Battle of Strasbourg - Background: Barbarian Invasion of Gaul

Background: Barbarian Invasion of Gaul

In January 350, the Roman empire was ruled by two sons of Constantine I the Great, the Augusti (joint emperors) Constans, who ruled the West, and Constantius II in the East. But in that month, Constans was overthrown and killed by the usurper Magnentius, a laetus from Gaul who was comes (commander) of the elite brigade in Constans' comitatus (imperial escort army). In the East, Constantius had been engaged in a lengthy war against the Persians under Shah Shapur II (337-350). But he immediately concluded a truce in order to deal with Magnentius. He led his own comitatus to Illyricum where he assumed also command of the local comitatus, bringing his combined strike force to about 60,000. Magnentius gathered an army consisting of the Gaul comitatus and probably some Frankish and Saxon foederati (allies) and marched into Illyricum to confront Constantius. (For explanation of the term comitatus, see Late Roman army).

The Franks and Alamanni on the Rhine frontier now seized the opportunity presented by the absence of the best Roman forces in the civil war to overrun much of eastern Gaul and Raetia. Libanius claims that they were incited to do so by letters from Constantius, in order to create a diversion in Magnentius' rear. The barbarians captured many of the Roman forts along the Rhine, demolished their fortifications and established permanent camps on the West bank of the river, which they used as bases to pillage Gaul during the four years that the civil war lasted (350-3). In excess of 20,000 Roman civilians were reported to have been abducted from Gaul and forced to work in the Alamanni's fields. In turn, this would have reinforced Alamanni raiding in Gaul by freeing many from the harvest cycle.

Meanwhile, a huge number of Rome's finest troops, including most of the Gaul comitatus and perhaps half the combined Eastern/Illyricum force, were wiped out in the civil war. At the Battle of Mursa in Pannonia (351), one of the bloodiest in Roman history, Magnentius lost an estimated 24,000 men (perhaps two-thirds of his army). Constantius' army, although victorious, suffered even greater casualties (about 30,000). A final encounter at the Battle of Mons Seleucus in the Alps saw further casualties. Such massive losses of first-grade troops could not quickly or easily be replaced. Constantius, now based in Milan, was left with an escort army of about 30,000, but Illyricum and the East had been stripped of their comitatus. With renewed Persian attacks, the East was the top priority for reinforcement and Illyricum second. In the circumstances, Constantius could only spare in the region of 13,000 men for the Gaul comitatus, about half its previous strength. The Frankish-born general Silvanus was appointed its commander (magister equitum).

Using his own comitatus, Constantius succeeded in driving the Alamanni out of Raetia (354), and binding the kings of southern Alamannia, Wadomar and Gundomad, with a treaty of alliance. Meanwhile, Silvanus made considerable progress in restoring the situation in Gaul. But the following year (355), Silvanus staged a coup d'état, proclaiming himself emperor at Colonia (Cologne). Constantius responded by despatching to Cologne a flying-squad of protectores (cadet officers), including the historian Ammianus himself, under the command of Ursicinus. These succeeded in preventing a general mutiny and in swiftly executing Silvanus. But the shaken emperor decided that he needed a member of his own imperial family (named the Flavii, after Constantine the Great's middle name) to share the burdens of governing the empire, a remarkable change of policy for a ruler who regarded all his relatives with intense suspicion and had put many of them to death. He appointed his cousin Julian as Caesar (deputy emperor) for the West and gave him overall command of forces in Gaul. The appointment was widely seen as unsuitable as Julian, who was just 23 years old, had no military experience and until that moment had spent his time studying philosophy at Athens. But Constantius' own paranoid purges had left him little choice: Julian was his sole surviving adult male close relative.

The task confronting Julian as he took up his command was daunting. The civil war had left Gaul in a chaotic state. The defensive line of the Rhine had largely collapsed. According to Ammianus, the Franks had taken Cologne by storm and razed it to the ground. Moguntiacum (Mainz), Borbetomagus (Worms), Nemetae Vangionum (Speyer), Tabernae (Saverne), Saliso (Brumat) and Argentorate (Strasbourg) were all in German hands. Only three strongpoints on the Rhine remained in Roman hands: a single tower near Cologne and two forts, at Rigodunum (Remagen) and Confluentes (Koblenz). Large barbarian bands were roaming and pillaging eastern Gaul at will, reaching as far as the river Seine. So many and so large were the marauding enemy bands that Silvanus was considered a brave man for having led a large force (8,000 men) along a wooded highway in the heart of Gaul because of the risk of ambush. Further, the limitanei (border protection forces) along the Rhine had been decimated by the fall of most of their forts to the Germans, while those units that survived intact had mostly retreated from the frontier to garrison Gaul's cities. Cynics at court whispered that Julian had been given an impossible mission to rid Constantius of a potential rival for the throne. In the event, however, he surprised everyone by proving an effective military leader.