Battle of the Milvian Bridge

On 28 October 312, just north of Rome where the Via Flaminia crosses the Tiber, the army of Constantine defeated that of his rival emperor Maxentius, who drowned in the river as his troops fled. The victory gave Constantine the whole western half of the Roman Empire and set him on the road to sole rule. Christian writers of the time linked it to a divine sign, and it has long been seen as the moment Constantine turned towards Christianity.

The battle grew out of the breakdown of the system of shared rule created by Diocletian. After Diocletian stepped down in 305, his successors soon fell out. When Constantine's father, the western emperor Constantius, died in July 306, his troops at York proclaimed Constantine emperor. A few months later, on 28 October 306, Maxentius, son of the former emperor Maximian, seized power in Rome. The senior emperor Galerius accepted Constantine, though only at the lower rank of Caesar, but treated Maxentius as a usurper. Two attempts to remove Maxentius failed: the army of the emperor Severus went over to him in 307, and Galerius himself could not take Rome later that year.

By 312 Constantine and Maxentius were open enemies, even though Constantine was married to Maxentius's sister Fausta. In the spring Constantine crossed into Italy with an army put at around 40,000; the ancient historian Zosimus gave much larger figures for both sides, which modern scholars treat as implausible. Constantine swept through the north, winning battles near Turin and at Verona, where Maxentius's leading general, the praetorian prefect Ruricius Pompeianus, was killed.

According to Christian authors, Constantine received a sign from God before the battle, but their accounts differ. Lactantius wrote that a dream told him to mark a heavenly symbol on his soldiers' shields. Eusebius, in his later Life of Constantine, said the emperor had told him of seeing a cross of light above the sun, with words usually rendered "in this sign, conquer", followed by a dream in which Christ explained it. Some historians reject the vision altogether, others suggest a natural halo around the sun, and Constantine's coins went on showing the sun god Sol for more than a decade.

Maxentius had stocked Rome with food and was expected to sit out a siege, as he had done successfully before. Instead he chose to fight in the open, perhaps encouraged because the date was the anniversary of his accession and an oracle had foretold that an enemy of Rome would die that day. His men crossed the Tiber on a temporary wooden or pontoon bridge beside the stone Milvian Bridge, which had probably been partly broken. They formed up with the river close behind them. Constantine's cavalry broke Maxentius's horsemen, and his infantry then pushed the enemy back towards the water. When Maxentius ordered a retreat into the city, the temporary bridge collapsed under the crowd of fleeing soldiers. Many of those left on the north bank were killed or captured, while the Praetorian Guard held its ground to the end. Maxentius drowned, either swimming for safety or thrown from his horse.

Constantine entered Rome on 29 October to popular celebration. Maxentius's body was recovered and beheaded, and his head paraded through the streets before being sent to Africa, which then submitted. Constantine promised the Senate there would be no reprisals against Maxentius's supporters, but he erased his rival's memory, took over his building projects and disbanded the Praetorian Guard.

The victory left Constantine master of the western empire. In 313 he and his eastern colleague Licinius issued the Edict of Milan, granting Christianity official toleration. The triumphal Arch of Constantine in Rome credits the victory to divine help, though it carries no openly Christian imagery.

Our 300 map shows the Roman Empire shortly before Constantine's rise; the 400 map shows it later in the century, when the Western Roman Empire had its own court.