Fall of Rome. Valentinian I (Latin: Valentinianus; 321 – 17 November 375), sometimes called Valentinian the Great, was Roman emperor from 364 to 375. He ruled the Western half of the empire, while his brother Valens ruled the East. During his reign, he fought successfully against the Alamanni, Quadi, and Sarmatians, strengthening the border fortifications and conducting campaigns across the Rhine and Danube. His general Theodosius defeated a revolt in Africa and the Great Conspiracy, a coordinated assault on Roman Britain by Picts, Scoti, and Saxons. Valentinian founded the Valentinianic dynasty, with his sons Gratian and Valentinian II succeeding him in the western half of the empire.