Theodosius I. The Great, a Roman emperor, son of the preceding, born in Italica or Cauca, Spain, about A. D. 346, died in Milan, Jan. 17, 395. He learned the art of war under his father, was early given a separate command and appointed duke of Moesia, and in 374 gained a victory over the Sarmatians. After the execution of his father he retired to Spain, where he led a private life until the emperor Gratian summoned him to take the supremo command, declared him Augustus, Jan. 19, 379, and assigned to him the administration of Thrace, Asia, and Egypt, with Dacia and Macedonia. Fixing his headquarters at Thessalonica, Theodosius carried on the war against the Goths during four campaigns (379-3S2). The Goths, divided by dissensions and jealousies after the death of their leader Fritigern, were again united under Athanaric, who made peace and visited Constantinople, where ho died; and the magnificent funeral honors paid him by Theodosius so won over his followers that they enlisted in the Roman army. In.383 Gratian, the emperor of the West, was dethroned and put to (loath by Maximum, and Theodosius entered into a treaty with the usurper, by which he recognized him as emperor of the countries north of the Alps, Valontinian, the brother of Gratian, being secured the possession of Italy, Africa, and western Illyricum. Theodosius now devoted his attention to the affairs of the church.
Fixing his residence at Constantinople, the stronghold of Arianism, he determined to do away with that creed, and gave to the archbishop Demophilus the alternative of subscribing to the Nicene creed or instantly resigning. Demophilus resigned, and Gregory Nazianzen was installed in his place. Six weeks afterward Theodosius commissioned his lieutenant Sapor to expel all the Arian clergy from the churches in his dominions, and gave him a military force sufficient to carry out the decree. In May, 381, he assembled the first council of Constantinople, to confirm and complete the Niceno creed; and during 15 years he issued at least 15 edicts against all heretics, especially against those disbelieving the doctrine of the Trinity. In the mean time Maximus had entered Italy, and dethroned Valontinian II. Theodosius, who had married a sister of Va-lentinian, marched against Maximus, then encamped at the Pannonian city of Siscia (now Sissek) on the Save, defeated him, and pursued him to Aquileia, where Maximus was given up by his own troops and put to death.
Theodosius entered Rome in triumph, June 13, 389. The people of Thessalonica having for a slight cause murdered Botheric and the other principal officers of the little garrison, the emperor sent thither an army of barbarians, who, when the inhabitants were assembled by invitation at the circus, massacred them to the number of many thousands. For this St. Ambrose forbade him to enter a church in Milan until he had done public penance. He remained in Italy three years. When Valentinian was strangled in 392 by his general Arbogastes, who had secured for himself all the real power of the government, and now set up as emperor the rhetorician Eugenius, Theodosius undertook again the conquest of the West. After a severe and long uncertain contest he defeated Arbogastes near the passes of the Julian Alps. Theodosius was now master of the whole Roman world. Honorius, his younger son, was called to Milan to receive the sceptre of the West, and here Theodosius died immediately after his arrival. In the eastern empire he was succeeded by his elder son Arcadius.