The USS Defiant in Star Trek: Deep Space Nine broke big Star Trek rules because it was equipped with a Romulan cloaking device. Originally, DS9 wasn't supposed to have a starship of its own, but as the Dominion was developed as the series' villains, executive producer Ira Steven Behr fought for a starship in order for DS9 to have the proper firepower to meet this emerging threat. Paramount ultimately acquiesced but decreed DS9 's starship needed to be smaller than Star Trek: The Next Generation and Star Trek: Voyager's signature ships.
The Defiant (NX-74205) debuted at the start of Star Trek: Deep Space Nine season 3. As then-Commander Benjamin Sisko (Avery Brooks) explained, the Defiant was a prototype of a new class of warship that was originally intended to defeat the Borg. The undersized but overpowered Defiant quickly became beloved by DS9 fans, and it played an integral role throughout the series. The original Defiant was destroyed in DS9 season 7, but it was replaced a few episodes later by an identical ship that was renamed Defiant. The "tough little ship" was also commanded by Worf (Michael Dorn) when it made its movie debut in Star Trek: First Contact. But the Defiant was also one of Starfleet's most controversial ships.
DS9's USS Defiant Broke Star Trek Rules With Its Cloaking Device
Star Trek: Deep Space Nine's USS Defiant was equipped with a Romulan cloaking device, which a Starfleet ship should not have. In 2311, the United Federation of Planets signed the Treaty of Algeron with the Romulan Star Empire. This forbade the Federation from using cloaking devices, an advantage the Romulans and Klingons continued to enjoy. Sisko explained in DS9 that the Defiant received a special dispensation by the Romulans to have a cloaking device out of concern for the Dominion threat. There was also a Romulan officer on the Defiant meant to watch the cloaking device, but he was forgotten after 2 episodes.
Not only was the Defiant's cloaking device an about-face of Star Trek's in-universe law, but it also defied Gene Roddenberry's edict that he didn't want Starfleet to use cloaking devices because "our heroes don't sneak around." Originally, the Defiant's cloak was only to be used when the ship was in the Gamma Quadrant, but soon, Captain Benjamin Sisko began cloaking the Defiant whenever he saw fit. Executive producer Ronald D. Moore said in a 1997 AOL chat that, "We wanted to throw in a line somewhere explaining that there was a follow-on agreement with the Roms [sic] that allowed us greater freedom with the cloak, but never found a comfortable place to put it."
Why DS9 Got Away With Breaking The Cloaking Device Rule
Star Trek: Deep Space Nine didn't face any significant blowback for the USS Defiant utilizing a cloaking device. DS9's status as the 'red-headed stepchild' of the 1990s Star Trek franchise could sometimes be a blessing in disguise because it allowed the series the freedom to push the boundaries of Trek without a great deal of oversight. The Defiant having a cloak also helped set it apart from the USS Enterprise-E and the USS Voyager.
The USS Defiant using prohibited Romulan technology opened the gateway for Star Trek: Voyager to also break a few Starfleet rules. Voyager was trapped in the Delta Quadrant millions of light-years from Earth, and Captain Kathryn Janeway (Kate Mulgrew) had to be flexible with regulations in order to survive. Voyager occasionally utilized alien and even Borg technology if it meant getting the crew back to Earth faster. But Star Trek: Deep Space Nine did enjoy the freedoms to tell serialized stories how it saw fit, and Captain Sisko's Defiant had a unique cool factor that was enhanced by its cloaking device.