After arranging for the assassination of Valentinian III, Petronius Maximus secured the throne of the Western Roman Empire, only to be killed 11 weeks later during the sack of Rome. Valentinian III: Valentinian III was Roman emperor in the West from 425 to 455. Starting in childhood, his reign over the Roman Empire was one of the longest, but was dominated by civil wars among powerful generals and the barbarian invasions. He was the youngest sole emperor in the Western Roman Empire. March 17, 455. Wiki link