Made emperor because of his military abilities, he settled the long-standing Gothic problem by allowing the Goths to settle South of the Danube as allies of Rome. His title comes from his vigorous championship of orthodox Christianity.

Theodosius was born in Spain and became co-emperor with Gratianus in 378. He solved the Gothic threat in the Balkans by fostering dissension among them and making the Visigoths allies rather than subjects. He allowed barbarian tribes to settle on Roman territory, and gave their leaders responsible positions.

At his death, his will divided the Roman empire between his two sons, Honorius in the West and Arcadius in the East. This meant the end of imperial unity. The East Roman Empire was ruled from Constantinople; the western empire from Ravenna (after 404).