In 2410 the USS Bajor carried a complement of T-204 tanks into the Delta Quadrant as part of the 103rd Expeditionary Force. Ten of these tanks were deployed against the Vaadwaur on Kobali Prime, and two were destroyed by enemy fire. ("Tinker, Golfer, Doctor, Trill", "Last Rights")
The T-204 measured 10 meters long and 4 meters wide, and weighed 24 metric tons. The tank was heavily armed, with a 120 millimeter smoothbore coilgun capable of putting a bunker-buster round through eight meters of ferrocrete, as well as sponson-mounted dual Type 5 phasers and a Type 4 phaser assault minigun on the turret. Power was supplied by a miniature fusion reactor much like that of a Type-10 shuttlecraft. ("Last Rights")
The Kobali did not possess any weapons capable of penetrating the T-204's armor. ("A Voice in the Wilderness, Part I")
StarSword mentioned in the author's notes for "Last Rights" that the T-204's design was meant to evoke a combination of a current-generation main battle tank such as the M1 Abrams or T-90, and a Leman Russ from Warhammer 40,000. The name was a combination of American and Russian tank nomenclature, with "Hayes" referring to MACO Major J. Hayes of Star Trek: Enterprise.