The specifications for the Defiant were stolen by Miles O'Brien during a visit to the primary universe. "Smiley" O'Brien was able to download the schematics from Deep Space 9's computers and brought them back to the mirror universe, where construction began at Terok Nor, now under rebel control. While construction proceeded well with materials stolen from the Alliance, the powerful warp core on the Defiant caused the ship to almost shake itself apart during test flights, much like had occurred with the counterpart version.