Shortly before the end of the Tionese War[1] between the Galactic Republic and the Honorable Union of Desevro & Tion[2] around 23,900 BBY,[3] a Tionese military force carried out an attack on the Republic settlements on the astronomical object Okator VIII.[1]
Sources
Star Wars: The Essential Guide to Warfare Author's Cut, Part 4: The Rise of the Republic on StarWars.com (backup link) (First mentioned)