Constantius III, d. 421, Roman emperor of the West (421). In 411, as general of Honorius, he defeated Gerontius and Constantine; thereafter he was the virtual ruler of the West. Aspiring to the hand of Honorius' sister Galla Placidia, he vied with his rival, Ataulf, the Visigothic king, and drove him from Gaul into Spain shortly after Ataulf's marriage (414) to Galla Placidia. In 416, after Ataulf was assassinated, he made peace with the new Visigothic king, Wallia, and in 417 he married Galla Placidia. He was the prime mover in granting (418) local government to Gaul and in settling (419) the Visigoths in Aquitaine. In 421 Galla Placidia persuaded Honorius to make Constantius coemperor, but Constantius died a few months after his accession. He was the father of Valentinian III.