The Battle of Milvian Bridge took place on October 28, 312 between the Roman Emperors Constantine the Great and Maxentius. When Constantine emerged victorious, the path of Western civilization as it had been known was about to be changed forever.

The underlying cause of the battle was the five-year-long dispute between Constantine and Maxentius over control of the western half of the empire. Although Constantine was the son of the western emperor Constantius Chlorus, the system in place at the time, the tetrarchy, did not necessarily provide for hereditary succession. When Constantius died on July 25, 306, his father's troops proclaimed him as Augustus (October 28, 306), but in Rome, the favorite was Maxentius, the son of Constantius' predecessor Maximian. Both men continued to claim the title afterwards, although a conference to resolve the dispute in 308 resulted in Maxentius being named a senior emperor along with Galerius. Constantine was allowed to maintain rule of the provinces of Britain and Gaul, but was officially only a "Caesar", or junior emperor.

By 312, the two men were engaged in open hostility to one another, although they were brothers-in‑law. Much of this was the work of Maxentius' father Maximian, who had been forcibly retired as emperor in 305 by Diocletian. Maximian schemed and double-crossed both his son and Constantine trying to regain power before the latter had him executed in 310. When Galerius died in 311, the power struggle was on. In the summer of 312, Constantine gathered his forces and decided to settle the dispute by force.

He easily overran northern Italy, and stood less than 10 miles from Rome when Maxentius chose to make his stand in front of the Milvian Bridge, a stone bridge (a successor of which stands today at the same site, by the Italian name Ponte Milvio or sometimes Ponte Molle) which carries the Via Flaminia road across the Tiber River into Rome. Holding it was crucial if Maxentius was to keep his rival out of Rome, where the Senate would surely favor whoever held the city.

Constantine, after arriving, realized he had made a miscalculation and that Maxentius had many more soldiers available than he did. Some sources say the advantage was 10‑to‑1 in Maxentius' favor, but it was probably more like four to one. In any case, Constantine had a tough challenge ahead of him.

It is commonly stated that on the evening of October 27, with the armies preparing for battle, Constantine reportedly had a vision as he looked toward the setting sun; although Eusebius records the event as occurring when Maxentius' army was still in Northern Italy. At any rate, a cross appeared emblazoned on the sun, and maybe the Greek letters XP ("Chi-Rho", the first two letters of "Christ") intertwined with it; and Constantine either saw or heard the Greek phrase "Εν Τουτω Νικα", often rendered in Latin as "In Hoc Signo Vinces" — "Under this sign, you will conquer." Constantine, who was a pagan at the time, is said to have put the symbol (the labarum) on his solders' shields.

The next day, the two armies clashed, and Constantine emerged victorious. Already known as a skillful general, Constantine began to push Maxentius' army back toward the Tiber, and Maxentius decided to retreat and make another stand at Rome itself. But there was only one escape route, via the bridge, and Constantine's men inflicted heavy losses on the retreating army. Finally, a bridge of boats 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, with Maxentius numbered among the dead.

Constantine entered Rome not long afterwards and was acclaimed as sole western Augustus. He credited his victory at the Milvian Bridge to the god of the Christians, and ordered the end of any religious persecution within his realm, a step he had already taken in Britain and Gaul in 306. With the emperor as a patron, Christianity, which was already quite common in the empire, exploded in popularity.