'St. Ambrose Absolves Emperor Theodosius', c.1673 (oil on canvas)

Theodosius I (347-395), also known as Theodosius the Great, was born into a military family in Hispania. He served with his father until his execution in 374 CE, after which Theodosius retired to Hispania until he was given the position of co-emperor by Emperor Gratian after Emperor Valens' death in 378 CE.- Theodosius ruled the East Roman Empire, and after Gratian himself was killed in 383 CE, appointed his son Arcadius as his co-ruler in the east while briefly acknowledging the usurper Magnus Maximus before agreeing to a marriage with Emperor Valentinian II's sister Galla and defeating Maximus in battle. He then appointed his trusted general Arbogast to watch and effectively rule over the young Valentinian II in the west, making Theodosius de facto ruler of both West and East.- Arbogast eventually killed Valentinian II and placed Eugenius as his puppet emperor in the west in 392 CE, forcing Theodosius to march against him, giving his son Honorius the title of co-emperor in the West instead. Eugenius and Arbogast were defeated in 394 CE, the latter executed while the former committed suicide, leaving Theodosius as the last sole emperor to truly rule over both halves of the Roman Empire. He eventually died in 395 CE from severe edema, leaving his sons ruling each half of the empire.