As the Emperor Gratian was at this period occupied with a war against the Alamanni, Maximus quitted Britain, with the design of usurping the imperial power. Valentinian was then residing in Italy, but as he was a minor, the affairs of state were transacted by Probus, a prætorian prefect, who had formerly been consul.
Justina, the mother of the emperor, having espoused the Arian heresy, persecuted Ambrose, bishop of Milan, and disquieted the churches by her efforts to introduce alterations in the Nicene doctrines, and to obtain the predominance of the form of belief set forth at Ariminum. She was incensed against Ambrose because he strenuously opposed her attempts at innovation, and she represented to her son that he had insulted her. Valentinian believed this calumny, and, determined to avenge the supposed wrongs of his mother, he sent a party of soldiers against the church. On their reaching the temple, they forced their way into the interior, arrested Ambrose, and were about to lead him into exile at that very moment, when the people assembled in crowds at the church, and evinced a resolution to die rather than submit to the banishment of their priest. Justina was still further incensed at this occurrence; and with a view of enforcing her project by law, she sent for Menivolus, one of the legal secretaries, and commanded him to draw up, as quickly as possible, an edict confirmatory of the decrees of Ariminum. Menivolus, being firmly attached to the Catholic Church, refused to write the document, and the empress tried to bribe him by promises of greater honors. He still, however, refused compliance, and, tearing off his belt, he threw it at the feet of Justina, and declared that he would neither retain his present office, nor accept of promotion, as the reward of impiety. As he remained firm in his refusal, others were intrusted with the compilation of the law. By this law, all who conformed to the doctrines set forth at Ariminum and ratified at Constantinople were exhorted to convene boldly; and it was enacted that death should be the punishment of those who should hinder or be running counter to this law of the emperor.
While the mother of the emperor was planning the means of carrying this cruel law into execution, intelligence was brought of the murder of Gratian, through the treachery of Andragathius, the general of Maximus. Andragathius obtained possession of the imperial chariot, and sent word to the emperor that his consort was traveling towards his camp. Gratian, who was but recently married and youthful, as well as passionately attached to his wife, hastened incautiously across the river, and in his anxiety to meet her fell without forethought into the hands of Andragathius; he was seized, and, in a little while, put to death. He was in the twenty-fourth year of his age, and had reigned fifteen years. This calamity quieted Justina's wrath against Ambrose.
Maximus, in the meantime, raised a large army of Britons, neighboring Gauls, Celts, and other nations, and marched into Italy. The pretext which he advanced for this measure was, that he desired to prevent the introduction of innovations in the ancient form of religion and of ecclesiastical order; but he was in reality actuated by the desire of dispelling any suspicion that might have been excited as to his aspirations after tyranny. He was watching and intriguing for the imperial rule in such a way that it might appear as if he had acquired the Roman government by law, and not by force. Valentinian was compelled by the exigencies of the times to recognize the symbols of his rule; but soon after, in fear of suffering, fled with his mother Justina, and Probus, the prætorian prefect in Italy, to Thessalonica.