The Third-Century Crisis and the Reforms of Diocletian and Constantine

Between 235 and 284 CE, Rome endured a period of military anarchy that began with the assassination of Emperor Alexander Severus. Over the following five decades, more than twenty emperors rose and fell, most of them violently deposed, and this relentless turnover steadily eroded the authority of the Senate against the very real power now wielded by the provincial armies. The frontiers came under mounting pressure from Germanic peoples such as the Alamanni, Goths, and Franks along the Rhine and Danube, while the Sasanian Empire threatened the east so persistently that it managed to capture Emperor Valerian himself in 260 CE. The strain briefly fractured the empire territorially: the breakaway Gallic Empire under Postumus lasted from 260 to 274 CE, while in the east Zenobia ruled an independent Palmyrene Empire from 270 to 273 CE. Economically, the crisis brought continuous devaluation of the silver coinage, runaway inflation, and a partial retreat into barter, alongside rural depopulation and a contraction of interprovincial trade.