Battle of Milvian Bridge
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's father, Constantinus Chlorus, was the Western emperor, the tetrarchial system in place at the time did not adhere to primogeniture. When Constantius died on July 25, 306, his father's troops proclaimed Constantine as Augustus (the Latin honorific for emperors, roughly meaning "venerable"). But in Rome, the favorite was Maxentius, the son of Constantius' predecessor Maximian. Both men continued to vie for the title, however 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 over provincial Britain and Gaul, but was officially only a "Caesar"—a little emperor.
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.
Much of this was the work of Maxentius' father Maximian, who had been forcibly retired as emperor on May 1, 305 following a coup by his abdicating co-ruler Diocletian. Maximian's execution in 310 and Galerius' death a year later exacerbated the power struggle between Constantine and Maxentius.
Events of the battle
In the summer of 312, Constantine gathered his troops and decided to settle the dispute by force. He easily overran north Italy, and stood at the Saxa Rubra on the Tiber, less than 10 miles from Rome. Maxentius chose to make his stand in front of the Milvian Bridge (today the Ponte Milvio), a stone bridge that 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 of Rome 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.Eusebius of Caesarae]'s Historia Ecclesiastica claims that Constantine also saw the Greek phrase "Εν Τουτω Νικα", often rendered in Latin as "In Hoc Signo Vinces" - In this sign you shall conquer. Constantine, a pagan at the time whose chief god was Sol invictus, is said to have ordered his soldiers to put the symbol on their shields. Constantine later made a standard, known as the labarum, bearing this symbol, which he used in his later battles.
The next day, the two armies clashed, and Constantine emerged the victor. 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—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. Maxentius was among the dead, having drowned in the river while trying to swim across it in a desperate bid to escape.
Constantine entered Rome not long afterwards and was acclaimed as sole Western Roman Augustus, disbanding 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, but specifically Christianity. The Edict would later give way to Christianity rising from a "faith of the catacombs," a repressed religion, to the faith of the Empire with Constantine's conversion. However, the Edict's effects were long term. Licinius soon marched against Constantine to gain control of the Empire, though his insurrerction was crushed by Constantine, and subsequently Licinius was executed. Furthermore, Constantine's nephew and future emperor Flavius Claudius Julianus would reinstate the pagan religio romana, though this change did not last long as Christianity was once again the Imperial faith after his reign.