In 2217, USS Valiant was lost while exploring star cluster NGC 321. The ship made first contact with the principal planet of an unnamed star system, Eminiar VII. While in orbit, the Valiant was warned to leave the system, but was "attacked" and subsequently declared a casualty in the war with neighboring Vendikar. The Valiant and the Earth expedition making the report failed to return from its mission. The ship's disposition was listed as "missing in space".
In 2267, on stardate 3192.1, the USS Enterprise was sent on a similar mission, only this time to open diplomatic relations with Eminiar VII. It was at this time that the fate of the Valiant was discovered. (TOS: "A Taste of Armageddon"; ENT: "In a Mirror, Darkly, Part II" display graphic)
|Starships named Valiant|
|SS Valiant • USS Valiant • USS Valiant (NCC-74210) • USS Valiant (NCC-75418)|
It should be noted that the designation "star cruiser" is one given by Anan 7 of the Eminian High Council in dialogue and then in reference to the USS Enterprise only. It is therefore assumed that Valiant was likewise designated by his predecessors.
According to the reference book Star Trek Spaceflight Chronology (p. 152), USS Valiant had the registry NCC-1102, whereas the Star Trek Encyclopedia (4th ed., vol. 2, p. 335) listed the registry of the Valiant as NCC-1223. The prior 1997 edition of the Encyclopedia stated that Valiant was a Daedalus-class starship, but that note was removed from later editions presumably due to the fact that, in TNG: "Power Play", Data mentioned that Daedalus-class vessels were taken from service years before the Valiant's mission to NGC 321. The first, 1994 edition of the Encyclopedia made no mention of class or registry at all, merely mentioning it as a "Federation starship" (p. 363).
At the start of second season of Star Trek: The Original Series the producers composed a fourteen ship list belonging to the Constitution-class, then still referred to as "Starship-class" by them, including Valiant. It should be noted that the inclusion of Valiant was a late addition and that it was not even considered in the first two proposals. This, canonically speaking at least, constituted somewhat of a conundrum, an observation not lost on Greg Jein, writer of the below-mentioned article, who noted that the ship in question was already destroyed in 2217, whereas the Constitutions only became operational a decade later as far as was established by the production staff. (The Making of Star Trek, p. 203; ) While another, new ship would have made more sense, the intent of the producers was clear, the class designation was retro-applied to the lost ship. All three lists were published in the The Making of Star Trek, pp. 164-165.
For his influential "The Case of Jonathan Doe Starship" article, published in the April 1973 issue 27 of the T-Negative fanzine, future Star Trek model maker Jein accepted the producer's intent and endowed Valiant with the appropriately lower conjectural registry number NCC-1623.  But unlike many of his other conjectural Constitution-class registry numbers, the 2006 remastered version of the series afforded no opportunity to bring this ship to canon. The reference book Star Fleet Technical Manual also listed a Constitution-class starship with the name Valiant but assigned with the registry NCC-1709.
Both role-playing games from Last Unicorn and Decipher list the Valiant as a member of the Ranger-class (β), a design based on a rejected NCC-1701 design by Matt Jefferies in the pre-production of the Original Series.