Valentinian II (371 - 392) was elevated as Roman emperor of the West at the age of four in 375, along with his half-brother Gratian who was seventeen.
They made Milan their home; and the empire was nominally divided between them, Gratian taking the trans-Alpine provinces, while Italy, Illyricum in part, and Africa were to be under the rule of Valentinian, or rather of his mother, Justina. Justina was an Arian, and the imperial court at Milan struggled against the Catholics of that city, led by their bishop Ambrose. But so great was his popularity that the court was decidedly worsted in the contest, and the emperor's authority materially shaken. In 387 Magnus Maximus, who had commanded a Roman army in Britain, and had in 383 (the year of Gratian's death) made himself master of the northern provinces, crossed the Alps into the valley of the Po and threatened Milan.
The emperor and his mother fled to Theodosius, the emperor of the East and husband of Galla, Valentinian's sister. Valentinian was restored in 388 by Theodosius, through whose influence he was converted to Orthodox Catholicism.
This entry is based on material from the 1911 Encyclopedia Britannica.