When his father was made caesar (subemperor), Constantine was left at the court of the emperor Diocletian, where he was under the watchful eye of Galerius, who was caesar with Constantius. When Diocletian and Maximian resigned in 305, Constantius and Galerius became emperors.
Constantius requested that Constantine be sent to him in Britain, and Galerius reluctantly complied. Constantius died at York the next year. There, his soldiers proclaimed Constantine emperor, but much rivalry for the vacated office ensued. In Italy, Maxentius, supported by the Romans and by his father Maximian, vied with Severus and Galerius. Constantine, accepting the lesser title of caesar from Galerius, remained aloof while Maxentius and Maximian defeated Severus and Galerius.
Constantine made an alliance with Maximian, marrying his daughter Fausta and recognizing Maxentius after a fashion. When Maximian, in dispute with his son, fled to Constantine, Constantine received and sheltered him until Maximian, in an attempt to regain the throne, undertook (310) a revolt against Constantine's rule in Gaul. Unsuccessful against Constantine, Maximian was forced to commit suicide.
Constantine, having already declared against Maxentius and ignoring the fact that Galerius had recognized Licinius in the East, now considered himself emperor. When Galerius died in 310, still another claimant to the imperial throne appeared in Maximin (d. 313), who allied himself with Maxentius against the alliance of Licinius and Constantine. While Licinius attacked Maximin, Constantine moved into Italy against Maxentius. The rivals for Italy met (312) at the Milvian or Mulvian Bridge over the Tiber near Rome. Before the battle Constantine, who was already sympathetic toward Christianity, is said by Eusebius of Caesarea to have seen in the sky a flaming cross inscribed with the words, "In this sign thou shalt conquer." He adopted the cross and was victorious. Maxentius was routed and killed. The battle is regarded as a turning point for Christianity.
In 313 Constantine and his fellow emperor, Licinius, met at Milan and there issued the so-called Edict of Milan, confirming Galerius' edict of 309, which stated that Christianity would be tolerated throughout the empire. The edict in effect made Christianity a lawful religion, although it did not, as is sometimes believed, make Christianity the official state religion.
No longer having Maximin to contend with, Licinius challenged Constantine, and a brief struggle followed. Constantine, victorious, took (315) control over Greece and the Balkans, and the uneasy peace that followed lasted until 324, when Licinius again vied with Constantine. This time Licinius lost his throne and ultimately his life.
The Columbia Electronic Encyclopedia, 6th ed. Copyright © 2012, Columbia University Press. All rights reserved.