The Defiant-class starship, officially an escort, began development in 2366 as a small, highly-powered, heavily-armed warship intended to defend the Federation against the Borg. The USS Defiant (NX-74205) was the first of what was to be a new Federation battle fleet. Starfleet abandoned the project when the Borg threat became less urgent; also, design flaws turned up, nearly destroying the Defiant during its initial shakedown cruise.
The Defiant prototype was pulled out of storage in 2371 when the Dominion threat emerged. After several upgrades by Operations Chief Miles O'Brien, the ship was deemed worthy of use and several more ships of the class were constructed.