gather intelligence on the Dominion. The ship proved decisive in multiple combat situations, like the Battle of the Omarion Nebula, the rescue of the Detapa Council during the Klingon-Cardassian War, and the Battle of Sector 001.

With the start of the Dominion War in 2373, Defiant was reassigned to the Second Fleet. Under the command of now Captain Sisko, it eventually spearheaded significant operations during the war such as Operation Return, and both Battles of Chin’toka. In 2375, during the Dominion’s attempt at retaking the Chin’toka system, Defiant was hit with a Breen energy-dampening weapon and disabled. While most of the crew managed to evacuate, Defiant was destroyed alongside most of the Federation Alliance fleet.