Valentinian I (văl´əntĬn´ēən), 321–75, Roman emperor of the West (364–75). He held high military rank under Julian and Jovian. After the death of Jovian, Valentinian was proclaimed emperor; he appointed his brother Valens coregent in the East. Valentinian defeated the Alemanni several times, and his general Theodosius successfully defended the empire in Britain and in Africa. To protect the frontiers of his empire, Valentinian ordered the construction of fortresses on the Rhine and the Danube rivers. He reduced taxation and promoted education. Although he was an orthodox Christian, he allowed religious freedom to Arians and to pagans. He was succeeded by his sons Valentinian II and Gratian.