Constantinople enforces a strict licensing system for physicians.
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.
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.
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
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 expions 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.