Emperor Constantine I dedicates Constantinople as the new capital of the Roman Empire.
Date given in the Julian calendar.
The first ecumenical council of Christianity, convened by Constantine, condemns Arianism and formulates the Nicene Creed.
Emperors Constantine and Licinius agree on religious toleration, ending the persecution of Christians in the Roman Empire.
Constantine defeats Maxentius near Rome; according to tradition he saw a Christian sign before the battle.
The Goths, many of whom had crossed the Danube fleeing the Huns, crush a Roman army and kill Emperor Valens.
Emperor Theodosius I proclaims Nicene Christianity the state religion of the Roman Empire.
After the death of Theodosius I, the empire is divided between his two sons into Western and Eastern halves, never to be reunited.