Diocletian and the Tetrarchy (293): Rule of Four Emperors

The Tetrarchy, or “rule of four”, was the system of government created by the Roman emperor Diocletian, completed on 1 March AD 293 when two junior emperors were appointed alongside two senior ones. Designed to end half a century of civil war and frontier crises, it gave the Roman Empire four rulers working as a team. Together with Diocletian’s sweeping reforms of the army, provinces and taxes, it reshaped the late Roman state for centuries, even though its plan for succession collapsed within a few years.

Background: an empire in crisis

Between 235 and 284 the empire went through what historians call the Crisis of the Third Century. More than twenty emperors were recognised in Rome in about fifty years, most of them killed by their own troops. Persians, Goths and Germanic peoples raided across the frontiers, the silver coinage collapsed and, for a time, breakaway states ruled Gaul and much of the East.

One lesson of the crisis was that a single emperor could not be everywhere at once. When he fought on the Danube, the Rhine or Euphrates armies often proclaimed their own commander. Usurpers appeared wherever the emperor was absent.

Diocletian’s rise

Diocletian was born in Dalmatia around 244, of humble origins, and rose through the army. In November 284, after the mysterious death of the emperor Numerian, the eastern army proclaimed him emperor near Nicomedia (modern İzmit in Turkey). In 285 he defeated Numerian’s brother Carinus in a battle near the river Margus in the Balkans, and became sole ruler.

Almost at once he shared power. In 285 he made his fellow officer Maximian a Caesar and sent him to deal with unrest in Gaul; in 286 Maximian was raised to Augustus. The two men took religious titles: Diocletian was Jovius, protected by Jupiter, and Maximian was Herculius, linked to Hercules, the god’s helper. Diocletian clearly remained the senior partner.

Creating the Tetrarchy in 293

Two Augusti were still not enough. A naval commander, Carausius, had seized Britain and part of northern Gaul in 286, and the Persian and Danube frontiers remained restless. On 1 March 293 two Caesars were appointed: Constantius, known as Chlorus, under Maximian in the West, and Galerius under Diocletian in the East. Some scholars date Galerius’s appointment a few weeks later, but the arrangement was in place by spring 293.

Each Caesar was tied to his Augustus by adoption and marriage. Constantius married Theodora, Maximian’s stepdaughter (some sources call her his daughter), and Galerius married Diocletian’s daughter Valeria. The intention was that the Caesars would one day become Augusti and choose new Caesars in turn, so that power passed to proven soldiers rather than to sons by birth.

The four rulers based themselves near the frontiers rather than in Rome. Diocletian usually resided at Nicomedia, Maximian at Milan, Constantius at Trier on the Moselle and Galerius at Sirmium on the Danube and later at Thessaloniki. Rome kept its prestige and its Senate, but it was no longer where the empire was governed.

Military successes

The new team quickly proved its value. Constantius recovered Britain in 296, ending the breakaway regime of Carausius’s successor Allectus. Maximian campaigned in Spain and North Africa, and Diocletian crushed a revolt in Egypt in 297–298 after a siege of Alexandria lasting several months.

In the East, Galerius was defeated by the Persian king Narseh in 297, but in 298 he won a crushing victory in Armenia and captured the king’s family. The resulting Peace of Nisibis, concluded in 298 or 299, gave Rome control of territories beyond the Tigris and a frontier that held for more than sixty years. Galerius celebrated the triumph on his arch at Thessaloniki, which still stands.

Diocletian’s reforms

The Tetrarchy was part of a broader rebuilding of the Roman state.

• Provinces: The roughly 50 provinces were split into around 100 smaller ones, grouped into 12 larger units called dioceses. Smaller provinces made governors easier to control and taxes easier to collect.
• Civil and military power: Provincial governors increasingly lost command of troops, which passed to separate military officers called duces. This made it harder for a single official to launch a rebellion.
• Army: The army was enlarged, perhaps substantially, though modern estimates vary widely. Frontier forts and roads were rebuilt along the Danube, Rhine and eastern frontiers.
• Taxation: A new system assessed land and people in standard units (iugatio and capitatio), with regular censuses, giving the state a predictable budget, collected largely in kind.
• Money and prices: A reformed coinage was introduced around 294, and in 301 the Edict on Maximum Prices tried to fix ceilings for more than a thousand goods, services and wages. It failed in practice, as goods vanished from the markets.

The imperial court also became more formal and remote. Subjects prostrated themselves before the emperor and kissed the hem of his robe. Later historians called this new style of rule the “Dominate”, from dominus, meaning lord, in contrast to the earlier “Principate”, although no Roman used those labels as names for periods.

Religion and the Great Persecution

The tetrarchs presented themselves as guardians of Rome’s traditional gods. In 303 Diocletian, urged on, according to the Christian writer Lactantius, by Galerius, launched the last and most severe persecution of Christians. The Great Persecution was enforced unevenly: harshly in the East, lightly in Constantius’s provinces in Gaul and Britain.

Abdication and collapse

In November 303 Diocletian celebrated twenty years of rule in Rome. Soon afterwards he fell seriously ill. On 1 May 305, in an act almost unheard of for a Roman emperor, he abdicated at Nicomedia, while Maximian, reluctantly, did the same at Milan.

Constantius and Galerius became Augusti, and two new Caesars were named: Severus in the West and Maximinus Daia, Galerius’s nephew, in the East. Passed over were Constantine, son of Constantius, and Maxentius, son of Maximian. This decision undid the system.

When Constantius died at York on 25 July 306, his army proclaimed Constantine emperor. In October 306 Maxentius seized power in Rome. A conference at Carnuntum on the Danube in 308, attended by the retired Diocletian, tried to restore order and named Licinius as Augustus, but at times six men claimed the imperial title. Diocletian himself refused to return to power and remained in his palace at Split, where he died around 311 or 312; the exact year is disputed.

The civil wars ended only with Constantine’s victories, first at the Battle of the Milvian Bridge in 312 and finally over Licinius in 324.

Myths and facts

Myth: Diocletian split the empire into four states. The empire stayed legally one. Laws, coins and inscriptions named all four rulers together, and the regions were zones of responsibility, not separate countries.

Myth: the Tetrarchy was a total failure. Its succession scheme failed, but most of Diocletian’s administrative, military and fiscal reforms survived and were extended by Constantine. The empire of the fourth century was in many ways Diocletian’s creation.

Fact: the famous statue was moved. The porphyry group of the four tetrarchs in Venice, showing them embracing in pairs as a sign of harmony, was probably made in Egypt around 300. It stood in Constantinople until crusaders took it to Venice after 1204; a fragment of its base, including a missing foot, was found in Istanbul in the twentieth century.

Timeline

• 284, 20 November: Diocletian is proclaimed emperor near Nicomedia.
• 285: He defeats Carinus near the river Margus and appoints Maximian as Caesar.
• 286: Maximian becomes Augustus; Carausius seizes Britain.
• 293, 1 March: Constantius and Galerius are appointed Caesars, completing the Tetrarchy.
• 296: Constantius recovers Britain.
• 298–299: Galerius defeats Narseh; the Peace of Nisibis follows.
• 301: Edict on Maximum Prices is issued.
• 303, February: The Great Persecution begins.
• 305, 1 May: Diocletian and Maximian abdicate.
• 308, November: Conference of Carnuntum attempts to restore the system.

Why it matters

The Tetrarchy pulled the Roman Empire out of its third-century spiral and laid the foundations of the late Roman state: smaller provinces, a separate military command, a regular tax system and a court centred on the emperor rather than on Rome. Its failure to manage succession opened the way for Constantine, whose reign transformed the empire’s religion and created a new eastern capital.