Battle of the Milvian Bridge
The Battle of the Milvian Bridge took place on October 28, 312, between the Roman Emperors Constantine I and Maxentius. Constantine won the battle and started on the path that led him to end the Tetrarchy and become the only ruler of the Roman Empire.
The battle is noteworthy also because Constantine's victory was attributed by Christian sources to God.
The underlying cause of the battle was the five-year-long dispute between Constantine and Maxentius over control of the Western Roman Empire. Although Constantine was the son of the western emperor Constantius Chlorus, the system in place at the time, the tetrarchy, didn't necessarily provide for hereditary succession. When Constantius died on July 25, 306, his father's troops proclaimed Constantine as Augustus. In Rome, the favorite was Maxentius, the son of Constantius' predecessor Maximian, who was made emperor on October 28 of the same year.
By 312, the two men were engaged in open hostility with one another, although they were brothers-in‑law through Constantine's marriage to Fausta, sister of Maxentius.
The most important ancient sources for the battle are Lactantius, de mortibus persecutorum 44; Eusebius of Caesarea, Ecclesiastical History ix, 9 and Life of Constantine i, 28-31 (the vision) and i, 38 (the actual battle); Zosimus ii, 15-16; and the Panegyrici Latini of 313 (anonymous) and 321 (by Nazarititus).
Events of the battle
In the spring of 312, Constantine gathered his forces and decided to settle the dispute by force. He easily overran northern Italy, and reached Rome at the end of October 312 on the Via Flaminia. He camped at the location of Malborghetto near Prima Porta, where remains of a Constantinian monument in honour of the occasion are still extant.
It was expected that Maxentius would remain within Rome and endure a siege, as he already had successfully employed this strategy during the invasions of Severus and Galerius in 307 and 308, respectively. He had already brought large amounts of food to the city in preparation. Surprisingly, he decided otherwise and met Constantine in open battle. Ancient sources about the event attribute this decision either to divine intervention (e.g. Lactantius, Eusebius) or superstition (e.g. Zosimus). They also note that the day of the battle was the same as the day of his accession (October 28), which was generally thought to be a good omen. Lactantius also reports that the populace supported Constantine with acclamations during circus games, though it isn't clear how reliable his account of the events is.
Maxentius chose to make his stand in front of the Milvian Bridge, a stone bridge that carries the Via Flaminia road across the Tiber River into Rome. (A successor of the ancient bridge stands today at the same site, by the Italian name Ponte Milvio or sometimes Ponte Molle.)Holding it was crucial if Maxentius was to keep his rival out of Rome, where the Senate would surely favour whoever held the city. As Maxentius had probably partially destroyed the bridge during his preparations for a siege, he had a wooden or pontoon bridge constructed to get his army across the river.
The next day, the two armies clashed, Constantine emerged victorious. Already known as a skillful general, Constantine began to push Maxentius' army back toward the Tiber, Maxentius decided to retreat and make another stand at Rome itself. But there was only one escape route, via the bridge, Constantine's men inflicted heavy losses on the retreating army. Finally, the provisional bridge set up alongside the Milvian Bridge, over which many of the troops were escaping, collapsed, and those men stranded on the north bank of the Tiber were either taken prisoner or killed. Maxentius was among the dead, having drowned in the river while trying to swim across it in a desperate bid to escape. His body was found and his head paraded through the city during Constantine's entry into Rome as proof of his death.
Vision of Constantine
It is commonly stated that on the evening of October 27, with the armies preparing for battle, Constantine had a vision which lead him to fight under the protection of the Christian God. The details of that vision, however, differ between the sources reporting it.
Lactantius states that, in the night before the battle, Constantine was commanded in a dream to "delineate the heavenly sign on the shields of his soldiers" (de mort. pers. 44,5). He obeyed and marked the shields with a sign "denoting Christ". Lactantius describes that sign as a "staurogram", or a Latin cross with its upper end rounded in a P-like fashion. There is no certain evidence that Constantine ever used that sign, opposed to the better known chi-rho sign described by Eusebius.
From Eusebius, two accounts of the battle survive. The first, shorter one in the Ecclesiastical History leaves no doubt that God helped Constantine but doesn't mention any vision. In his later Life of Constantine, Eusebius gives a detailed account of a vision and stresses that he had heard the story from the emperor himself. According to this version, Constantine with his army was marching somewhere (Eusebius doesn't specify the actual location of the event, but it clearly isn't in the camp at Rome), when he looked up to the sun and saw a cross of light above it, and with it the Greek words "Εν Τούτω Νίκα" ("In this sign you will conquer"). At first, unsure of the meaning of the apparition, but in the following night, he had a dream in which Christ explained to him that he should use the sign against his enemies. Eusebius then continues to describe the labarum, the military standard used by Constantine in his later wars against Licinius, showing the chi-rho sign.
Those two accounts can hardly be reconciled with each other, though they have been merged in popular notion into Constantine seeing the Chi-Rho sign at the evening before battle. Both authors agree that the sign wasn't readily understandable to denote Christ, which corresponds to the fact that there is no certain evidence of the use of chi-rho as a Christian sign before Constantine. Its first appearance is on a Constantinian silver coin from c. 315, which proves that Constantine did use the sign at that time, though not very prominently. He made extensive use of the Chi-Rho and the Labarum only later in the conflict with Licinius.
As the god Sol Invictus, the Unconquered Sun, featured prominently on Constantinian coins and monuments in the years before and after the battle, the vision has been interpreted in a solar context (e.g. as a halo phenomenon), which would have been reshaped to fit with the Christian beliefs of the later Constantine.
Constantine entered Rome the next day and was acclaimed as sole Western Roman Augustus. He disbanded the 300-year-old Praetorian Guard. In 313, Constantine and Licinius joined forces against Maximinus. Their alliance would lead to the Edict of Milan, which legalized all religions within the Empire. Many still erroneously believe today that this decree established Christianity as the sole faith of Rome, a change which didn't take place until the early 390s, under Theodosius I. Instead, the Edict abandoned the persecution of Christians and restituted their property.