Flavius Valentinianus was born to Emperor Valentinian I and his second wife, Justina. He was the half-brother of Valentinian’s other son, Gratian, who had shared the imperial title with his father since 367. He had three sisters Galla, Grata and Justa. The elder Valentinian died on campaign in Pannonia in 375. Neither Gratian (then in Trier) nor his uncle Valens (emperor for the East) were consulted by the army commanders on the scene. Instead of merely acknowledging Gratian as his father’s successor, Valentinian I’s generals acclaimed the four-year-old Valentinian augustus on 22 November 375. The army, and its Frankish general Merobaudes, may have been uneasy about Gratian's lack of military ability, and so raised a boy who would not immediately aspire to military command.

Gratian, forced to accommodate the generals who supported his half-brother, governed the trans-alpine provinces (including Gaul, Hispania, and Britain), while Italy, part of Illyricum, and North Africa were under the rule of Valentinian. In 378, their uncle, the Emperor Valens, was killed in battle with the Goths at Adrianople, and Gratian invited the general Theodosius to be emperor in the East. As a child, Valentinian II was under the influence of his Arian mother, the Empress Justina, and the imperial court at Milan, an influence contested by the Nicene bishop of Milan, Ambrose.

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