Battle of the Milvian Bridge
312 CE.
Shown on the map at 41.936°, 12.467°.
The Battle of the Milvian Bridge took place between the Roman Emperors Constantine I and Maxentius on 28 October AD 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; his body was later taken from the river and decapitated, and his head was paraded through the streets of Rome on the day following the battle before being sent to Africa. 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.
Text from the Wikipedia article Battle of the Milvian Bridge, licensed CC BY-SA 4.0.