Ancient Rome · CHAPTER 9 ·
Diocletian and Constantine
Chapter 9 of The Rise and Fall of Rome, 264 BC – AD 476: “Diocletian and Constantine”
From 284 Diocletian rebuilt the Roman state: the Tetrarchy of four rulers, about a hundred provinces, separate civil and military commands, and a larger army and tax system. The Tetrarchy collapsed after he retired in 305, and Constantine won the civil wars that followed, including the Battle of the Milvian Bridge in 312.
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The map above is the world of The Roman Empire, AD 285 — The world restored.
SHORT EXPLANATION
In 284 Diocletian inherited an empire repeatedly shaken by invasion and civil war. One ruler could not be present at every threatened frontier, and a vacant throne invited rival armies to put forward their own candidates. His Tetrarchy put two senior and two junior emperors in charge, with succession planned between them. He also enlarged the army, reorganized provinces and taxation, and separated civil from military command. Order returned, but the larger government cost more, and the succession broke down after his retirement.
Constantine emerged from the ensuing wars as the victor. After winning at the Milvian Bridge in 312, he supported Christianity, which Diocletian had tried to destroy. In 330 he dedicated Constantinople on the Bosporus. Its defenses, access to eastern wealth and position near the Danube and Persian fronts gave the empire a strong new center in the east.
The full story
The third-century crisis had exposed two connected problems. Armies needed emperors close enough to respond to danger, yet every army that raised its own candidate threatened to start another civil war. Diocletian’s Tetrarchy gave the empire four legitimate rulers stationed near frontiers: two senior emperors and two junior colleagues intended to succeed them. Government itself acquired a more imposing appearance. From Nicomedia in the Greek east, Diocletian presented the emperor as a sacred, half-divine figure, surrounded by ceremony. Augustus had posed as the first citizen; the new monarchy, often called the Dominate from the Latin for “lord and master,” made the ruler’s exalted position unmistakable.
Diocletian also changed the machinery beneath the throne. He roughly doubled the number of provinces to around a hundred, allowing closer supervision, and separated civilian governors from military commanders so that a governor had fewer forces with which to rebel. The army grew toward half a million men, with mobile troops held behind the frontier to reinforce threatened areas. Taxes assessed in goods and organized around an annual budget helped supply this force despite debased coinage. These arrangements brought roughly twenty years of stability and victories over Persia. They also left more soldiers and officials to be supported by a tax base that had not grown.
The succession plan faced its test in 305, when Diocletian voluntarily retired and required his senior colleague to do the same. It soon failed. Constantine, an emperor’s son passed over in the arrangement, was acclaimed by his late father’s army and went on to defeat his rivals. At the Milvian Bridge in 312 he won under a sign he later associated with Christianity. Christian worship was legalized in 313. Constantine funded churches and summoned the Council of Nicaea in 325; his motives probably combined sincere belief with political calculation.
In 330 Constantine dedicated Constantinople on the Bosporus. The choice made visible a change already under way: the richer, more urban east supplied revenues more readily than the west and lay close to the Danube and Persian fronts. Sea routes and strong defenses helped protect the new capital. Rome kept its symbolic standing, but the working center of government moved toward the resources and dangers that occupied its rulers. The heavier state had helped restore the empire; its eastern half would have a stronger foundation for sustaining it.
FULL EXPLANATION
Government after the crisis
The reforms form a connected answer to the third century. Several rulers could attend to distant frontiers; smaller provinces and separate civil and military offices made rebellion harder for any one governor. Taxes on land, people and goods supplied the state when debased silver could not be relied upon, and mobile field armies offered a reserve behind the frontier. Each measure addressed a weakness the crisis had exposed. The Tetrarchy, however, depended on soldiers accepting appointed successors. Family loyalty remained powerful, as Constantine’s acclamation by his father’s army showed. The new institutions also cost more without making every province richer. The urban east could support them more easily than the west. A common solution to imperial disorder thus imposed an uneven burden.
Constantine’s belief and its evidence
The accounts of Constantine’s experience before battle do not agree. Lactantius describes a dream on the previous night; Eusebius, writing later and citing Constantine’s own testimony, tells of a cross of light in the sky. The experience itself cannot be independently verified. Constantine’s subsequent policy is easier to follow. He gave Christianity property, legal privileges and great churches, although he retained some traditional practices and postponed baptism until he was dying. Belief and political advantage could work together. Bishops, councils and charitable networks offered an emperor organized partners throughout the empire; imperial patronage in turn brought him into their doctrinal disputes. At Nicaea imperial power helped bishops define orthodoxy and gave their decisions state backing. The church gained wealth and jurisdiction, but also a dangerous closeness to coercive power.
Key places on the map
On this day
Why it happened
Answers to the third century. The repeated usurpations and failures of the third century shaped Diocletian’s choices. Four rulers addressed the struggle for a single throne; separate civil and military offices limited the power of governor-generals. Taxes in goods answered debased coinage, while sacred ceremony raised the dignity of an office whose holders had so often been killed. These measures were designed for dangers already experienced. Barbarian settlement and the west’s thinner economy would present further tests.
Persecution and the church. From 303 to 311 the Great Persecution sought to break Christianity. Authorities burned scriptures, imprisoned clergy and required Christians to sacrifice. Yet Christians were connected across communities, held official posts and included people willing to die for their faith. The campaign failed; an eastern edict ended persecution in 311. Constantine’s support, a decade after the campaign began, brought the emperor into partnership with a network the state had been unable to destroy.
An eastern centre. Constantinople did not create the east’s importance. Diocletian had governed from Nicomedia and visited Rome only once: cities, revenues, recruits and dangerous frontiers had already drawn the court eastward. Constantine gave that orientation a new capital. When eastern and western courts were formally separated in 395, the east had a defensible capital and resources to garrison it; the west retained Rome’s prestige and its exposure. Chapter 11 follows their unequal fortunes.
An organised Christian community. Roman traditional religion joined many local cults to civic life. Christianity connected communities differently. Bishops in cities, councils, letters and charitable work gave the church an infrastructure that crossed social and ethnic divisions. Its universal membership and account of authority under one God also offered a way to relate religious and imperial power. Constantine gained organized partners, and their disputes came with them. Chapter 12 follows the church beyond the western state it joined.
THE TURNING POINT
The Milvian Bridge, 28 October 312. On 28 October 312 Constantine won at the Milvian Bridge under a sign he later linked to Christianity. The sources differ about what he experienced, but his public support for the church continued for the remaining twenty-five years of his rule. Christians were perhaps a tenth of the empire; their proportion in the army is uncertain. Their faith moved from persecution to legal recognition and imperial sponsorship. Later, under Theodosius, imperial law increasingly favored orthodox Christianity and restricted traditional worship. The battle’s consequences lay in the choices its winner and his successors made with power.
What it changed
The cost of government. A larger government had to be paid for. Courts, officials and soldiers depended on taxes and compulsory obligations, including those placed on guilds and tenant farmers, or coloni, tied to estates. A contemporary pamphleteer complained of “more tax-eaters than taxpayers,” Diocletian’s Price Edict of 301 attempted to control inflation by fixing prices, but a prescribed price could not ensure that goods would be offered at it. Did the heavier state preserve the empire or damage the economy and loyalty it needed? Chapter 11 returns to the question in the west.
Church and imperial power. By sponsoring the church, emperors became involved in its disputes: Constantine called bishops to Nicaea to address one of them. Church leaders could also challenge imperial conduct. Within eighty years Bishop Ambrose required Theodosius to perform public penance after Thessalonica. The alliance made cooperation possible, while giving religious and political authorities reasons to contest one another. Chapter 12 considers that relationship’s longer history.
An eastern capital. Constantinople acquired its own court, senate, mint and grain supply, including grain redirected from Egypt. The eastern government could therefore operate from a center less dependent on Rome. Compare its foundation in 330 for a united empire with the formal division in 395 and the end of the western imperial line in 476. The eastern empire survived until 1453. The capital was only part of that difference, but its defensible position and resources helped the eastern court govern and defend itself.
DISCUSSION QUESTION — Did the larger state created under Diocletian and Constantine weaken the empire it helped preserve?
The reformed state could defend the empire: after 284 its frontiers held for a century, Persia was repeatedly defeated, and the east retained the apparatus for centuries longer. Those results give the case for paying for a larger government its force. The costs matter too. Sources describe tax flight, local elites trying to escape council duties and burdens on farmers. Salvian later reported people preferring barbarian rulers to Roman exactions; his complaint cannot establish what every taxpayer thought. The east’s greater wealth made the demands easier to sustain there than in the west. We must weigh the protection against its cost, and ask whether a lighter government could have provided comparable defense.
An interesting fact
Diocletian’s fortified retirement palace on the Dalmatian coast became part of a living city. In the seventh century refugees from nearby Salona settled within its walls, helping to form Split, Croatia. Roughly 3,000 people still live in the palace complex, with laundry hanging between imperial columns. His mausoleum became Split’s cathedral, dedicated to a bishop martyred during his persecution of Christians. The emperor’s body vanished from history.
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