Emperor Gratian was thus left with what is now France, Britain and Spain while the nominal Emperor Valentinian II reigned over Italy, most of the Balkans and Africa. His imperial court was in Milan but tensions were thick. The most influential figure in his life was his mother Justina, an adherent of the Arian heresy, who clashed with the preeminent religious authority, St Ambrose, Bishop of Milan and prefect of Liguria and Emilia who was an adamant opponent of Arianism. This was a dangerous combination as the young Valentinian II became the pawn of those wishing to push their own agendas. Justina used his authority to try to suppress the orthodox Christians in favor of the Arians, though St Ambrose had the local populace on his side. Magnus Maximus, who was trying to claim the throne for himself, also used Arianism as his tool, casting himself as a champion of orthodoxy in order to take power from Valentinian II or, more precisely, those who ruled in his name. After Emperor Valens in the east came the reign of Emperor Theodosius the Great who was also opposed to Arianism and so would be of no help.
Emperor Valentinian II occupied himself with peaceful pursuits in Vienne while Arbogast marched off to fight the Germans along the Rhine in his name. However, it was clear that Arbogast was the real ruler in the west, not Valentinian II who he jealously guarded. Though nominally answerable to Emperor Theodosius, it is likely that the Eastern Emperor would not have approved of his tyrannical treatment of Valentinian. Anyone who became too close to the isolated young man could be expected to meet an unhappy end, always with the justification that the person had been guilty of some crime. Harmonius, for example, was a friend of Valentinian who was murdered by Arbogast personally and in the presence of the young emperor himself. Arbogast justified this by asserting that Harmonius had been guilty of taking bribes, but then as now many suspected that he simply wished to keep Valentinian isolated and under his power.
However, none of that was to happen and it may be that Arbogast was informed of what had been written in those letters for on May 15, 392 Emperor Valentinian II was found dead, hanging in his palace in Vienne. He was only twenty-one years old. Arbogast said that he had committed suicide and certainly his life had been one to encourage depression and despair. However, many people, then as now, believe that Arbogast had the young emperor murdered, with many accepting this as a matter of fact. Given what he had just written to Theodosius and St Ambrose, it does seem that, while in despair, Valentinian still held out hope for redress of his situation. The timing of his sudden death certainly points to Arbogast being the guilty party. We also know that St Ambrose eulogized him and praised him highly as a virtuous, young monarch, a Christian Roman emperor who should be an example to others. It seems highly unlikely he would have done so if he had harbored any suspicion that the Emperor had killed himself.
So it was with that air of mystery and suspicion that the reign of Emperor Valentinian II came to an end. His body was carried, with full imperial Roman pomp and ceremony, to Milan where his remains were met by St Ambrose and his weeping sisters Justa and Grata. He was buried alongside Emperor Gratian in the Basilica of San Lorenzo. Arbogast, though first claiming to remain loyal to Theodosius by proclaiming his son Arcadius emperor, eventually set up his own puppet usurper in the person of Eugenius. Emperor Theodosius came west, defeated Eugenius and Arbogast and placed his son Honorius on the throne as Western Roman Emperor.