312

Year 312 was a leap year starting on Tuesday of the Julian calendar. In the Roman Empire, it was known as the Year of the Consulship of Constantinus and Licinianus. The denomination 312 for this year has been used since the early medieval period, when the Anno Domini calendar era became the prevalent method in Europe for naming years.

Events

Constantine I crosses the Cottian Alps with an army and defeats Maxentius's generals in three battles at Turin, Brescia and Verona. Maxentius's Praetorian Prefect Ruricius Pompeianus is killed in the fighting outside Verona.
October 28 - Battle of the Milvian Bridge: Constantine defeats Maxentius at the Milvian Bridge, becoming the only Roman emperor in the West. Prior to the battle, he reportedly has a vision of a cross with the phrase "in hoc signo vinces". This encourages him to convert to Christianity.
October 29 - Constantine enters Rome; he stages a grand adventus in the city, and is met with popular jubilation. Maxentius' body is fished out of the Tiber and decapitated.
Constantine forges an alliance with co-emperor Licinius, and offers him his half-sister, Constantia, in marriage. The Praetorian Guard and Imperial Horse Guard are disbanded.
Emperor Maximinus Daza campaigns unsuccessfully against the Armenians.
By topic

Constantine I adopts the words "in hoc signo vinces" as a motto, and has the letters X and P emblazoned on the shields of his soldiers.
The Council of Carthage supports Donatism, which espouses a rigorous application and interpretation of the sacraments. These doctrines will be condemned by the Council of Arles.
Constantine I promotes a policy of state sponsorship of Christianity, perhaps even becoming a Christian himself.