He was at first junior co-ruler of his half-brother, then was sidelined by a usurper, and finally became sole ruler after 388, albeit with limited de facto powers.

A son of emperor Valentinian I and empress Justina, he was raised to the imperial office at the age of 4 by military commanders upon his father's death.

Until 383, Valentinian II remained a junior partner to his older half-brother Gratian in ruling the Western empire, while the East was governed by his uncle Valens until 378 and Theodosius I from 379.

When Gratian was killed by the usurper emperor Magnus Maximus in 383, the court of Valentinian in Milan became the center of Italy where several religious debates took place.

[10] According to Zosimus, Gratian governed the trans-alpine provinces (including Gaul, Hispania, and Britain), while Italy, part of Illyricum, and North Africa were under the rule of Valentinian.

However, Gratian and his court was essentially in charge of the whole Western empire, including Illyricum, and Valentinian did not issue any laws and was marginalized in textual sources.

As a lesser partner to Gratian in the West, Valentinian and his court in Milan had remained ineffectual and obscure until his brother's tragedy finally brought them to the forefront.

[20] However, not only Justina, but the wider imperial court also opposed Ambrose's claim, since the praetorian prefect and the emperor's counsellors met him and demanded that he turn over the basilica.

[20][21] When Ambrose was summoned to be punished to the Imperial palace, the orthodox populace rioted, and Gothic troops were prevented by the arch-bishop himself, standing in the doorway, from entering the Basilica.

Rufinus continues that the imperial troops besieged him, but Ambrose held on, reinforcing the resolution of his followers by allegedly unearthing, beneath the foundations of the church, the bodies of two ancient martyrs.

In a panegyric for Theodosius, the orator Pacatus asserted that the empire belonged to his two sons, Arcadius and Honorius, while barely mentioning the newly restored Valentinian.

[29][30] On the Eastern emperor’s coinage, Valentinian continued to be represented with the “unbroken” legend like Arcadius, depicting both of them as Theodosius’ junior colleagues.

[31][32][7] Modern scholars, observing Theodosius’ actions, suspect that he had no intention of allowing Valentinian to rule, due to his plan for his sons to succeed him.

[33][34][35] When Theodosius decided to return to the East, his trusted general, the Frank Arbogast, was appointed magister militum for the Western provinces (bar Africa) and guardian of Valentinian.

[40] The crisis reached a peak when Arbogast prohibited the emperor from leading the Gallic armies into Italy to oppose a barbarian threat.

His reign was a harbinger of the fifth century, when young emperors were controlled by powerful generals and officials until mid-century (Honorius, Arcadius, Theodosius II, Valentinian III and Romulus Augustulus).