August 7
Birth of Constantius II to Constantine the Great and Fausta.
November 8
Constantius II made Caesar by Constantine the Great.
Constantius II sent to recapture Mesopatamia by Constantine the Great. He overruns it, captures Amida, and kills their leader Narses.
September 9
Death of Constantine the Great. Constans, Constantine II, and Constantius II become emperors and divide the empire.
July
Delmatius is killed by his own soldiers, possibly orchestrated by Constantius II.
Constans and Constantius II hold the Council of Serdica to settle a dispute over the bishop Athanasius of Alexandria. There is no agreement.
Constantius II and Constans nearly start a civil war, until Constantius allows Athanasius to return to his position in Alexandria.
December 25
Vetranio capitulates to Constantius II and is released to private life.
March 15
Constantius II raises Constantius Gallus to Caesar and marries him to his sister Constantina. Constantius Gallus takes up residence in Antioch.
Constantius II defeats Magnentius at the Battle of Mursa Major, which is costly for both sides.
Constantius II campaigns against the Sarmatians along the Danube.
August 8
Constantius II defeats Magnentius at the Battle of Mons Seleucus.
August 10
Magnentius commits suicide, leaving Constantius II in complete control.
Constantius Gallus is arrested by Constantius II and ordered put to death. He then reconsiders and orders Gallus spared, but the orders are delayed intentionally by Eusebius.
Constantius II campaigns agains the Alamanni.
August 11
Claudius Silvanus rebels against Constantius II in Gaul.
November 6
Constantius II elevates Julian to Caesar.
Athanasius is exiled from Alexandria by Emperor Constantius II due to his opposition to Arianism.
April
Constantius II visits Rome for the only time in his life.
Shapur II demands Constantius II return the lands taken earlier, but is rejected. He then attacks.
Shapur II destroys Singara, forcing Constantius II to march east.
November 3
Constantius II dies of fever in Cilicia, leaving Julian as the sole Augustus.
Constantius II unsuccessfully attempts to take Ad Tigris.
February
The troops of Julian declare him Augustus in Paris, thus rebelling against Constantius II.