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 exaccerbated 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.
[[image:Labarum.jpg|thumb|right|The Labarum, Constantine's new standard bearing the Chi Rho, the first two letters in the name Χ