Diocletian and the Tetrarchy: Saving Rome from Collapse

When the soldier Diocles, the son of a freedman of Dalmatia, was proclaimed emperor by his troops in 284 CE, the Roman Empire was on the brink of dissolution. In the half-century since Marcus Aurelius died in 180 CE, the empire had seen at least twenty-six emperors, most of them army generals raised by their troops and killed by their rivals. The provinces of Gaul, Britain, and Palmyra had each broken away under their own imperial claimants. The empire had been sacked by Goths. A devastating plague had killed perhaps a third of the population. The currency had been debased almost to worthlessness. The army and the bureaucracy were at war with each other. And the old senatorial aristocracy, which had governed the empire through the long peace of the second century, was exhausted.

The soldier-emperor who took the purple in 284 CE was Diocletian (Gaius Aurelius Valerius Diocletianus, 244 – 311 CE). He would rule for twenty years, defeat every rival, reorganize the entire imperial state, and then do something no Roman emperor had ever done: he would voluntarily abdicate. By the time he died, he had transformed the Roman Empire from a Mediterranean commonwealth into an autocratic state that would last, in the east, for another thousand years. For a broader survey of all the emperors, see Roman Emperors. For the crisis that preceded him, see the Crisis of the Third Century.

The Rise of Diocletian

Diocles was a career soldier from the province of Dalmatia, on the eastern Adriatic coast. He served as a member of the protectores domestici, the imperial bodyguard, and rose through the ranks. When the emperor Numerian, the son of the great Carus, was found dead in his tent in 284 CE, the army at Nicomedia proclaimed Diocles as his successor. Within months he had defeated and killed Carinus, Carus’s surviving son, at the Battle of the Margus.

Diocletian — the name he took upon becoming emperor — recognized immediately that a single emperor could not defend an empire that stretched from the Scottish Highlands to the Syrian desert, with a hostile Persia to the east and restless Germanic tribes on the Rhine and the Danube. He made a series of appointments that built a system of joint rule, the Tetrarchy, that would last as long as he did. For the background of the crisis that made this necessary, see the Crisis of the Third Century.

The Tetrarchy

The Tetrarchy — literally “rule of four” — was Diocletian’s solution to the problem of governing an overextended empire. In 285 CE he appointed his friend Maximian as Caesar, and a year later as Augustus, with the understanding that the two would rule jointly, Diocletian taking the eastern half and Maximian the western. Each emperor was to have a junior partner, a Caesar, who would marry the senior emperor’s daughter and succeed him. In 293 CE Diocletian appointed Galerius as his Caesar, and Maximian appointed Constantius Chlorus as his. The four were to rule together, two Augusti and two Caesars, with the Augusti retiring after twenty years and the Caesars becoming the new Augusti.

Each of the four tetrarchs had a capital. Diocletian ruled from Nicomedia in Asia Minor, Maximian from Milan in northern Italy, Galerius from Sirmium on the Danube, and Constantius from Trier in Gaul. The system was designed to be flexible and to put an emperor within reach of every threatened frontier. It worked remarkably well during Diocletian’s lifetime. The breakaway empire of Postumus in Gaul was crushed. The Palmyrene Empire of Zenobia in the east was reconquered by Aurelian. A great invasion of the empire across the Rhine was repelled. The Sassanid Persians, the new and powerful enemy of Rome in the east, were driven back across the Tigris. By 298 CE Diocletian had ended the worst of the crisis, and the empire was once again a single state under four co-emperors.

The Tetrarchy was more than a military command arrangement. It was a complete reorganization of the imperial office. Diocletian transformed the emperor from a princeps, a “first citizen” of the senatorial class, into a dominus, an absolute monarch on the Persian model. He required his subjects to prostrate themselves in his presence, gave himself the elaborate Persian court title Jovius — “the Jovian,” the earthly representative of Jupiter — and dressed in purple silk and gold. The Senate of Rome, which had been the most prestigious political body in the empire since the Republic, was reduced to the government of a single city. Real political power passed to the four imperial courts and the bureaucracy that served them.

The Administrative Reforms

The Tetrarchy required an empire reorganized to match it. Diocletian expanded the imperial bureaucracy to something like four or five times its previous size, dividing the old provinces into smaller units, each governed by a separate official answerable to the emperor. He separated civil and military command, so that no general could ever again raise his own legions and march on Rome. He reformed the tax system, basing taxation on the capitatio-iugatio, a census of land and people that was meant to be revised every fifteen years. He reformed the currency, issuing a new gold coin, the aureus’s later replacement, and a new silver coin of high purity, the argenteus, that for a generation restored confidence in Roman money.

The army was expanded to perhaps six hundred thousand men, organized into smaller field units, the comitatenses, and frontier units, the limitanei, under a separate chain of command. The mobile field army was meant to respond to any invasion; the frontier units were meant to hold the line until the field army arrived. The system was expensive. The tax burden on the empire increased dramatically, and the imperial estates and the municipal councils were forced to make up the difference. The consequence was the gradual decay of the old municipal governing class and the rise of a new imperial aristocracy of service.

The Edict on Maximum Prices (301 CE)

The most famous of Diocletian’s economic reforms, and the most spectacular failure, was the Edict on Maximum Prices, issued in 301 CE. The edict fixed the maximum price at which more than a thousand different goods and services could be sold in the empire, with the death penalty for violators. Diocletian believed that inflation was the result of greed, and that the empire would prosper if merchants were not allowed to charge unreasonable prices.

The edict was a disaster. Goods disappeared from the markets rather than be sold below the cost of production, and within a few years the edict had to be quietly abandoned. The price of grain, in particular, skyrocketed. Diocletian’s attempt at wage-and-price control is the first well-attested example of inflation in European history, and it set a precedent that medieval and early modern governments would repeat for centuries.

The Great Persecution

In 303 CE, in the eighteenth year of his reign, Diocletian launched the last and most severe of the Roman persecutions of the Christians. The Great Persecution began with the burning of the church in Nicomedia and a series of four imperial edicts requiring that Christian churches be destroyed, that Christian scriptures be confiscated and burned, that Christian clergy be imprisoned and forced to sacrifice, and that all Christians be required to sacrifice to the gods on pain of death.

The persecution was not uniform. It was applied most harshly in the eastern provinces, particularly in Palestine, Egypt, and the Balkans, and most lightly in the west, where Constantius Chlorus made a show of destroying a few churches while leaving the Christian community essentially undisturbed. The persecution produced a generation of Christian martyrs, including the British saints Alban and Julius and Aaron of Caerleon, and the Spanish saint Vincent of Saragossa. The edict was never formally repealed; it was simply allowed to lapse when Constantius’s son Constantine the Great became emperor in 306 CE.

Why Diocletian launched the persecution is still debated. The emperor’s chief priest, the Pontifex Maximus, had been his official title since 286 CE, and Christians were refusing to sacrifice to the gods on whom the prosperity of the empire was understood to depend. The edict of 303 CE may have been a sincere attempt to restore the religious foundations of the Roman state. Whatever his motivation, the persecution did not save the old religion. It ended with Diocletian’s death in 311 CE, and within two years Constantine would legalize Christianity with the Edict of Milan.

The Division of the Empire

In 285 CE, when he appointed Maximian, Diocletian reorganized the empire into two administrative halves. The eastern half, with its capital at Nicomedia, included the wealthier and more populous provinces of the Balkans, Asia Minor, Syria, Palestine, and Egypt. The western half, with its capital first at Milan and later at Ravenna, included Italy, Gaul, Spain, Britain, and the African provinces. The division was meant to be administrative, not permanent. But it proved to be the most enduring institutional change of the entire imperial period.

The eastern half was, in resources, larger than the western half, and the contrast between the two would grow over the following centuries. By 400 CE the western empire was unable to pay for its own defense, while the eastern empire was still the richest state in the Mediterranean. The division of the empire in 285 CE set the stage for the survival of the east and the fall of the Western Roman Empire two centuries later.

The Abdication of Diocletian

On 1 May 305 CE, in a ceremony at Nicomedia, Diocletian abdicated the purple. He had been emperor for twenty years, and by his own system he had been required to retire. He forced Maximian to abdicate on the same day. The two Augusti were replaced by their Caesars, Galerius and Constantius, who were expected to appoint their own Caesars in turn.

Diocletian retired to his palace at Spalatum (modern Split, in Croatia), a huge fortified residence that had been built for him on the Adriatic coast. He lived there as a private citizen for the remaining six years of his life, tending his cabbages and refusing, with one famous exception, to re-enter public life. The exception was the conference at Carnuntum in 308 CE, when the collapse of the succession system was already in view. Diocletian, called out of retirement, persuaded Maximian to abandon his second attempt at the throne. He then returned to Spalatum and lived quietly until his death in 311 or 312 CE.

The palace of Diocletian at Split is one of the best-preserved Roman imperial residences, and roughly half the modern city of Split is built inside its walls. Its octagonal mausoleum, the Cathedral of Saint Domnius, still stands in the city center.

The Crisis of Succession

Diocletian’s abdication broke his own system. The new Augusti, Galerius and Constantius, were supposed to appoint their own Caesars, but the army on the Rhine, in the camp of Constantius at York, simply proclaimed his son Constantine as Augustus in 306 CE, ignoring Galerius’s wishes. Galerius accepted, reluctantly. Maximian’s son Maxentius was proclaimed in Rome. A son of Maximian, also called Maxentius, was proclaimed in Africa. By 308 CE, with the conference at Carnuntum, the Tetrarchy had collapsed into a free-for-all, and it took another twenty years of civil war — including the battles at the Milvian Bridge and Chrysopolis — for Constantine to emerge as the sole Augustus in 324 CE.

Diocletian had intended a system of orderly succession. He got, instead, a return to the bloody succession wars of the third century, with the twist that one of the warring factions was now Christian. Constantine’s victory at the Milvian Bridge in 312 CE, where he fought under the labarum of Christ, marked the beginning of the Christian Roman Empire and the end of the pagan empire Diocletian had tried to save.

The Diocletianic Legacy

Diocletian transformed the Roman state. He expanded the army, the bureaucracy, and the tax base. He reorganized the provinces. He separated civil and military command. He turned the emperor from a first citizen into an absolute monarch. He divided the empire into eastern and western halves. He created the system of imperial ceremonies, the sacra auditorium, the elaborate rituals of the imperial court, that would outlast him by a thousand years.

Many of these reforms would not have been possible without the crisis of the third century, and many of them made the empire harder to govern. The bureaucracy that Diocletian expanded would, in time, become the salaried officialdom that made the late empire possible but slow. The court ceremonial would make the emperor increasingly remote from his subjects. The religious changes of the fourth century would split the empire along a new fault line between Christian and pagan. The division of the empire into east and west would become permanent.

But the empire Diocletian saved in 284 CE lasted, in the east, until 1453 CE, more than a thousand years after his death. The state of Justinian and the empire of the Comneni were, in a real sense, his creation. He could not save the western empire, and the fall of the West came within two centuries of his death, but the eastern half of the Roman Empire — the Byzantine state that historians call the surviving Roman Empire — was the child of his reforms.