Battle of the Milvian Bridge, AD 312: the battle that made Rome Christian

On a single day in October AD 312, two Roman emperors fought for the city of Rome, and the winner changed the course of Western history. The battle at the Milvian Bridge was not the largest Rome ever saw, but few have echoed longer. It decided who would rule the western empire, and, if the Christian writers are to be believed, it was won under a sign from heaven that would carry a persecuted faith to the throne of the Caesars.³

Background: the Tetrarchy breaks apart

The war of 312 grew out of a system built to prevent wars. In 286 the emperor Diocletian split the empire between himself and a fellow emperor, Maximian, and in 293 each of the two augusti took a junior partner, a caesar. This Tetrarchy, rule by four, was meant to end the civil wars over the succession that had torn the third century apart. In 305 Diocletian and Maximian stepped down in favor of their caesars, Galerius in the East and Constantius in the West.⁴

The arrangement broke within a year. Constantius died at York in July 306, and his troops at once acclaimed his son Constantine as his successor. That autumn in Rome, Maximian's son Maxentius, passed over in the succession, had himself proclaimed with the backing of the Praetorian Guard, the old imperial bodyguard in the city. The senior emperor Galerius sent Severus against him; Severus's soldiers deserted to Maxentius, and Severus was captured and put to death. Galerius then invaded Italy himself and had to retreat when his own legions began to waver.⁵

Family ties only tangled the quarrel further. Maximian came out of retirement, gave his daughter Fausta in marriage to Constantine, tried to overthrow his own son in Rome and then plotted against his son-in-law in Gaul, and died soon after, by suicide or murder. At a conference at Carnuntum the emperors declared Maxentius a usurper and made Licinius emperor in the West, though Maxentius still held Italy and Africa. When Galerius died in 311, the last restraint was gone. Constantine drew Licinius to his side with the promise of his half-sister Constantia in marriage, while Maxentius, blaming Constantine for his father's death, declared war.⁶

Armies and commanders

Constantine was a soldier who had served under Galerius and campaigned with his father in Britain, and he ruled Britain, Gaul and Spain with an army recruited and hardened on the Rhine frontier. Maxentius held Rome itself, Italy and Africa. Lactantius, a Christian writer of the day, says that Maxentius had the larger army: his father's veterans who had deserted Severus, and troops he had raised in Mauritania and Italy.⁷

The numbers are uncertain. Britannica puts Constantine's invading army at nearly 40,000 men. The Catholic Encyclopedia notes estimates for it running from 25,000 to 100,000, against a much larger force for Maxentius. The pagan historian Zosimus, writing nearly two centuries later, gives Constantine 90,000 foot and 8,000 horse and Maxentius 170,000 foot and 18,000 horse, figures no modern estimate accepts.⁸

The battle in phases

Over the Alps

Constantine moved first and fast. Early in 312 he crossed the Alps into Italy, stormed Segusium (Susa), the first town that resisted him, and destroyed a strong Maxentian army near Augusta Taurinorum (Turin). At Verona he found the Praetorian prefect Ruricius Pompeianus with a large army and laid siege to the city; when a relief force arrived, Constantine turned on it with part of his army and beat it, and Verona surrendered. With the north of Italy in his hands, the road to Rome lay open.⁹

Maxentius leaves the walls

Maxentius had meant to sit out the war behind the walls of Rome, and he cut the bridges over the Tiber to slow the invader. But the city grew restless as Constantine came on from the north, and the day of decision fell on the anniversary of Maxentius's accession. Maxentius had the Sibylline books, Rome's ancient prophecies, consulted. They promised that on that day the enemy of the Romans would perish. He took it as a sign and marched out. Zosimus tells the same story of the oracle and adds the obvious irony: the enemy of Rome was Maxentius himself.¹⁰

To cross the river, Maxentius had a temporary bridge thrown over the Tiber near the cut stone Milvian Bridge. Eusebius calls it a bridge of boats; Zosimus says it was built in two parts held together with iron pins, so that it could be pulled apart under the enemy. Maxentius's army crossed and drew up on the far bank, with the river at its back.¹¹

The sign in the sky

This is where the most famous story of the battle belongs, and it survives in two different versions. Lactantius, a Christian writer of the time, wrote that while Constantine's army lay encamped opposite the Milvian Bridge, he was told in a dream to mark the heavenly sign of God on his soldiers' shields, and that the troops went into battle bearing the Chi-Rho, the monogram of Christ. Eusebius of Caesarea, writing after the emperor's death and repeating what he said Constantine had told him under oath, described a cross of light seen in the sky above the sun by Constantine and his whole army, with the words “Conquer by this,” followed by a dream in which Christ told him to copy the sign as a battle standard, later known as the labarum. The Latin motto in hoc signo vinces, “in this sign you will conquer,” is a rendering of those words.¹²

The two accounts do not agree, and Britannica notes that Eusebius does not date the vision to the eve of the battle and that its absence from the earlier accounts has led some scholars to doubt it. The Catholic Encyclopedia, for its part, points out that it was the result of the battle that gave the vision its full importance. Whatever a modern reader makes of it, Constantine went into battle under a new emblem.¹³

Disaster at the river

The ancient accounts of the fighting itself are brief. According to Zosimus, Constantine sent his cavalry against Maxentius's horse and charged so hard that he threw it into disorder; his infantry then advanced in good order. Lactantius says the armies fought hard at first and held their ground, and Britannica, following the sources, says simply that Constantine's army broke Maxentius's.¹⁴

When the cavalry gave way, the rest of the army fled for the river. Thousands of men crowded onto the temporary bridge, and under their weight it gave way. Maxentius was thrown into the Tiber and drowned along with many of his soldiers. Eusebius compared the scene to Pharaoh's army swallowed by the Red Sea.¹⁵

Turning point

The battle was lost before it began, when Maxentius gave up the walls. Behind the defenses of Rome he might have worn down an army smaller than his own; on the open ground north of the river, with the Tiber behind him and a single makeshift bridge as his line of retreat, one broken charge was enough to turn defeat into catastrophe. Every ancient account agrees on that bridge: whatever the cause of the rout, it was the collapse of the crossing that killed the emperor and destroyed his army.¹⁶

Aftermath

Maxentius's body was pulled from the river, and his head was carried through Rome on a spear. Constantine entered the city the day after the battle and was acclaimed by the Senate and people of Rome.¹⁷

The victor punished few. Zosimus says only a handful of Maxentius's closest friends suffered, and the Catholic Encyclopedia that no bloody executions followed. The Praetorian Guard, which had made Maxentius emperor, was another matter: Constantine disbanded it and pulled down its fortress in the city. In the heart of Rome he set up a statue of himself holding a cross-shaped standard, with an inscription that, Eusebius records, credited this saving sign with freeing the city from the yoke of tyranny.¹⁸

Early in 313 Constantine met Licinius at Milan, where Licinius married Constantia. The policy the two emperors agreed there, known as the Edict of Milan, gave Christians and everyone else freedom to worship as they chose and ordered confiscated church property returned. It did not make Christianity the state religion; that step came only under Theodosius I, whose edict of 380 bound his subjects to the Catholic faith.¹⁹

Peace between the two victors did not last. Licinius destroyed Maximinus Daia in the East in 313, and then he and Constantine fell out. They fought in 314 and again later, and in 324 Constantine defeated Licinius at Adrianople and Chrysopolis and became sole master of the Roman world.²⁰ › Battle of Chrysopolis, 324

Legacy and memory

The Milvian Bridge is remembered less for its scale than for its consequences. Britannica judges it almost certain that the victory confirmed Constantine's belief that the Christian God had protected him, and Britannica argues that the toleration he granted set the stage for Christianity to become, within a few generations, the dominant religion of the Mediterranean world. For that reason the battle stands among the true turning points of the ancient world.²¹

Rome itself kept the memory in stone. The Senate raised a triumphal arch for Constantine's victory over Maxentius between the Palatine and the Caelian, near the Colosseum. It was put together in haste, and much of its sculpture was taken from older monuments of Trajan, Hadrian and Marcus Aurelius.²²

The Church made the battle a founding story. The labarum, the standard topped with the Chi-Rho, became one of the best-known emblems of Christian Rome, and the words of the vision passed into Latin as in hoc signo vinces. In the Vatican, the Hall of Constantine was painted by Raphael's school after his drawings, with the Vision of the Cross and the Battle of the Milvian Bridge on its walls; the battle scene at the top of this page, by Giulio Romano, is one of them. See how Rome's armies were built to fight in how the Roman legion worked, revisit the earlier struggle for Rome at Actium, or check the terms in our military history glossary.²³