The Battle of Cheron was a pivotal battle that took place in 2160, between the Romulan Star Empire and an Earth/Andorian/Vulcan/Tellarite alliance. The outcome of the battle effectively ended the Earth-Romulan War, with the "humiliating defeat" of the Romulans. (ENT: "In a Mirror, Darkly, Part II" production art)
Even into the mid-24th century, the battle remained an intense embarrassment for the Romulans, which, according to Alidar Jarok, would be retributed by "the new leaders" on Romulus, who "vowed to discard the treaty and claim the Neutral Zone," with Nelvana III being the first step. (TNG: "The Defector")
After the Battle of Cheron had first been mentioned in "The Defector" in 1990, it was not until 2005 that a computer screen seen in "In a Mirror, Darkly, Part II" presented the first canon reference to when and in which context it took place. The same computer screen, which displayed a historical archive entry mentioning a "Human/Vulcan/Andorian/Tellarite alliance" winning the Battle of Cheron in 2160, is also the only canon reference that these races fought the Romulans alongside Earth forces.
An article in the Picard family album that wasn't seen on screen was titled "Foe in retreat after Cheron debacle" in reference to this battle.
According to the reference book Star Trek: Federation - The First 150 Years, the planet Cheron seen in TOS: "Let That Be Your Last Battlefield" was the namesake of the battle. ((citation needed • edit))
However, according to the Star Trek: Star Charts (pp. 60 and 67) and the Stellar Cartography: The Starfleet Reference Library ("Federation Historical Highlights, 2161-2385"; "The Romulan War: An Astrographic Brief"; "Portion of the Romulan Star Empire"), the Battle of Cheron was fought in the Cheron (Omicron Gruis) star system. The primary was a F-class star and was located in the Romulan War Theater of Operations (2156-2160). The Battle of Cheron was fought in this system on January 10, 2060. According to a map of the Romulan Star Empire – prepared by the Stellar Archive Artist Lsel Essik, from the Romulan Master Data Catalog, in 2366 – Cheron was identified as a Romulan system. (The homeworld of the Cheron natives was located in the Cheron (83 Leonis) system.}