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.
Sources
By far the most detailed and reliable source for the battle, and Julian's Gallic campaign (355–60) generally, is the Res Gestae (Histories) of Ammianus Marcellinus, a contemporary historian. Ammianus was a Greek career soldier who joined the army before 350 and served until at least 363.Ammianus, Rerum Gestarum Libri XXXI, XXXI.16.9 Translated by John C. Rolfe, Ammianus Marcellinus (Harvard: University Press, 1935-1939), vol. 3 pp. 504f Enlisted as a protector (cadet senior officer), he served as a staff officer under magister equitum Ursicinus and then under Julian himself in the latter's Persian campaign. Although he was not present at Strasbourg, he had experience of the Gallic front as he was involved in the suppression of the revolt of Claudius Silvanus, the magister equitum (commander-in-chief) in Gaul (355).Ammianus, Res Gestae XV.5.22. Translated by Rolfe, Ammianus, vol.1 pp 146-149 However, his narrative reveals a passionate admiration of Julian and occasionally descends to the level of eulogy. Furthermore, as he was writing some 40 years after the event, it is likely that Ammianus relied heavily, if not exclusively, on Julian's own memoir of the Strasbourg campaign (which we know he published, but has been lost).Penguin Classics Amm Notes p450 Thus Ammianus' account probably reflects Julian's own propaganda. In addition, Ammianus' account is of uneven quality, with many gaps and some contradictory elements.
The late 5th-century Byzantine chronicler Zosimus's Nova Historia deals with the battle, and Julian's Gallic campaign in a summary fashion and adds little to Ammianus' account. However, Zosimus is useful on two accounts. One is that his account of the revolt of Magnentius (350–3) survives, unlike Ammianus', which was covered in the 13 lost books of his history. The other is that for the reign of Julian we have on the authority of the Patriarch Photius that for this section of his history Zosimus is an epitomator of the account of Eunapius of Sardis, who drew upon the memoirs of Julian's physician Oribasius.G.W. Bowersock, Julian the Apostate (Cambridge: Harvard University, 1978), pp. 7f
The contemporary rhetorician Libanius delivered Julian's funeral oration in 363, whose text survives. This contains some details about the battle which are missing in Ammianus, which he presumably learnt from members of Julian's entourage. But because his oration was intended as a eulogy, not a historical narrative, his account of Julian's campaign is unreliable, and Ammianus' version is to be preferred where there is a contradiction.
The emperor Julian himself published a memoir of his campaigns on the Rhine, now lost. His Letter to the Athenians, an attempt to justify his rebellion against his cousin and senior emperor Constantius II, contains some details of the Rhine campaigns.
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).Goldsworthy (2000) 178 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).Drinkwater (2007) 223 (map) 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.Drinkwater (2007) 122
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.Drinkwater (2007) 143; Elton (1996) 73 Alamanni society was a violent warrior-society based on feuding clans, a fine breeding-ground for good warriors.Drinkwater (2007) 121
At this time of Strasbourg, the Alamanni confederation appears to have been under the presidency of two paramount kings, Chnodomar and Westralp.Ammianus XVI.12.1 Chnodomar was the driving force. A man of prodigious stature, strength and energy, he was nicknamed Gigas ("the Giant") by the Romans.Libanius 143 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.Ammianus XVI.12.24 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.Speidel (2004) Each nobleman could raise an average of about 50 warriors.Drinkwater (2007) 120
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).Zosimus II.58 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.Zosimus II.59 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.Elton (1996) 231 (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.Ammianus XV.5.2 Libanius claims that they were incited to do so by letters from Constantius, in order to create a diversion in Magnentius' rear.Libanius 133 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.Julian Epistulae ad Athenienses 280Libanius 18.34 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).Britannica Online Battle of Mursa 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.Jones (1964) 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.Ammianus XV.4 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.Ammianus XV.8.1 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.Ammianus, XV., 8; XVI., 11Libanius 132 But Constantius' own family purges had left him little choice: Julian was his sole surviving adult male close relative.