USS Defiant's history began after the USS Enterprise was hurled into system J-25 in the Delta Quadrant, where it encountered the Borg for the first time. The data that Enterprise collected on the Borg projected a grim shadow on the defense systems of the Federation. After the Enterprise presented its report on the capabilities of the Borg, Starfleet began a project to develop new technologies to combat the Borg threat. Design and research was nearing completion Stardate 43998.0, when the Borg invaded the Sol system. Unfortunately, the advancements made under Project Defiant were still only conceptual, with no working prototype at the time. Had production been completed, perhaps the incident at Wolf 359 could have been avoided. The Battle of Wolf 359 only clarified the need for better defensive weapons and sped up production. Less than one year after the Borg ship was destroyed, USS Defiant was completed in its secret construction facility at Utopia Planitia Fleet Yards, Mars. The Defiant's weapons systems make it the most powerful warship in the Alpha Quadrant: twin fountain stream phaser cannons capable of destroying the shields of an enemy vessel and then devastating its hull, and new developments of photon torpedoes which remain classified. Added to these (and other classified systems), the Defiant is extremely maneuverable and quick -- the fastest sublight ship ever constructed. The Defiant is small and very automated, requiring a crew of 10-12, with a maximum crew compliment of 50. Not only is Defiant a technological marvel, it is also a diplomatic marvel -- the Federation was able to convince the Romulan Empire to fit it with a cloaking device. USS Defiant was stationed at Deep Space 9, under the command of Captain Benjamin Sisko, who was assigned to Defiant's enginnering team after his former ship, USS Saratoga NCC-31911, was lost at Wolf 359. Defiant currently is the only Federation ship assigned to explore the Gamma Quadrant. It is no longer the only ship of its class. The advancements made during Project Defiant are currently being integrated into new ship classes currently under construction. The Soverign class is an example, she's fitted with ablative armor, carries quantum torpedos and has an auto modulating deflector shield system.

While leadng a Federation assault force to retake the ChinToka system from the new Dominion/Breen alliance, the orginal Defiant was destroyed. Most of the crew made it make to DS9 and took over a new USS Defiant presented to Captain Sisko, by Admiral Ross.