The Defiant was a prototype vessel for the Defiant class of warships, originally developed to counter the threat of the Borg. Designed to be fast, highly maneuverable, and have extremely powerful weaponry, the ship was almost too powerful and needed extensive work done to keep itself from being torn apart. The ship was designed specifically for the task of battle, and was relatively spartan by Starfleet standards of the time. The ship was not designed to carry family members, had no science labs, and had a limited infirmary.
Additionally, the Defiant was the first Starfleet ship to legally carry a cloaking device. Supplied by the Romulan government, it was meant to be operated by an agent of that government for the crew of the ship. Primarily, the cloak would only be used in the Gamma Quadrant, but the cloaking device also ended up being used (illegally) in the Alpha Quadrant.
Upon encountering the Jem'Hadar (the shock troops of the Dominion), Captain Benjamin Sisko requested the Defiant, and it was moved to Deep Space Nine (aka Terok Nor) as an addition to the station's defenses. The ship also allowed the crew of DS9 to travel faster and farther than they could with the Runabout-class ships that had originally been supplied to the station.
There were actually two ships to carry the name Defiant in the Deep Space Nine series. The ship first appeared as USS Defiant (NX-74205) at the beginning of that series' third season. After its destruction during the Dominion War, it was replaced with the USS Sao Paulo (NCC-75633), which was renamed to carry on the USS Defiant name.
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