THEODOSIUS I., "the Great," son of Theodosius, Valentinian's great general, who in 368-69 saved Britain from the Picts, and suppressed the revolt of Firmus in Mauretania (372). Shortly after (376), the elder Theodosius was put to death, perhaps by order of Valens. The younger Theodosius was born about the year 346. He was a native of Spain, but the exact place of his birth is uncertain. He accompanied his father into Britain (368). and a little later defeated the Sarmatians who had invaded Moesia (374). On his father's death he retired to his native place, where he lived quietly till after the great battle of Adrianople (Aug. 9, 378), when Gratian summoned him to share the empire. After gaining some fresh victories over the Sarmatians, Theodosius was made Augustus at Sirmium on Jan. 19, 379, and was assigned all the eastern provinces, including part of Illyricum. In 379 Theo dosius, after reorganizing the army at Thessalonica, carried on a successful campaign of skirmishes along the Danube and induced numerous Gothic bands to give in their allegiance; his lieutenant Modares, a Gothic refugee, defeated the invaders severely in Thrace. In 381 he was called upon to meet two armies of invaders. He conducted in person the war against the Visigoths under Fritigern, this campaign, being only ended by Fritigern's death. The defence of the Danube against the Ostrogoths under Alatheus and Safrax was entrusted to the general Promotus, who severely defeated the enemy in an attempt to cross the river. Theodosius attained even greater successes by his diplomacy. He persuaded the fugitive Visigoth king Athanaric to enter his service, and en listed 40,000 of his former enemies as foederati, providing them with settlements in various parts of the realm.
In 383 Theodosius created his eldest son Arcadius Augustus. The same year saw the revolt of Maximus in Britain and the murder of Gratian. For five years Theodosius consented to accept the usurper as his colleague; but when Maximus attempted a few years later to make himself master of Italy Theodosius advanced against the invader and overthrew him near Aquileia (July 28, 388). This victory was followed by the murder of Maximus and his son Victor, after whose death Theodosius conferred upon Val entinian II. all that part of the empire which his father had held.
After celebrating a triumph in Rome (389) he stayed to arrange the government of Italy for another two years. If we may trust the evidence of Zosimus, from the end of the year 388 Theodosius resigned himself to gluttony and voluptuous living, from which he was only roused by the news that in the 'Western empire Arbogast had slain the young Emperor Valentinian and set up the gram marian Eugenius in his stead (May 15, 392).
Theodosius at once marched out against Eugenius. The armies met near the river Frigidus, some thirty-six miles distant from Aquileia. On the first day Theodosius' barbarians, engaging with those of the hostile army, were almost destroyed, and the victory seemed to be with Eugenius. After a night of prayer, towards cockcrow the emperor was cheered by a vision of St. Philip and St. John, who, mounted on white steeds, promised him success. On the second day the issue was doubtful till, if we may trust the concurrent testimony of all the contemporary church historians, a sudden gust of wind blew back the enemy's arrows on themselves. This was the turning-point of the battle : Eugenius was slain by the soldiers; and two days later Arbogast committed suicide (Sept. 5-9, 394). From the north-eastern parts of Italy Theo dosius passed to Rome, where he had his son Honorius pro claimed emperor under the guardianship of Stilicho. Thence he retired to Milan, where he died of dropsy (Jan. 17, 395), leaving the empire to be divided between his two sons Honorius and Arcadius.
The *chief authorities for the age of Theodosius are Ammianus Marcellinus, Zosimus, Eunapius and the ecclesiastical historians (Socrates, Sozomen, Theodoret). Much information may also be gleaned from the writings of St. Ambrose, St. Gregory of Nazianzus, Isidore of Seville, and the orators Pacatus, Libanius, Themistius.