January 18 – Western Roman Emperor Constans I makes himself extremely unpopular; one of his generals, Magnentius, is proclaimed emperor at Augustodunum in the Diocese of Galliae, with the support of the army on the Rhine frontier.
January – Constans I flees towards Spain, where he is subsequently assassinated at Castrum Helenae. Magnentius rules the Western portion of the Roman Empire and is far more tolerant towards Christians and Pagans alike.
June 30 – Nepotianus is defeated and killed by Marcellinus, a trusted general sent by Magnentius. His head is put on a spear and carried around the city.
December 25 – Vetranio meets Constantius II at Naissus (Serbia) and joins forces with him. Vetranio is forced to abdicate his title, and Constantius allows him to live as a private citizen on a state pension.
The Alamanni and the Franks cross the Rhine and defeat the depleted Roman units left at the frontier. The Germans take control of around 40 towns and cities between the Moselle and the Rhine.[citation needed]
May 17 – Pope Julius I dies after a 15-year reign in which he has made himself the chief opponent of Arianism. He is succeeded by Pope Liberius as the 36th pope, who immediately writes to Constantius II requesting a council at Aquileia to discuss the former Alexandrian patriarch Athanasius, who opposes the Arian belief to which the emperor subscribes.[citation needed]
Constantius II sends his official Paulus Catena to Britain, to hunt down the opponents supporting Magnentius. Flavius Martinus, vicarius of Britain and supporter of Constantius, opposes the persecutions; he is then accused by Catena of being a traitor. In response, Martinus tries to kill Catena with a sword; he fails and then commits suicide.
As a result of the armies of the West having been largely withdrawn by the usurper Magnus Magnentius, to fight Constantius II, hordes of barbarians (Franks and Alemanni) cross the upper Rhine into Gaul and invade the lands of the Helvetians.
Emperor Constantius II builds new forts to secure upper Mesopotamia. Persia's king Shapur II sends an emissary to Constantinople with gifts and a letter wrapped in white silk. He requests that Constantius return the lands of his ancestors from the Euphrates to the frontier of Macedonia. Constantius tactfully refuses to cede any territories.
Siege of Amida: Shapur II besieges the Roman fortress of Amida (modern Diyarbakir). After seventy-three days the city is conquered and the population is massacred by the Persians. Ammianus Marcellinus is a fortunate survivor and flees to Singara (Iraq).
Famine in Upper Rhineland: A fleet of 800 river boats, built for the Rhine, cross to the British east coast, and carry back enough corn to raise the famine.[13]
Winter – Shapur II halts his campaign, due to heavy casualties during the Persian invasion.
July – Emperor Constantius II convenes the Council of Rimini, to resolve the crisis over Arianism in the Church. Some 400 bishops of the Western portion of the Roman Empire attend, while the Eastern bishops simultaneously hold a meeting at Seleucia. Given Saint Jerome's comment that, "The whole world groaned in astonishment to find itself Arian", it appears to have failed. Pope Liberius rejects the new creed at Rimini.