Emperor Julian tries to organize a pagan church and substitute it for Christianity. Pope Liberius repudiates the Arian creed, and declares that the Council of Arminium has no authority to issue decrees.
July 18 – Emperor Julian arrives at Antioch with an expeditionary force (60,000 men), and stays there for nine months, to launch a campaign against the Persian Empire. He secures the co-operation of King Arsaces of Armenia, who fights a bloody guerrilla war against the Persians.
February 21 – Athanasius returns to Alexandria and convenes a council, at which he appeals for unity among Christians who differ in terminology, but Emperor Julian orders Athanasius to leave Alexandria. He will remain in exile in Upper Egypt, until after Julian's death the next year.
October 22 – The temple of Apollo at Daphne, outside Antioch, is destroyed in a mysterious fire.
March 5 – Emperor Julian departs from Antioch with his army (90,000 men) and heads north towards the Euphrates. En route he creates a diversion and sends a force of 30,000 soldiers under his cousin Procopius to Armenia.
June 16 – The Roman army starts its retreat northward to Corduene (Armenia). Julian marches back up the Tigris and burns his fleet of supply ships. During the withdrawal Julian's forces suffer several attacks from the Persians.
June 26 – Battle of Samarra: Julian is mortally wounded in a skirmish and dies from a wound received during the fighting near Samarra (Iraq). Jovian, general of the Guard, succeeds him and is proclaimed Emperor by the troops.
The Council of Laodicea, which deals with constricting the conduct of church members, is held. The major canon approved by this council is Canon 29, which prohibits resting on the Sabbath (Saturday), restricting Christians to honoring the Lord on Sunday.
March 28 – Valens, brother of Valentinian I, is appointed co-emperor (Augustus) in the palace of Hebdomon (Turkey). He rules the Eastern portion of the Roman Empire, from the Danube to the Persian border, and begins the first anti-pagan persecutions.
Britain is forced to endure fierce barbarian raids.
Emperor Valens orders the expulsion of the Alexandrian bishop Athanasius from his see, but instead of going into exile Athanasius, now about 67, moves to the outskirts of Alexandria.
Valens builds a pontoon bridge across the Danube, and drives the Visigoths farther north, where they will come under pressure from the advancing Huns.
Winter – Emperor Valentinian I appoints Jovinus, his Master of the Horse (Magister Equitum), general of the army. He defeats the Alamanni in three successive battles, and pushes them out of Gaul.
Winter – Valentinian I mobilises a massive army for his campaign against the Alamanni and the Franks. He summons the Italian and Illyrian legions for a spring offensive.
In the region of the constellation Perseus, a star not visible to the naked eye, and 1,533 light years distant from Earth, explodes in a nova. The light from the star, now called GK Persei, was first detected on Earth on February 21, 1901.[8]
Spring – Emperor Valentinian I and his 8-year-old son, Gratian, cross the Rhine with an army into Alamannic territory. He defeats the Alemanni and burns food stores along the border. A temporary peace is signed with Macrian, king of the Bucinobantes, and Valentinian returns to his capital Augusta Treverorum (modern Trier).
Winter – The barbarians are driven back to their homelands, Hadrian's Wall is retaken and order returns to the Roman diocese. Theodosius reorganises the abandoned forts and mounts punitive expeditions in Hibernia (Ireland).
Fritigern becomes king of the Visigoths; amidst hostilities with his rival Athanaric, he asks Valens and the Thracian field army to intervene. They end the civil war, and Fritigern converts to Christianity.