Bede's Ecclesiastical History of England, ed. by A.M. Sellar, [1907], at sacred-texts.com
In the year of our Lord 377, Gratian, the fortieth from Augustus, held the empire for six years after the death of Valens; though he had long before reigned with his uncle Valens, and his brother Valentinian. Finding the condition of the commonwealth much impaired, and almost gone to ruin, and impelled by the necessity of restoring it, he invested the Spaniard, Theodosius, with the purple at Sirmium, and made him emperor of Thrace and the Eastern provinces. At that time, Maximus, a man of energy and probity, and worthy of the title of Augustus, if he had not broken his oath of allegiance, was made emperor by the army somewhat against his will, passed over into Gaul, and there by treachery slew the Emperor Gratian, who in consternation at his sudden invasion, was attempting to escape into Italy. His brother, the Emperor Valentinian, expelled from Italy, fled into the East, where he was entertained by Theodosius with fatherly affection, and soon restored to the empire, for Maximus the tyrant, being shut up in Aquileia, was there taken by them and put to death.