Constantine the Great and the Founding of Constantinople

No single person is more responsible for the existence of the Byzantine Empire than Constantine I, and he would not have understood what the phrase meant. He thought of himself as a Roman emperor, he ruled a Roman state, and the city he dedicated on the Bosphorus in 330 was in his own mind a second Rome rather than the capital of something new. What he did, in the space of about twenty-five years, was to attach the Roman imperial office to the Christian church and to move the centre of gravity of the empire eastward. Everything that follows in this site’s history of the empire proceeds from those two decisions.

The evidence for Constantine is unusually good and unusually partisan. Eusebius of Caesarea, who wrote both an Ecclesiastical History and a Life of Constantine, knew the emperor and admired him without reservation; the pagan Zosimus, writing a century and a half later, detested him. Between the two lies a figure who is genuinely hard to read: pious and ruthless, a reformer of coinage and taxation who also executed his eldest son and his wife.

Born	c. 272–273, Naissus (modern Niš, Serbia)
Proclaimed emperor	York, 25 July 306, by his father’s troops
Milvian Bridge	28 October 312
Sole ruler	324, after the defeat of Licinius at Chrysopolis
City dedicated	11 May 330
Died	22 May 337, near Nicomedia

The Road to Sole Rule

The Tetrarchy and Its Collapse

Constantine was born into the system Diocletian had built to stop the third-century civil wars: two senior emperors (Augusti) and two juniors (Caesars), each with a region and an army. His father Constantius I was one of the four. The system depended on orderly succession, and it failed at the first attempt. When Constantius died at York in July 306, his soldiers ignored the rules and acclaimed his son, and within a few years there were as many as six claimants at once.

Constantine spent the next eighteen years eliminating them. He was, by any measure, the best general of the group — fast, willing to take risks with small forces, and unusually good at holding the loyalty of Rhine and Danube troops.

The Milvian Bridge

The decisive battle was fought outside Rome on 28 October 312, against Maxentius, who held Italy and Africa. Maxentius had the larger army and the walls of Rome; he chose to come out and fight with the Tiber at his back, and when his line broke, the pontoon bridge his men were retreating across collapsed. He drowned.

The battle matters less for its tactics than for the story attached to it. Lactantius, writing within a few years, says Constantine was told in a dream to mark his soldiers’ shields with a heavenly sign. Eusebius, writing much later and claiming the emperor told him so himself under oath, describes a vision of a cross of light above the sun with the words In this sign, conquer. The two accounts do not agree on what was seen, when, or where, and scholars have argued about them for four hundred years. What is not in doubt is the consequence: from 312 Constantine treated the Christian God as the source of his victories, and he never stopped.

Licinius

Toleration came first, in the agreement Constantine reached with his eastern colleague Licinius at Milan in 313 — traditionally called the Edict of Milan, though it survives as a letter to provincial governors rather than an edict. It restored confiscated church property and granted freedom of worship to everyone, Christians explicitly included.

The alliance did not last. Constantine and Licinius fought in 316–317 and again in 324, when Constantine won at Adrianople and then, after his son Crispus destroyed the eastern fleet in the Hellespont, at Chrysopolis on the Asian shore opposite Byzantion. Licinius surrendered on a promise of his life and was executed the following year. Constantine was sole ruler of the Roman world for the first time, at about fifty-two.

The Emperor and the Church

Constantine’s relationship with Christianity was practical as much as devotional. He gave the church legal privileges, exempted clergy from civic burdens, funded basilicas at Rome, Jerusalem and Bethlehem, and allowed bishops to act as judges in civil suits. In return he expected a religion that could function as a source of unity, and he was repeatedly annoyed to discover that it could not.

The Donatist dispute in North Africa was the first disappointment. The Arian controversy was far worse, and it drew him into summoning the Council of Nicaea in 325, where he presided over the assembled bishops as a layman, pressed for a formula that would end the argument, and left believing the matter settled. It was not settled; his own sons would take opposite sides. The pattern established here — an emperor convening councils, enforcing their decisions, and being frustrated by theologians — runs through the whole of the Orthodox Church’s Byzantine history.

He was himself baptised only on his deathbed, by Eusebius of Nicomedia, a bishop of Arian sympathies. Deathbed baptism was common practice in the fourth century and implies nothing about the sincerity of his belief; it was a way of dying with a clean slate after a career that necessarily involved killing.

The New City

Why the Bosphorus

The choice of site was strategic before it was symbolic. The Greek colony of Byzantion, founded by settlers from Megara in the seventh century BC, sat on a triangular promontory where the Bosphorus meets the Sea of Marmara. It had deep, sheltered water in the inlet called the Golden Horn; it controlled the strait between the Black Sea and the Mediterranean and the land route between Europe and Asia; and it could be walled across its landward side in a few kilometres.

The two fronts that mattered in the fourth century were the Danube and the Persian border in Mesopotamia. From Byzantion an emperor could reach either in weeks. From Rome he could reach neither quickly, which is why emperors had already been ruling from Milan, Trier, Sirmium and Nicomedia for decades. Constantine did not abandon Rome for the Bosphorus out of contempt; he formalised something that had been happening for a generation.

The Building

Work began in 324, immediately after Chrysopolis, and the dedication took place on 11 May 330 — a date the city kept as its birthday for more than a thousand years. The new walls enclosed an area perhaps four or five times that of old Byzantion. Inside went the standard equipment of a capital: an enlarged Hippodrome adjoining a new imperial palace, a Senate house, baths, porticoed avenues, and a circular Forum of Constantine with a porphyry column carrying the emperor’s statue. The column still stands in Istanbul, burnt and banded with iron, known as Çemberlitaş.

Constantine stripped the Greek world of statuary to furnish it. The bronze Serpent Column from Delphi, cast to commemorate the Greek victory over Persia at Plataea in 479 BC, was brought to the Hippodrome and is there still. Jerome’s contemporary complaint was that Constantinople was dedicated “by stripping almost all other cities bare.”

To fill it he offered free grain from Egypt, land grants, and senatorial rank to families willing to move. The population grew fast enough that Constantius II had to expand the water supply, and by the early fifth century the city had outgrown Constantine’s walls entirely, which is why Theodosius II’s prefect built the land walls about a mile and a half further out.

New Rome

The title New Rome appears in the third canon of the Council of Constantinople of 381, which gave the city’s bishop precedence immediately after the bishop of Rome “because it is New Rome.” That single clause caused a millennium of trouble; it is one of the roots of the estrangement between the Greek and Latin churches, since Rome’s claim rested on Peter rather than on the presence of an emperor.

Constantine’s own city was not, at its foundation, a purely Christian one. Old temples remained in the pre-Constantinian core, and the dedication ceremonies included elements that any pagan would have recognised. The thorough Christianisation of the urban landscape took another century.

What Constantine Actually Changed

Three things outlived him.

A Christian imperial office. After Constantine, with the brief and unsuccessful exception of Julian, every Roman emperor was a Christian, and the emperor’s duty to protect and define orthodoxy became part of the job description. Byzantine political theology — the emperor as God’s viceroy, the earthly order mirroring the heavenly — begins here and was still being asserted in the fifteenth century.

A stable gold coin. The solidus, struck at 72 to the pound of gold, held its weight and purity for seven hundred years and became the reserve currency of the Mediterranean and beyond. The fiscal stability that underwrote the Byzantine state is Constantine’s least romantic and most durable achievement; the story is told under Byzantine coinage.

An eastern capital. When the western provinces fell away in the fifth century, the eastern half had a fortified, well-supplied administrative centre that no invader could take. Constantinople survived sieges by Avars, Arabs, Bulgars and Rus. It fell to the Fourth Crusade in 1204 and to the Ottomans in 1453, and both of those dates are remembered precisely because the city had been thought impregnable for so long.

Where to See Constantine’s City

Very little of the fourth-century fabric survives above ground, which surprises visitors. The Serpent Column and the Egyptian obelisk on the Hippodrome spine, the burnt porphyry column, the substructures of the Great Palace with their mosaic floors, and the line of the streets are what remain; everything else is Theodosian, Justinianic, Komnenian, Ottoman or modern. The best way to read the Constantinian city is on a map, following the ceremonial route from the Golden Gate along the Mese to the Forum and the palace — a route that the tenth-century Book of Ceremonies still describes in detail.

The Sources and Their Bias

Constantine is the best-documented Roman emperor before the medieval period and one of the hardest to read, because the two fullest accounts were written by men with opposite commitments.

Eusebius of Caesarea wrote the Ecclesiastical History, revising it repeatedly as Constantine’s position improved, and after the emperor’s death the Life of Constantine — a panegyric that includes documents apparently copied from imperial archives. Papyrus finds in the twentieth century confirmed the authenticity of at least one of these, which raised the credibility of the rest considerably. Eusebius is nonetheless writing hagiography, and his Constantine is a providential instrument rather than a politician.

Lactantius, writing On the Deaths of the Persecutors around 315, is closer to the events and interested in demonstrating that God destroys persecutors. His account of the Milvian Bridge vision — a dream, a sign on the shields — is the earlier version, and it does not match Eusebius’s celestial cross.

Zosimus, a pagan official writing around 500, blamed Constantine for the empire’s decline, attributed his conversion to a search for absolution after the killings of Crispus and Fausta, and is hostile at every point.

Against these stand the non-literary sources, which are the useful ones: the coinage, which shows the gradual disappearance of pagan imagery over two decades rather than a sudden break; the legal texts preserved in the Theodosian Code; the inscriptions; and the buildings. The Arch of Constantine in Rome, dedicated in 315, credits his victory to “the prompting of the divinity” — a formula carefully vague enough to satisfy a Christian and a pagan senate simultaneously.

What Is Still Argued

Four questions remain genuinely open.

Whether the conversion was sincere. The older cynical reading — that Constantine adopted a useful religion — has lost ground. His legislation, his correspondence with bishops, his personal involvement in doctrinal disputes and the sheer expense of his church-building suggest a man who believed something. What he believed, and how orthodox it was, is another matter.

Whether he intended a Christian empire. He did not ban paganism, kept the title pontifex maximus, and continued to use solar imagery on his coins into the 320s. The systematic Christianisation of the state came under his successors.

The killings of 326. Constantine had his eldest son Crispus executed and his wife Fausta killed shortly afterwards, and the sources are evasive about why. The story of an accusation, an affair or a plot has been reconstructed a dozen ways; no version is secure.

Whether Constantinople was meant as a Christian capital. The evidence for pagan elements in the foundation ceremonies is real, and the thoroughgoing Christianisation of the city’s public spaces belongs to the fifth century.

• History of the Byzantine Empire — the full thousand-year narrative
• The Walls of Constantinople — the defences that outlasted the founder’s
• The First Council of Nicaea — the council Constantine summoned
• The Hippodrome of Constantinople — the civic heart of the new capital
• The Great Palace — what stood beside it
• Why “Byzantine”? — the name Constantine’s subjects never used
• The Donation of Constantine — the forgery built on his memory