Gaius Aurelius Valerius Diocletianus (245?–312?), born Diocles, was Roman Emperor from November 20 284 to May 1 305. Diocletian brought to an end the period popularly known to historians as the "Crisis of the Third Century" (235–284). He established an autocratic government and was responsible for laying the groundwork for the second phase of the Roman Empire, which is known variously as the "Dominate" (as opposed to the Principate), the "Tetrarchy", or simply the "Later Roman Empire". Domitian's reforms helped ensure the survival of the Roman imperium, in the East at least, for several centuries.

Background

An Illyrian of low birth (from the province of Dalmatia in today's Croatia), Diocles rose through the ranks to the consulship. He was chosen by the Roman army on November 20 284 to replace Numerian and after the assassination of Carinus in Spring 285 became sole ruler of the Roman Empire. He changed his name to Diocletianus upon his ascension.

Previously, between 235 and 284, there had been some 20 to 25 successive Emperors in a period of about 50 years - an average of a new Emperor every two to three years.

Diocletian seemed at first to be following in the footsteps of his short-lived predecessors in the years between 284 and 298, as he fought a lengthy series of wars from one end of the Empire to the other, maintaining the extended boundaries of the frontiers and stamping out domestic uprisings. By 298, however, Diocletian had succeeded in repelling Germanic intrusions from across the Danube and Rhine, had put a halt to Persian invasions in Syria and Palestine, and had defeated his political rivals within the Empire.

Diocletian's reforms

His position secure, a remarkable feat after over fifty years of internal instability that nearly saw the collapse of the Roman Empire (what has become known as the Crisis of the Third Century), Diocletian believed that going forward under the current system of Roman Imperial government was unsustainable. He initiated a number of reforms to prevent a return to the anarchy of previous generations and maintain the viability of the Empire. These included splitting the Empire into two in order to be more manageable, creating a new system of Imperial succession, ruling as an autocrat and stripping away any remaining facade of republicanism, and economic reforms aimed at the problem of hyperinflation.

The Roman Emperorship, had originally been a military dictatorship, elaborately disguised as a constitutional monarchy. While it drew much of its legitimacy from a complex array of republican titles and practices, it drew most of its actual power from command over the legions and the Praetorian Guard. This is reflected in the most important of all Imperial titles, Imperator (Supreme Commander), from which the word emperor itself is derived. These arrangements, while awkward at times and followed more closely by some emperors than others, worked for the first two centuries of the empire's existence. However, starting with the reign of Septimius Severus, rulers began to strip away or simply ignore many of the republican niceties, and reigned more as dictators than constitutional monarchs. This process undermined the office's foundations and legitimacy. Diocletian, recognized that the title had to be based on something more than simply military force, in order to be more recognized and stable. So he sought to build a new basis for imperial legitimacy in the state religion, with himself as semi-divine monarch and high priest. The old republican title of Pontifex Maximus, would begin to take on a new importance.

Diocletian chose a new title for himself, calling himself Dominus et deus, or "Lord and God". This was in contrast to previous Emperors, who were known as Princeps or "First Citizens", a name which implied some level of equality and democracy, if in name only. Diocletian through his new title removed any such facade, installing himself as a supreme overlord. He was not to be seen in public, and if an audience was required, he had elaborate ceremonies in which the visitor would be required to lie on the ground prostrate and never to look at the Emperor, allowed perhaps to kiss the bottom of his robe. In this way he created a remote, mysterious, theocratic and autocratic office.

The Tetrarchy

Diocletian's experiences during his first nine years of running around the Empire putting out fires brought him to the conclusion that the Empire was simply too big for a single Emperor to rule—that it was not feasible to address barbarian invasions along the Rhine and Egyptian problems at the same time, for example. His radical solution was to split the Empire in two, drawing a line straight down the middle of the map with the axis just east of Rome into eastern and western halves. While this division did not last in the short term, it did eventually become permanent.

The question of Imperial succession had never been solved in the Roman system; there was no clear principle of succession, which often led to civil wars. Prior Emperors had preferred the system of adoption, whereby they would adopt a son in order to be the chosen heir. The military did not like the system of adoption and preferred biological succession, with the emperor's son being the rightful heir. The Senate believed they should have the right to elect a new Emperor. Thus there were usually at least three, if not many more, rightful heirs of succession.

In order to solve the problem of succession, and to answer the question of who would be Emperor of the newly divided East and West, Diocletian created what has become known as the system of "Tetrarchy", or "rule of four", whereby a senior emperor would rule in the East and West, and each would have a junior emperor. Among the many titles traditionally bestowed on Roman emperors, the most important was that of Augustus and therefore only the two senior emperors took this title, with the junior emperors receiving the lesser title of Caesar. Diocletian intended that when the senior emperor retired or died, the Caesar would take his place and choose a new junior emperor Caesar, thus solving the problem of succession.

By 292 Diocletian had the system in place and chose the Eastern Empire for himself and gave Maximian the Western Empire. The imperial power was now divided between two people. The two men established separate capitals, neither of which was at Rome. The ancient capital was too far removed from the places where the empire's fate was decided by force of arms. While improving the ability of the two emperors to rule the empire, the division of power further marginalized the Senate, which remained in Rome. In 293, Diocletian and Maximian each appointed a Caesar (Galerius and Constantius, respectively), formally adopting them as their heirs. However, these were not merely successors - each was given authority over roughly a quarter of the Empire.

Considering that during the half-century preceding Diocletian's ascension the Empire had been in a constant state of simmering civil war, it is remarkable that the Tetrarchy did not immediately fall apart due to the greed of any one of the four emperors. However, the opportunistic nature of Roman Imperial politics soon brought about the disintegration of the Tetrarchy and the reinstitution of one-man rule. When in 305, Diocletian retired (and his western counterpart was persuaded to do the same), the two Caesars became the senior emperors as designed, but when it came time to choose new Caesars, the military and Senate intervened and brought forward their own candidates. In 306 Constantine started a civil war in the west, which he won in 312, and took the eastern half by 324, thus ruling as a united Empire until his death in 337. However, by 395 the division occurred again and the two halves would never be united again.

Economic reforms

Economically Diocletian made reforms as well. In 301, Diocletian attempted to curb the rampant inflation of the 3rd century, and issued his Edict on Maximum Prices. This Edict fixed prices for over a thousand goods, fixed wages, and threatened the death penalty to merchants who overcharged. It was unable to stop the inflation and was eventually ignored, but it is an important document for an understanding of Roman economics.

Military reforms

Militarily Diocletian divided the army into two major portions: The frontier troops (limitanei or ripenses) and mobile field forces (comitatenses) to provide a reserve. About two-thirds of the army's strength was frontier forces. The remainder was the mobile units which the Augusti and Caesars kept centrally located in their territories. Since they were closer to the centers of power, and therefore more politically dangerous, the mobile troops were better paid than the frontier forces. This proved a cause for resentment and, later on, trouble.

The experience with the vexillatio system lead Diocletian to reduce the legions of the field forces to about 1,000 men each, to assure greater strategic and tactical flexibility without the need for detachments. The legions of the frontier were kept at full strength (4,000-6,000 men). Auxiliary units in both mobile and frontier forces were usually 1,000 men each.

Also under Diocletian, the post of Praetorian prefect was greatly reduced in power. Instead, each Augustus and Caesar had two major military commanders: a Magister militum (master of soldiers) and a Magister Equitum (master of cavalry). This not only divided military responsibilities, thus reducing political dangers, but it also acknowledged the increased importance of cavalry in the Roman army.

Many of the military reforms started by Diocletian were continued by his successors and largely completed under Constantine, who abolished the Praetorian Guard, replacing it with a smaller, more controllable personal bodyguard of about 4,000 men.

In 303, the last and greatest persecution of Christians by the Roman Empire began. In the earlier part of Diocletian's reign, Galerius was more the instigator of such persecution than Diocletian himself. However, in the later part of Diocletian's reign, Diocletian embraced the policy of persecution with unequivocal zeal. This wave of persecution lasted until 311.

Retirement and death

In 305, at the age of fifty-nine, after almost dying from a sickness, Diocletian retired to his palace near the administrative center of Salona on the Adriatic Sea, taking up his beloved hobby of growing cabbages. When solicited at a later date to resume the honours which he had voluntarily resigned, his reply was, "Would but you could see the vegetables planted by my hands at Salona, you would then never think of urging such an attempt." He was the only Roman Emperor to remove himself from office; all of the others either died of natural causes or were removed by force.

Diocletian's Palace later became the seed of modern Split, Croatia.

Dioceses of Diocletian

Diocletian in fiction

• Diocletian is the main character of the novel Numerius, written by V. Martucci (2005)
• The novel Fabiola, or The Church of the Catacombs, written in 1852 by Cardinal Wiseman, takes place during the reign of Diocletian.