The Battle of Strasbourg, also known as the Battle of Argentoratum, was fought in 357 between the Western Roman army under Julian and the Alamanni tribal confederation led by the joint paramount King Chnodomar. The battle took place near Strasbourg, called Argentoratum in Ammianus Marcellinus' account, Argentorate in the Tabula Peutingeriana (section 2).
Although possibly outnumbered, the Roman army won a decisive victory after a hard-fought struggle with the Alamanni. With negligible casualties of their own, the Romans drove the Alamanni beyond the river, inflicting heavy losses. The Roman force, the imperial escort army of Julian, was small but of high quality. The battle was won by the skill of the Roman infantry, with the Roman cavalry initially performing poorly.
The battle was the climax of Julian's campaigns in 355–57 to evict barbarian marauders from Gaul and to restore the Roman defensive line of fortifications along the Rhine, which had been largely destroyed during the Roman civil war of 350–53. In the years following his victory at Strasbourg, Julian was able to repair and garrison the Rhine forts and impose tributary status on the Germanic tribes beyond the border.
Alamanni
During the 3rd century, the small and fragmented tribes of Germania Libera ("free Germany" i.e. Germany outside the empire) apparently coalesced into large, loose confederations: the Franks (NW Germany), Alamanni (SW Germany) and Burgundians (Central Germany). Although riven by internal feuding, these confederations could mobilise large forces and may have presented a greater threat to the empire than previously thought.
The Alamanni, who were originally from the Main valley of central Germany, had colonised the Agri Decumates (roughly the modern state of Baden-Württemberg in SW Germany) when the region was evacuated by the Romans in the mid-3rd century after belonging to the Roman province of Germania Superior for over 150 years. The Alamanni established a series of small pagi ("cantons"), mostly strung along the East bank of the Rhine (although a few were in the hinterland). The exact number and extent of these pagi is unclear and probably changed over time. Pagi, usually pairs of pagi combined, formed kingdoms (regna) which, it is generally believed, were permanent and hereditary.
The total Germanic population of Alamannia at this time has been estimated at a tiny 120,000 – 150,000. This compares with the about 10 million inhabitants of Roman Gaul. Alamanni society was a violent warrior-society based on feuding clans, a fine breeding-ground for good warriors.
At this time of Strasbourg, the Alamanni confederation appears to have been under the presidency of two paramount kings, Chnodomar and Westralp. Chnodomar was the driving force. A man of prodigious stature, strength and energy, he was nicknamed Gigas ("the Giant") by the Romans. He was a formidable sight in his "flashing" helmet (probably gold-leafed) and full parade armour. He is described by Ammianus as the "evil mastermind" behind the invasion of Gaul. Under the paramount king were 7 other kings (reges), including Vestralp, Urius, Ursicinus, Suomar and Hortar. It is possible that the petty kings (reguli) mentioned by Ammianus were the rulers of the pagi. Underneath the regal class were the nobles (called optimates by the Romans) and warriors (armati). The warriors consisted of professional warbands and levies of free men. Each nobleman could raise an average of about 50 warriors.
Background: Barbarian invasion of Gaul
In January 350, the Roman empire was jointly ruled by two sons of Constantine, the Augusti (joint emperors) Constans, who ruled the West, and Constantius II in the East (the third brother, Constantine II, had been deposed and murdered by Constans' agents in 340).
But in that month, Constans was in his turn overthrown and killed by the usurper Magnentius, a laetus from Gaul who was comes (commander) of an 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 command of the local comitatus also, bringing his combined strike force to about 60,000. Magnentius gathered an army consisting of the Gaul comitatus (c. 25,000 men) 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 heavy 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, Vadomarius, and Gundomadus, with a treaty of alliance. Meanwhile, Silvanus made considerable progress in restoring the situation in Gaul. But the following year (355), Silvanus was driven by false accusations against him at the Imperial court into proclaiming himself emperor at Colonia (Cologne) and leading his men in a rebellion against Constantius. Constantius responded by dispatching to Cologne a flying-squad of protectores domestici (imperial staff-officers), including the future historian Ammianus himself, under the command of Ursicinus. These soon contrived to murder Silvanus and prevented a wider mutiny. But the shaken emperor decided that he needed a member of his own imperial dynasty (the House of Constantine, known to the Romans as the Flavii, after Constantine the Great's clan name) to share the burdens of governing the empire. This was a difficult decision for a paranoid ruler who regarded all his relatives with intense suspicion and had already put to death 2 of his uncles and 7 cousins, including Julian's half-brother Constantius Gallus. He appointed his cousin Julian as Caesar (deputy emperor) for the "Three Gauls" (the dioceses of Gaul, Spain, and Britain) and gave him overall command of forces in Gaul. The appointment, though greeted with enthusiasm by the troops at Milan, was more generally 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 family 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, Mogontiacum (Mainz), Borbetomagus (Worms), Nemetae Vangionum (Speyer), Tabernae (Saverne), Saliso (Seltz), and Argentorate (Strasbourg) were all in German hands. Apart from the major fortified city of Colonia Agrippina (Cologne), 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 northeastern 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 Roman limitanei (border defence 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 Constantius' court in Milan 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.
Prelude
As a personal cavalry escort, Constantius provided Julian with 200 scholares, a regiment of cataphractarii (heavily armoured cavalry), and some mounted archers (in total 360 men). En route to Gaul from Milan, at Taurini (Turin), he received the calamitous news that Cologne, Rome's most important city and military fortress on the Rhine, had fallen to the Franks. He spent the winter of 355/356 with his escorting troops at Vienna (Vienne), not far south of Lugdunum (Lyons). For the 356 campaigning season, Julian's first task was to link up with the main Gaul comitatus, which had wintered at Remi (Reims) under the command of the magister equitum, Ursicinus' recently appointed successor, Marcellus. This involved a long march through country swarming with Alamanni raiding bands, many of which were as large or larger than Julian's own escort and expert at ambuscades. On the way, Julian surprised and drove off a large barbarian force that had surrounded Augustodunum (Autun) and defeated a raiding band in the Morvan wilderness.
At Reims, Julian showed his characteristic boldness by deciding, in conference with his senior commanders, to deal with the Alamanni problem at the source by marching straight to Alsace and restoring Roman control of the region. On the way, however, his army was ambushed and nearly destroyed at Decem Pagi (Dieuze) by a large German band who fell on two rearguard legions which had lost contact with the rest of the column in dense mist. They were rescued by auxilia palatina regiments that heard the uproar. Proceeding to Brocomagus (Brumath) in Alsace, Julian's army routed another German band in the field.
But after assessing the situation in Alsace, Julian evidently decided that his force was insufficient to prevail over the Alamanni alone. Instead, he set out to recover Cologne. From Metz, he led his army via Treviri (Trier) to Roman-held Koblenz and thence along the Rhine to Cologne. Entering the ruined city unopposed, Julian's men were set to work to rebuild the city walls. Julian then concluded a peace treaty with the Franks. This had the important result of removing half the opposition from the equation and allowing Julian to focus his resources on dealing with the Alamanni.
For the winter of 356/7, he chose Senones (Sens) near Paris as his own wintering base, but quartered most of his troops in other towns, including the main body at Reims under Marcellus, to spread the burden. A large band of Alamanni heard of his reduced escort, however, and besieged him at Sens. Julian's forces were able to hold out until, after a month, the Germans withdrew. He was so outnumbered by the enemy, however, that he was unable to sally forth and give chase. During the siege, Marcellus had failed to come to his assistance. For this omission, denounced as cowardice by Ammianus, Marcellus was dismissed as magister equitum by Constantius and replaced by Severus, a distinguished officer who was more compatible with Julian.
For the 357 campaign season, a plan was laid down at Constantius' headquarters in Mediolanum (Milan) to trap the Alamanni in eastern Gaul in a pincer movement. Julian would advance eastward from Reims, while the major part of Constantius' comitatus in Italy (25,000 strong) was despatched under magister peditum (field marshal) Barbatio to Augusta Rauracorum (Augst) in Raetia, from which he was to advance northward to meet Julian. The Alamanni bands would be cornered and destroyed in the southern part of Germania I province (Alsace).