This is part of a series that will present the registry numbers and names (and ship pennants) of notable Starfleet Vessels throughout the Star Trek franchise, starting at the beginning.
The U.S.S. Defiant NX-74205 was the prototype for a new class of Federation Starship, named after the original Constitution-Class USS Defiant. Officially designated as an escort vessel, the Defiant was unofficially a warship. Although the Federation and Starfleet are peaceable entities, they do resort to certain measures when the times warrant them. The Defiant was initially designed to battle the Borg threat, but as the threat lessened over time after the inital invasion, the Defiant project became a low priority. Coupled with more armament and engine power than a ship its usual size, the initial tests had the Defiant nearly tearing itself apart. So Starfleet mothballed the ship and developmental project. However, when the Dominion started gaining ground into the Alpha Quadrant, and the threat of an attack increased, with Deep Space Nine still unable to defend itself against a potentially massive and heavily armed mobile force, at the request of then Commander Benjamin Sisko, the Defiant was pulled from storage and brought to Deep Space Nine to be the first line of defense against such an attack. As many have stated, the Defiant may have flaws, but she also has teeth, armed with photon and quantum torpedoes, heavy pulse phaser banks, highly powerful and maneuverable engines, and a cloaking device on loan from the Romulan government.
Unfortunately the Defiant met her end at the Battle of Chintoka engaging Dominion forces, which had been reinforced by Breen ships and their energy-dampening technology.