Theodosius I

Theodosius I was the last emperor to exercise effective rule over both halves of the Roman Empire, and the arrangement made at his death fixed its permanent division. He came from Hispania, born in 347, the son of the elder Theodosius, a distinguished general under Valentinian I who was executed amid court intrigue in 376. The son's career survived his father's fall: after the catastrophic Roman defeat by the Goths at Adrianople in 378, the western emperor Gratian appointed Theodosius Augustus for the East in January 379.

His first task was the Gothic war, which he ended not by reconquest but by the treaty of 382, settling the Goths within the empire as allied soldiers under their own leaders, a precedent of long consequence. In religion his reign was decisive. Baptized during an illness in 380, he issued in that year the Edict of Thessalonica declaring Nicene Christianity the faith of the empire, convened the Council of Constantinople in 381, and in the early 390s issued laws against public sacrifice that marked the legal end of state paganism. After his troops massacred civilians at Thessalonica in 390, he performed public penance at the prompting of Ambrose of Milan, an episode much elaborated later but resting on contemporary evidence.

Dynastic marriage tied his new house to the old. His second wife, Galla, was the daughter of Valentinian I, joining the Theodosian and Valentinianic lines; their daughter Galla Placidia became mother of a western emperor. Twice Theodosius marched west against usurpers, defeating Magnus Maximus in 388 and Eugenius at the river Frigidus in September 394, after which he briefly ruled the whole empire alone.

He died at Milan in January 395, aged forty-seven. The East passed to his elder son Arcadius and the West to the ten-year-old Honorius, under the guardianship of the general Stilicho, who was himself bound into the family by marriage to the emperor's niece Serena. The Theodosian dynasty endured in both empires for two further generations.