The Provincial List of Verona
§ 1. The exact measure of the originality of Diocletian's statesmanship has not yet been taken. ‘Like Augustus,’ said Gibbon, ‘Diocletian may be considered the founder of a new empire’ and these words express the accepted view. In the whole work of pulling the Empire together, which went on from A.D. 270 to 330, the three outstanding actors were Aurelian, Diocletian, and Constantine, and the part played by Aurelian was indispensable for the restitutio orbis. It was he who destroyed the Principate, notwithstanding the negligible episode of Tacitus. It was he who founded the autocracy; Diocletian who regularized and systematized it. Two new things Diocletian certainly did, one of which was a success and the other a failure though not a fruitless one. His division of the Empire into Dioceses was permanent for nearly three hundred years. His throne system led to disaster and disappeared; yet the territorial quadripartition which it involved was afterwards stereotyped in the four Prefectures, and Nicomedia pointed to Constantinople. But in many of the other changes which distinguished the Empire of Constantine from the Empire of Severus and which have generally been regarded as inventions of Diocletian, it is becoming clear that he was not the initiator but was only extending and systematizing changes which had already been begun. The separation of civil from military powers in provincial government had been initiated by Gallienus (the importance of whose reign has in recent years been emerging). Some of the characteristics which mark the military organization of the fourth century had come before Diocletian's accession. Mr. Mattingly's studies in the numismatic history of the third century have been leading him, as he tells us, to similar conclusions.