407–411 AD – Constantine III

What Happened

In AD 407, the Roman garrison in Britain, feeling abandoned by the central government in Ravenna and facing increasing threats from barbarian incursions, took matters into their own hands. The Western Roman Empire was in a state of profound crisis. On December 31, 406, a massive coalition of Vandals, Alans, and Suebi had crossed the frozen Rhine, devastating Gaul and severing communication between Britain and Italy.

In response to this existential threat, the British legions mutinied and elevated a series of usurpers. After quickly murdering the first two (Marcus and Gratian), they proclaimed a common soldier named Constantine as emperor, likely chosen simply because he shared the auspicious name of Constantine the Great, who had also been elevated in Britain a century earlier.

Constantine III immediately recognized that the key to securing Britain was to control the opposite coast of Gaul and prevent the barbarians from crossing the Channel. He stripped Britain of its remaining mobile field army (comitatenses) and crossed to Gesoriacum (Boulogne). He successfully rallied the surviving Roman forces in Gaul and Hispania, establishing his capital at Arelate (Trier or Arles) and effectively creating a new, independent western empire.

For four years, Constantine III managed to hold the line against the barbarian invaders in Gaul and even secured grudging recognition as co-emperor from the legitimate Western Emperor, Honorius, who was besieged in Ravenna by Alaric the Goth. However, Constantine's position was inherently unstable. He faced constant rebellions from his own generals, most notably Gerontius in Hispania, and relentless pressure from the barbarians.

In AD 411, Honorius's capable general, Constantius (the future Emperor Constantius III), marched on Arelate and besieged Constantine. After his general Edobich was defeated attempting to relieve the siege, Constantine surrendered, having first been ordained as a priest in a desperate bid to save his life. Constantius ignored this and had him beheaded on the journey to Ravenna.

Why It Mattered

The usurpation of Constantine III (AD 407–411) was the final, fatal blow to Roman military control in Britain. By taking the last effective field army across the Channel, he left the island virtually defenceless against the Picts, Scots, and Saxons. This withdrawal is traditionally seen as the definitive end of Roman Britain, as the central government never again sent a significant military force to reoccupy the province.

The events of 407–411 also highlighted the complete collapse of the Western Roman Empire's strategic position. The inability of Honorius to defend Gaul or Britain forced the provincial armies to rely on usurpers for their survival. Constantine III's brief success demonstrated that a coordinated defence of the west was still possible, but his ultimate failure underscored the overwhelming military and political challenges facing the empire. The loss of Britain and the devastation of Gaul permanently crippled the western empire's tax base and manpower, accelerating its final disintegration.

Human Perspective

For the Roman soldiers in Britain, the elevation of Constantine III was a desperate act of self-preservation. They were cut off from Rome, facing imminent barbarian attack, and led by a government that seemed incapable of protecting them. Their decision to follow a common soldier across the Channel was a massive gamble, driven by the hope that he could restore order and secure their pay. The subsequent campaigns in Gaul and Hispania were brutal and chaotic, ending in defeat and execution for their chosen leader. The loss of these troops left their families and communities in Britain entirely exposed to the violence of the invading tribes.

For the Romano-British population, the departure of Constantine III and his army was the final abandonment by Rome. The stripping of the garrison removed the last vestige of imperial protection, leaving them to face the Picts, Scots, and Saxons alone. The realization that the central government would not—or could not—send help must have been a profound and terrifying shock. The events of 407–411 marked the beginning of a dark and violent period in British history, as the island descended into post-Roman fragmentation and the long struggle for survival against the Anglo-Saxon migrations.