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 Romulan Wars, with the "humiliating defeat" of the Romulans. (ENT: "In a Mirror, Darkly, Part II" production art)
Even into the 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")