Theodosius I (Ancient Greek: Θεοδόσιος Theodosios; 11 January 347 – 17 January 395), also known as Theodosius the Great, was a Roman emperor from 379 to 395. He won two civil wars and was instrumental in establishing the Nicene Creed as the orthodox doctrine for Nicene Christianity. Theodosius was the last emperor to rule the entire Roman Empire before its administration was permanently split between the Western Roman Empire and the Eastern Roman Empire. He ended the Gothic War (376–382) with terms disadvantageous to the empire, with the Goths remaining within Roman territory but as nominal allies with political autonomy.
Born in Hispania, Theodosius was the son of a high-ranking general of the same name, Count Theodosius, under whose guidance he rose through the ranks of the Roman army. Theodosius held independent command in Moesia in 374, where he had some success against the invading Sarmatians. Not long afterwards, he was forced into retirement, and his father was executed under obscure circumstances. Theodosius soon regained his position following a series of intrigues and executions at Emperor Gratian's court. In 379, after the eastern Roman emperor Valens was killed at the Battle of Adrianople against the Goths, Gratian appointed Theodosius as a successor with orders to take charge of the military emergency. The new emperor's resources and depleted armies were not sufficient to drive the invaders out; in 382 the Goths were allowed to settle south of the Danube as autonomous allies of the empire. In 386, Theodosius signed a treaty with the Sasanian Empire which partitioned the long-disputed Kingdom of Armenia and secured a durable peace between the two powers.[9]
Theodosius was a strong adherent of the Christian doctrine of consubstantiality and an opponent of Arianism. He convened a council of bishops at the First Council of Constantinople in 381, which confirmed the former as orthodoxy and the latter as a heresy. Although Theodosius interfered little in the functioning of traditional pagan cults and appointed non-Christians to high offices, he failed to prevent or punish the damaging of several Hellenistic temples of classical antiquity, such as the Serapeum of Alexandria, by Christian zealots. During his earlier reign, Theodosius ruled the eastern provinces, while the west was overseen by the emperors Gratian and Valentinian II, whose sister he married. According to historian Andreas Schwarcz, the marriage was specifically arranged to consolidate Theodosius' political and military power.[10] Theodosius sponsored several measures to improve his capital and main residence, Constantinople, most notably his expansion of the Forum Tauri, which became the biggest public square known in antiquity.[11] Theodosius marched west twice, in 388 and 394, after both Gratian and Valentinian had been killed, to defeat the two pretenders, Magnus Maximus and Eugenius, who rose to replace them. Theodosius's final victory in September 394 made him master of the entire empire; he died a few months later and was succeeded by his two sons, Arcadius in the eastern half of the empire and Honorius in the west.