The Battle of the Milvian Bridge took place between the Roman Emperors Constantine I and Maxentius on 28 October 312. It takes its name from the Milvian Bridge, an important route over the Tiber. Constantine won the battle and started on the path that led him to end the Tetrarchy and become the sole ruler of the Roman Empire. Maxentius drowned in the Tiber during the battle.
According to chroniclers such as Eusebius of Caesarea and Lactantius, the battle marked the beginning of Constantine's conversion to Christianity. Lactantius recounts that Constantine and his soldiers had a vision of the Christian God promising victory if they daubed the sign of the cross on their 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.
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 an usurper. Galerius sent his subordinate emperor, Severus, against him 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 his forces 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.
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