By 268 CE, the Roman Empire was running on borrowed time. Fifty years of near-constant civil war, plague, invasion, and economic collapse had left the empire fractured into three competing pieces — and the man who would put it back together had not yet risen above the rank of a frontier cavalry commander.

The Crisis of the Third Century is the name historians give to the half-century between roughly 235 and 284 CE, when Roman imperial authority came closer to permanent collapse than at any point before the empire's final fall two centuries later. The period opened with the murder of the last Severan emperor, Alexander Severus, by his own troops in 235 CE. What followed was a cascade of catastrophes, each feeding the next. Military commanders seized power, ruled for months or a few years, and were killed — usually by the same soldiers who had raised them up. Historians count somewhere between twenty and fifty emperors, co-emperors, and usurpers in this period depending on how they tally disputed claimants; the accepted candidates alone number more than twenty in five decades. These rulers are sometimes called barracks emperors, a phrase that captures their defining characteristic: they rose from the barracks, not from the Senate, and they held power only as long as their legions believed in them.

The military chaos was compounded by two simultaneous external pressures. On the northern and eastern frontiers, reorganized barbarian confederacies — the Goths along the Danube, the Alamanni along the Rhine and upper Danube, the Franks further west — began mounting sustained raids and occupations rather than the hit-and-run incursions of earlier centuries. At the same time, the Sassanid Persian Empire replaced the weaker Parthian kingdom in the East and proved a far more capable opponent. In 260 CE, the Sassanid king Shapur I captured the emperor Valerian at the Battle of Edessa — the single most humiliating event in Rome's history to that point, a reigning emperor taken prisoner in the field and never ransomed home.

The Plague of Cyprian, which struck roughly 249–262 CE, added biological catastrophe to military and political disaster. Named for the bishop of Carthage who described it, the plague (its exact pathogen remains debated by modern historians) killed an estimated five thousand people a day in Rome at its peak, devastated the tax base, and stripped frontier garrisons of manpower at precisely the moment they needed it most. Population decline meant fewer soldiers, fewer taxpayers, less revenue for the army, and therefore less ability to stop the invasions that generated more population decline. The feedback loop was vicious.