Reign from Milan (375–387)
Gratian was forced to accommodate the generals who supported his half-brother; he purportedly enjoyed seeing to Valentinian's education. According to Zosimus, Gratian governed the trans-alpine provinces including Gaul, Hispania, and Britain; Valentinian was nominally ruler of Italy (part of Illyricum) and North Africa. In fact, however, Gratian governed the whole Western empire; Valentinian was marginalized and did not issue any laws. In 378 their uncle Emperor Valens was killed by the Goths in Adrianople; Gratian invited the general Theodosius to be emperor in the East. As a child, Valentinian II was under the pro-Arian influence of his mother the empress Justina and of the courtiers at Milan. Such influence was opposed by the Nicene bishop of Milan, Ambrose.
In 383, Magnus Maximus, commander of the armies in Britain, declared himself Emperor and established his headquarters in Gaul and Hispania. Gratian was killed while fleeing him. As a lesser partner in the West, Valentinian was largely ineffectual and obscure until the usurpation by Maximus and death of Gratian. For a time the court of Valentinian, through the mediation of Ambrose, came to an accommodation with the usurper, and Theodosius recognized Maximus as co-emperor of the West.
Valentinian tried to restrain the despoiling of pagan Roman temples. In 384 the pagan senators, led by the Prefect of Rome Aurelius Symmachus, petitioned the emperor for the restoration of the Altar of Victory in the Senate House which had been removed by Gratian in 382. Valentinian refused the request and, in so doing, rejected the traditions and rituals of pagan Rome to which Symmachus had appealed. While Ambrose participated in the campaign against the reinstatement of Altar of Victory, he admitted he was not the cause of the decision to remove the altar in the first place.
In 385 Ambrose refused an imperial order to hand over the Portian Basilica (now the Basilica of San Vittore al Corpo) for the celebration of Easter by the Imperial court. His refusal angered not just Valentinian but also Justina, high-ranking officials, and court Arians including Goths. Ambrose argued in his letter that Justina influenced her young son to oppose the Nicean party championed by Ambrose, and framed her motivation as purely selfish.
But the wider imperial court also opposed Ambrose's claim, and the praetorian prefect and the emperor's counsellors demanded that he turn over the basilica. When Ambrose was summoned to the court for punishment the orthodox populace rioted. Gothic troops attempted to enter the Basilica; Ambrose stood in the doorway, and the Goths did not enter. Rufinus claimed that when Ambrose was found guilty of breaking the new laws Justina persuaded Valentinian to have him banished, and that Ambrose barricaded himself within the walls of the Basilica, with the enthusiastic backing of the people. Rufinus says that the imperial troops besieged Ambrose but he held on, and that he found the bodies of two ancient Christian martyrs beneath the foundations of the church which raised the spirits of the people. Magnus Maximus tried to use the emperor's heterodoxy against him in a war of public opinion. To this end, he wrote a scathing missive attacking Valentinian for plotting against God.
In 386 to 387, Maximus crossed the Alps into the Po valley to threaten Milan. Valentinian II and Justina fled to Emperor Theodosius I in Thessalonica. The latter came to an agreement, cemented by his marriage to Valentinian's sister Galla, to restore the young emperor in the West. Theodosius and his forces marched west in 388 and defeated Maximus.
