In late-2373, the Valiant began a three month training cruise mission to circumnavigate the entire Federation before returning home. The plan was for the cadets to run the ship while the officers aboard observed and critiqued their performance.
The ship was transiting the Kepla sector when the Dominion War began, and subsequently trapped behind enemy lines after a Dominion fleet conquered that sector on the first day. In attempting to return to Federation territory, the Valiant encountered a Cardassian battle cruiser near El-Gatark. In the first fifteen minutes, all seven of the regular officers were killed or critically wounded, including the captain. The Valiant lost main power and was adrift; fortunately, the Cardassian cruiser was no better off. Despite having a punctured lung and massive internal injuries, Captain Ramirez refused to go to sickbay and instead, stayed on the bridge to direct the entire damage control effort. The cadets got weapons and impulse engines back on-line within three hours, and were able to destroy the cruiser. The next day, just before Ramirez died, he appointed Cadet Tim Watters to take command of the Valiant.
Early in the war, Starfleet ordered the Valiant to track and collect technical data on a previously unidentified Jem'Hadar battleship that was believed to be operating in the area. However, because the Valiant was operating under complete radio silence and limited to a speed of warp 3.2, Starfleet was unaware that the ship's officers were dead, and Watters decided to undertake the mission.
Around 2374, on stardate 51825, the Valiant rescued Jake Sisko and Ensign Nog from the runabout USS Shenandoah, which was fleeing from a Jem'Hadar attack on Starbase 257. Nog assisted the Valiant's crew in repairing their warp drive, and the ship finally managed to catch up to the Dominion battleship and obtain detailed intelligence concerning the enemy's capabilities.
Captain Watters' overconfidence, however, proved to be the Valiant's undoing. Rather than returning the valuable information to Starfleet, Watters decided to attempt to destroy the battleship by exploiting a perceived flaw in her construction.
Vastly outgunned, the Valiant was quickly devastated after an unsuccessful attempt to destroy the enemy's antimatter storage system primary support braces. With Captain Watters and most of the bridge crew killed, Nog gave the order to abandon ship. Two of the four escape pods launched were destroyed by the battleship, while a third was destroyed when it was unable to clear the exploding Valiant. The remaining pod was able to flee the battle area and was eventually recovered by the Valiant's sister ship, the USS Defiant.
"Valiant" was the name that Ronald D. Moore originally wanted to give to the Defiant. However, with the development of Star Trek: Voyager, the producers asked him to avoid a name that began with 'V'. (Star Trek: Deep Space Nine Companion (p. 158))
In the reference book Star Trek: Celebrations, it was established that Starfleet Academy created a service award to cadets who show exceptional teamwork and named it the Valiant commendation to commemorate the Valiant and its crew.