Valentinian II was born at Treviri in AD 371, the son of Valentinian and Justina, as the half-brother to Gratian.
At the death of Valentinian in AD 375, Gratian became sole emperor of the west. But within a mere five days Valentinian II, who was only four years old at the time, was hailed emperor at Aquincum by the Danubian troops. This was due to intense rivalry between the Danubian legions and those on the Rhine, feeling the German legions had too much say, this was a demonstration of Danubian power. Though Gratian accepted his brother as co-emperor and a serious crisis was averted. Realizing that the four your old Valentinian II was an innocent part in these events, Gratian didn't take offence and remained kind toward the child, overseeing his education and allotted him, at least in theory, the dominions of Italiae, Africa and Pannoniae.
Valentinian II was still a young child, too young by far to play any role, when Valens met his end at the fateful battle of Adrianople. And even when Magnus Maximus revolted in Britain and Gratian was assassinated Valentinian II was only eight years old.
The eastern emperor now negotiated a peace with Magnus Maximus, both on his own as well as on Valentinian II's behalf. According to this agreement Maximus had control of the west, but for Valentinian II's domains of Italiae, Africa and Pannoniae.
during this time of peace the west experienced a very tolerant and lenient religious policy. Leading pagan senators who came to hold powerful positions ensured no drastic steps were taken to enforce Christianity.
But the fragile peace would not last, it only served to allow Maximus to strengthen his position before seeking to grab more power for himself.
Theodosius moved on the usurper in AD 388, defeated, captured and executed him. Did Theodosius not like the tolerance which had been shown towards the pagans under Valentinian II, then he still reinstated him as emperor of the west. Although Valentinian II's power remained largely theoretical, as Theodosius remained in Italy until AD 391, most likely as a deterrent to any other potential rebels. Therefore Valentinian II's limited powers only really effected Gaul while the rest remained under the rule of the eastern emperor.
But during the very time when Theodosius was in Italy, the man who should bring down Valentinian II, was arising. Arbogast, the overbearing, Frankish 'Master of the Soldiers' grew in influence to be the power behind the throne of Valentinian II. Theodosius must have deemed him a safe pair of hands to assist the young western emperor in ruling his half of the empire, as he left him in place when he finally departed for the east in AD 391.
But the domineering Arbogast soon began to worry Valentinian II. As the emperor handed Arbogast a letter of dismissal he only had it insolently thrown at his feet. Arbogast felt himself invincible by now, so much so that he could publicly defy his own emperor.
There is the possibility that he committed suicide, but generally it is believed that the emperor was murdered on behalf of Arbogast.