The House of Constantine: Fifty-Seven Years That Remade the Empire

In July 306 CE, soldiers at Eboracum in northern Britain acclaimed Constantine emperor over the body of his father, Constantius I, one of the four rulers of Diocletian‘s Tetrarchy. The proclamation had no legal basis, and Constantine spent eighteen years supplying one, defeating his rival Maxentius at the Milvian Bridge outside Rome in 312, then defeating his eastern colleague Licinius at Chrysopolis in 324 to rule alone. The family he founded held the empire until 363.

Constantine extended legal protection and patronage to the Christian church, and in 325 he convened the Council of Nicaea in Asia Minor, the first gathering of bishops from the whole empire. It met to settle a quarrel begun by Arius, a priest of Alexandria, over whether Christ had always existed or had been created by God the Father. The council ruled that the Son shared the Father’s substance, and eastern churches disputed that formula for sixty years. In 330 he founded Constantinople on the site of the Greek city Byzantium. He introduced the solidus, a gold coin of about four and a half grams that held its value for centuries. He separated civil from military command and built a mobile field army distinct from the frontier garrisons. Together these measures produced a new imperial system, with taxation, justice, and defense running on principles the earlier empire had not used.

The succession was violent. Constantine died in May 337, and within weeks soldiers killed most of his male relatives, sparing his three sons and two young cousins, Gallus and Julian. Constantine II, Constans, and Constantius II divided the empire between them. Constantine II died near Aquileia in 340 invading his brother’s territory. Constans was killed in 350 during the usurpation of Magnentius, a commander in Gaul. Constantius II defeated Magnentius at Mursa in 351 in a battle so costly that later writers remembered it as a wound the empire inflicted on itself.

Constantius ruled alone from 353, fighting the Persian king Shapur II on the eastern frontier and intervening repeatedly in the argument Nicaea had left open. Timothy Barnes’s study of his long conflict with Athanasius, bishop of Alexandria, reads these interventions as ordinary imperial politics conducted in theological language. Constantius designated Gallus as caesar in 351 and executed him in 354, then raised Julian, Gallus’s half-brother, to the same rank in 355 and sent him to govern Gaul.

Julian’s troops proclaimed him Augustus at Paris in 360, and civil war was averted when Constantius died in November 361. Julian, trained in Greek philosophy and privately committed to the traditional gods, spent his brief reign restoring temple cults, reorganizing pagan priesthoods, and barring Christians from teaching classical literature. He was killed on campaign in Mesopotamia in June 363, and the army chose Jovian, an officer outside the family, to replace him.

Constantine had tried to settle the succession by dividing the empire among his sons. Within three years two of them were at war, and by 353 only Constantius remained. Julian’s restoration of the old cults lasted twenty months, and Jovian reversed it within weeks.

FURTHER READING

1. Lenski, Noel Emmanuel, ed. The Cambridge companion to the age of Constantine. Cambridge university press, 2012.
2. Lewis, William. Division of Empire: The Reign of the Sons of Constantine. Oxford University Press, 2024.
3. Harries, Jill. Imperial Rome AD 284 to 363. Edinburgh University Press, 2012.
4. Tougher, Shaun. Julian the Apostate. Edinburgh University Press, 2019.
5. Barnes, Timothy David. Athanasius and Constantius: theology and politics in the Constantinian empire. Harvard University Press, 1993.