Originally named Dioclēs. Of obscure origins, b. in Dalmatia perhaps in the early 240s ad, he rose to command the bodyguard of the emperor Numerianus on the Persian campaign of 283/4. When Numerianus was killed by his praetorian prefect Aper, the army proclaimed Diocles Augustus at Nicomedia; he killed Aper. Before long Diocletian was sole emperor. Visiting Italy, he proclaimed his comrade‐in‐arms Maximian as Caesar. Maximian was soon made Augustus (286) and spent the next years defending Gaul. Diocletian spent most of his reign on the Danube (see danuvius) or in the east.
In 293 Diocletian established the ‘tetrarchy’. To the two Augusti, now known as Iovius and Herculius respectively to emphasize their quasi‐divine authority, were added Caesars, Constantius I and Galerius; they were adopted into the Jovian or Herculian houses by the marriage of Galerius to Diocletian's daughter Valeria and of Constantius to Maximian's (?step‐)daughter. The arrangement would provide an imperial presence in different areas; it might deter usurpers; and the Caesars might become acceptable to the armies and live to succeed as Augusti. To raise the dignity of the imperial office Diocletian adopted an oriental court ceremonial and seclusion. Each tetrarch had his own staff, and was often on the move in his territory. In practice the empire was divided into two; Maximian and Constantius ruled the west, Diocletian and Galerius the east. After five years of sucessful fighting there was a lull in rebellions and wars; tetrarchic authority was secure.
Diocletian pursued systematically a long‐established policy of dividing provinces into smaller units; by 314 there were about 100, twice the number of a century earlier (see provincia). The purpose was to ensure closer supervision, esp. over law and finance, by governors and their numerous staffs; critics saw it as leading to never‐ending condemnations and confiscations. In the later part of his reign, Diocletian began an important reform, separating military from civil power in frontier provinces. Senators remained excluded from military commands. His conception of defence was conservative; he made little or no effort to increase the size of the élite field army, which had been formed in the late 3rd cent. But a huge programme of building and reconstruction of defensive works was undertaken on all frontiers, and they were to be held by sheer force of numbers; the size of the Roman army was perhaps nearly doubled.
The army and the increase of administrative personnel were a heavy financial burden. Diocletian reformed the system of taxation to take inflation into account and to regularize exactions in kind. Most revenue and expenditure was now in kind. By the Currency Edict (301) Diocletian attempted to create a unified currency, doubling the value of at least some coins, but he could not establish confidence in this revaluation. Late in 301 he tried to halt inflation by the Price Edict. In great detail it fixed maximum prices and wages; despite savage penalties it became a dead letter, as goods disappeared from the market.
Many legal decisions show Diocletian's concern to maintain or resuscitate Roman law in the provinces. He was an enthusiast for what he understood of Roman tradition and discipline, to reinforce imperial unity. This policy forms the backdrop to the persecution of Christians, undertaken possibly on the insistence of Galerius. (See christianity.) Earlier attempts had been made to purge the court and the army, but the first persecuting edict was designed to prevent the Church from functioning, by requiring the burning of Scriptures and the demolition of churches, and the banning of meetings for worship; recusants were deprived of any rank, and thus made liable to torture and summary execution and prevented from taking action in court; imperial freedmen were re‐enslaved. The later edicts were not promulgated outside the areas controlled by Diocletian and Galerius.
Subjects: Classical Studies.