350s (decade): civil war in Rome, Julian's campaigns, and Christian councils

The 350s saw a Roman civil war after the death of Constans, the rise and fall of the usurper Magnentius, Julian’s Gallic campaigns, and important church councils shaping Arian disputes.

Overview

The 350s (350–359 AD) were a turbulent decade in the late Roman Empire marked by internal conflict, military campaigning on the Rhine frontier, and intense disputes within Christianity. The period began with a violent change of imperial power in the West and ended with the consolidation of authority by the eastern emperor, even as religious controversies produced regional synods with lasting effects.

Major political and military events

The decade opened when the general Magnentius seized power in the West after the murder of Emperor Constans. Several short-lived usurpations and counter-moves followed; a series of civil wars between Magnentius and the eastern emperor Constantius II dominated the first half of the 350s. Key battles and outcomes included:

350: Magnentius’s revolt and the brief challenge posed by Nepotianus in Rome.
351: The Battle of Mursa Major — a costly victory for Constantius II that weakened both sides.
353: The final defeat of Magnentius and his subsequent death, leaving Constantius II as sole emperor.
355–358: Julian, appointed Caesar in the West, conducted successful campaigns to stabilize Gaul and repel Germanic incursions, notably winning a major engagement at Argentoratum (Strasbourg) in 357.

Religion and church politics

The decade was also decisive in the Arian controversy. Emperor Constantius II favored policies that moved the imperial church away from Nicene formulations, prompting synods and forced settlements in various regions. In 359 large assemblies — including the councils held at Ariminum (Rimini) and Seleucia — attempted to impose compromise creeds, producing outcomes that remained contested and contributed to ongoing theological division.

Frontiers and other regions

On the Rhine and Danube frontiers the empire faced pressure from Germanic groups such as the Franks and the Alamanni; Julian’s operations in Gaul temporarily restored frontier security. To the east, the Sasanian Persian realm under Shapur II continued to be an important regional power, and beyond the Roman and Persian borders other areas experienced political fragmentation and local upheaval characteristic of the fourth century.

Significance and legacy

The 350s reshaped imperial leadership and highlighted the limits of military solutions: civil war inflicted heavy losses and accelerated political centralization under Constantius II. The decade’s church councils influenced later theological alignment and episcopal politics, while Julian’s military successes in the West set the stage for his elevation shortly after the decade closed. Together, these developments illustrate how military, political, and religious pressures intersected during Late Antiquity.