Battle of the Milvian Bridge

According to Christian chroniclers Eusebius of Caesarea and Lactantius, the battle marked the beginning of Constantine's conversion to Christianity. Eusebius of Caesarea recounts that Constantine and his soldiers had a vision sent by the Christian God. This was interpreted as a promise of victory if the sign of the Chi Rho, the first two letters of Christ's name in Greek, was painted on the soldiers' shields. The Arch of Constantine, erected in celebration of the victory, certainly attributes Constantine's success to divine intervention; however, the monument does not display any overtly Christian symbolism.

Background

The underlying causes of the battle were the rivalries inherent in Diocletian's Tetrarchy. After Diocletian stepped down on 1 May 305, his successors began to struggle for control of the Roman Empire almost immediately. Although Constantine was the son of the Western Emperor Constantius, the Tetrarchic ideology did not necessarily provide for hereditary succession. When Constantius died on 25 July 306, his father's troops proclaimed Constantine as Augustus in Eboracum (York). In Rome, the favorite was Maxentius, the son of Constantius' imperial colleague Maximian, who seized the title of emperor on 28 October 306. But whereas Constantine's claim was recognized by Galerius, ruler of the Eastern provinces and the senior emperor in the Empire, Maxentius was treated as a usurper. Galerius, however, recognized Constantine as holding only the lesser imperial rank of Caesar. Galerius ordered his co-Augustus, Severus, to put Maxentius down in early 307. Once Severus arrived in Italy, however, his army defected to Maxentius. Severus was captured, imprisoned, and executed. Galerius himself marched on Rome in the autumn, but failed to take the city. Constantine avoided conflict with both Maxentius and the Eastern emperors for most of this period.

By 312, however, Constantine and Maxentius were engaged in open hostility with one another, although they were brothers-in‑law through Constantine's marriage to Fausta, sister of Maxentius. In the spring of 312, Constantine gathered an army of around 40,000 soldiers (although the ancient historian Zosimus "claimed, quite implausibly, that Constantine invaded Italy with 90,000 soldiers and 8,000 cavalry, and that Maxentius commanded an army of 170,000 soldiers and 18,000 cavalry, including 80,000 Italians and 40,000 Carthaginians") and decided to oust Maxentius himself. He easily overran northern Italy, winning two major battles: the first near Turin, the second at Verona, where the praetorian prefect Ruricius Pompeianus, Maxentius' most senior general, was killed.

Vision of Constantine

It is commonly understood that on the evening of 27 October with the armies preparing for battle, Constantine had a vision which led him to fight under the protection of the Christian God. Some details of that vision, however, differ between the sources reporting it.

Lactantius states that, on the night before the battle, Constantine was commanded in a dream to "delineate the heavenly sign on the shields of his soldiers" (On the Deaths of the Persecutors 44.5). He followed the commands of his dream and marked the shields with a sign "denoting Christ". Lactantius seems to describe that sign as a "staurogram", or a Latin cross with its upper end rounded in a P-like fashion, though his wording is imprecise. There is no certain evidence that Constantine ever used that sign, as 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 promotes the belief that the Christian God helped Constantine but does not 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 (Eusebius does not specify the actual location of the event, but it was clearly not 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 "Ἐν Τούτῳ Νίκα", En toutōi níka, usually translated into Latin as "in hoc signo vinces". The literal meaning of the phrase in Greek is "in this (sign), conquer" while in Latin it's "in this sign, you shall conquer"; a more free translation would be "Through this sign [you shall] conquer". At first he was 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.

The accounts of the two contemporary authors, though not entirely consistent, have been merged into a popular notion of Constantine seeing the Chi-Rho sign on the evening before the battle. Both authors agree that the sign was not widely understandable to denote Christ (although among the Christians, it was already being used in the catacombs along with other special symbols to mark and/or decorate Christian tombs). Its first imperial appearance is on a Constantinian silver coin from c. 317, which proves that Constantine did use the sign at that time, though not very prominently. He made more extensive use of the Chi-Rho and the Labarum later, during the conflict with Licinius.

Some have considered the vision in a solar context (e.g. as a solar halo phenomenon called a sun dog), which may have preceded the Christian beliefs later expressed by Constantine. Coins of Constantine depicting him as the companion of a solar deity were minted as late as 313, the year following the battle. The solar deity Sol Invictus is often pictured with a nimbus or halo. Various emperors portrayed Sol Invictus on their official coinage, with a wide range of legends, only a few of which incorporated the epithet invictus, such as the legend SOLI INVICTO COMITI, claiming the Unconquered Sun as a companion to the emperor, used with particular frequency by Constantine. Constantine's official coinage continues to bear images of Sol until 325/6. A solidus of Constantine as well as a gold medallion from his reign depict the Emperor's bust in profile jugate with Sol Invictus, with the legend INVICTUS CONSTANTINUS. The official cults of Sol Invictus and Sol Invictus Mithras were popular amongst the soldiers of the Roman Army. Statuettes of Sol Invictus, carried by the standard-bearers, appear in three places in reliefs on the Arch of Constantine. Constantine's triumphal arch was carefully positioned to align with the colossal statue of Sol by the Colosseum, so that Sol formed the dominant backdrop when seen from the direction of the main approach towards the arch.

However, other historians have discounted Eusebius's later account of a vision entirely. John Julius Norwich argued that "the vision of the Cross above the battlefield ... never occurred. Had it done so, it is unthinkable that there should not be a single reference to it in any of the contemporary histories until the Life of Constantine", and that Eusebius's "specific statement that 'the whole army ... witnessed the miracle'" is implausible. On the other hand, Norwich did concede that shortly before the battle, Constantine must have undergone "some profound spiritual experience", and that Eusebius's story was likely not so much "a deliberate falsehood" as "a possibly unconscious exaggeration", with Constantine adding "a gentle gloss" to his recollections which the author then uncritically noted down or embellished.