In The Fate of Rome: Climate, Disease, and the End of an Empire Kyle Harper argues that the Plague of Cyprian, between 249 to 262 A.D., served as a massive exogenous shock to the Roman Empire that changed history. Harper observes that the structures of Roman society were reordered in the face of near collapse and exhaustion due to the onslaught of disease. The Plague of Cyprian, at least in Harper’s telling, plays a major role in the rise of Christianity and the fading away of the traditional religion (more through the inability of the old pagan institutions to persist in the face of social instability as opposed to a crisis of faith).
But the change was more than cultural. It is well known that Augustus, the first of what we call Roman Emperors, styled himself Princeps, and maintained the external fiction that he restored the republic. The term Imperator was not applied regularly to Roman Emperors until the reign of Vespasian, in the last quarter of the 1st century A.D., nearly a century after Augustus came to power. But even then the rulers of the Roman world maintained a conceit and fiction that they were scions of the old republican world, the first among the aristocrats. This was certainly true of Marcus Aurelius, who famously styled himself something of a philosopher-king as well.
After the disastrous reign of Marcus Aurelius’ son, Commodus, the dynasty founded by Septimius Severus moved in a more nakedly autocratic direction. Severus notably presented laws to the Senate as expressions of his fiat will. But Severus was from the old aristocracy of Rome. He underwent the cursus honorum under the Antonines.
The true shift came during the late 3rd-century and the rise of the Tetrarchs. These military rulers, who came out of the barracks of the Illyrian legions, ushered in the Dominate. This is the despotic later phase of the Roman Empire and derives from the fact that Diocletian added dominus, lord or master, to one of his titles. Diocletian and his successors did not see the need for the pretense that their world was that of the Republic. It was fundamentally different. They accrued to themselves the powers and styles of despotic eastern rulers.
Why? The shock of the Plague of Cyprian induced instability in the Roman world, which a powerful ruler stabilized. But according to Peter Heather in The Fall of the Roman Empire the Romans were reacting to the emergence of the Sassanians, who had reconfigured Persia to be a more formidable rival to Rome.* The irony here is that just the Persians became the great enemy of Rome, the Emperors of Rome began to resemble their eastern rivals in their external form and internal self-identity.
* Adrian Goldsworthy disagrees that Sassanian Persia was so formidable, ascribing the military parity more to Roman decay than the rise of Iran.