Justin II inherited Justinian's overextended empire, stopped paying its neighbours to stay away, lost Italy to the Lombards and Syria to Persia, and went mad.
Justin II succeeded his uncle Justinian in 565 and inherited an empire that looked enormous on a map and was insolvent underneath it.
His response was to stop paying. Justinian had bought peace on several frontiers with subsidies, and Justin refused them — most consequentially to the Persians, which restarted a war the empire could not afford, and to the Avars, who had to be dealt with anyway.
Within a decade Italy was largely lost to the Lombards, the Balkans were open to Avars and Slavs, and the eastern war was going badly. In 574 he suffered a mental collapse and never fully recovered.
He was a nephew of Justinian through the emperor's sister, and married Sophia, a niece of the empress Theodora — so the succession kept the family arrangement doubled.
He held the office of curopalates in the palace during his uncle's last years and took the throne quickly on Justinian's death in November 565, forestalling a rival cousin of the same name.
The reign began with a public gesture that set its tone: he paid off his uncle's debts personally and announced an end to the corruption and the subsidies of the old regime.
The sources describe a proud, rigid man with a strong sense of imperial dignity and a poor sense of what the empire could sustain — which is a combination that appears repeatedly among successors to great reigns.
His refusal of tribute to the Persians and the Avars was defensible as principle and disastrous as policy, and the fact that he framed it in terms of Roman honour rather than of arithmetic is the clearest window onto how he thought.
His illness is described with unusual specificity: periods of violent derangement, lucid intervals, and the famous detail that organ music was played to calm him. In one of the lucid intervals he adopted Tiberius and delivered a speech of abdication that John of Ephesus reports as an admission that he had governed badly — which, if it is anything like accurate, is the most self-aware statement any Byzantine emperor left.
The Lombards entered Italy in 568, three years into the reign, and took most of the north within a decade. Justinian's twenty-year war to recover Italy was undone in less time than it had taken to win.
In the east, Justin's refusal of the annual payments and his intervention in Armenia restarted the Persian war in 572. Dara, the key fortress of the Mesopotamian frontier, fell in 573, and the news is what the sources connect to his collapse.
In the Balkans the Avars pressed south and Slavic groups began settling permanently. Every frontier Justinian had stabilised was in trouble at once, and the treasury he had emptied could not answer any of them.
His reign is the hinge between Justinian's empire and the smaller, harder state that fought Persia to exhaustion under Heraclius fifty years later.
The judgement usually passed on him — that he destroyed his uncle's achievement — is too simple. The reconquests were unaffordable before he touched them, and the collapse he presided over had been prepared by the plague, the wars and the finances he inherited.
What he did contribute was the choice of Tiberius, made while ill and made well. The succession he arranged carried the empire to Maurice and gave it two decades of competent government before the crisis of 602.