Constantius II

The longest-reigning son of Constantine the Great, Constantius II governed some part of the Roman world for twenty-four years and, for the last eight, all of it. Born in 317 to Constantine and Fausta, he was made Caesar as a child and received the eastern provinces when his father died in 337. That summer the army massacred most of the male descendants of Constantius I's second marriage, leaving only Constantine's three sons and two young cousins, Gallus and Julian, alive; the sources disagree on how far Constantius instigated or merely tolerated the killings, and his precise responsibility remains debated.

The brothers' division of the empire did not last. Constantine II died invading Constans's territory in 340, and Constans was killed by the usurper Magnentius in 350. Constantius defeated Magnentius in the exceptionally bloody battle of Mursa in 351 and by 353 ruled alone. His eastern frontier was consumed by intermittent war with the Persian king Shapur II, a grinding contest of sieges in Mesopotamia that defied resolution throughout the reign.

Needing colleagues, he turned to the dynasty's survivors. Gallus, appointed Caesar in the East in 351, governed badly and was executed in 354. Julian, made Caesar for Gaul in 355, was bound to the emperor by marriage to Constantius's sister Helena, a characteristic use of matrimony to cement the Constantinian house. Constantius involved himself deeply in church affairs, convening councils and promoting creeds that avoided the Nicene formula, and repeatedly exiling bishop Athanasius of Alexandria; ecclesiastical writers of the Nicene tradition consequently judged him harshly.

In 360 Julian's troops in Paris proclaimed him Augustus. Constantius, then engaged with Persia, marched west to confront him but fell ill and died in Cilicia in November 361, reportedly naming Julian his successor and so sparing the empire a civil war. Ammianus Marcellinus, the best source for his later years, portrays a suspicious but conscientious ruler, dignified in public ceremony and effective in civil administration.