Magnentius Proclaims Magnus Decentius as Caesar (Winter 350/351 CE)

Following his usurpation of the Western Roman Empire in 350 CE, Magnentius faced the looming threat of Constantius II, who ruled the Eastern Roman Empire and sought to avenge the murder of his brother, Constans. To strengthen his rule and secure the northern frontier, Magnentius proclaimed Magnus Decentius, likely his brother, as Caesar, probably during the winter of 350/351 CE.

1. The Role of Magnus Decentius

• Decentius was entrusted with the defense of Gaul and the Rhine frontier, a critical region vulnerable to Germanic invasions.
• By appointing a Caesar, Magnentius sought to consolidate his power in the West, allowing him to focus on fighting Constantius II in the East.
• The move also served to legitimize his rule, mirroring the Tetrarchic system established under Diocletian, where emperors had junior co-rulers.

2. The Struggle Against Constantius II

• Despite these strategic moves, Magnentius suffered a crippling defeat at the Battle of Mursa (351 CE) against Constantius II, greatly weakening his position.
• As Magnentius’ power declined, so too did Decentius’ authority in Gaul.
• By 353 CE, after Constantius' final victory, both Magnentius and Decentius faced collapse.

3. The End of the Usurpers (353 CE)

• Magnentius, seeing no path to victory, committed suicide in August 353 CE.
• Decentius, realizing his position was hopeless, also took his own life shortly after.
• With their deaths, Constantius II became the sole ruler of the Roman Empire, ending the last major civil war of the mid-4th century.

4. Conclusion: A Short-Lived Attempt at Stability

• The appointment of Magnus Decentius as Caesar was a practical move to protect Gaul and the Rhine frontier, but it ultimately failed to stop Constantius II's advance.
• Their defeat marked the reunification of the empire under Constantius II, but the losses suffered in the civil war severely weakened the Western Roman military, making it more vulnerable to future external invasions.

Magnentius’ attempt to maintain a breakaway empire ultimately collapsed, demonstrating the chronic instability of the late Roman Empire, where military usurpations frequently led to destructive civil wars.