The German and Sarmatian campaigns of Constantine were fought by the Roman Emperor Constantine I against the neighbouring Germanic peoples, including the Franks, Alemanni and Goths, as well as the Sarmatian Iazyges, along the whole Roman northern defensive system to protect the empire's borders, between 306 and 336.
After becoming controller of the western provinces along the Rhine limes (in 306) following the death of his father Constantius Chlorus (Augustus of the west) in 306, Constantine initially concentrated his forces on defending this area of the frontier against the Franks and Alemanni, making Augusta Treverorum his first capital for this purpose. Having defeated the usurper Maxentius at the Battle of the Milvian Bridge in 312,Eutropius, Breviarium ab Urbe condita, 10.4. all Italia passed under Constantine's control and he thus became the sole Augustus of the West.
In February 313, Constantine (who had spent the winter in Rome) formed an alliance with the Emperor of the East, Licinius, reinforced by Licinius' marriage to Constantine's sister, Flavia Julia Constantia.Zosimus, New History, 2.17.2. However, this alliance survived for only a few years, before the two Augusti came into conflict in 316. Constantine defeated Licinius, who was forced to cede Illyricum to Constantine, but not Thrace. Constantine advanced ever further east with his territorial acquisitions, now having to defend the important strategic region of the limes sarmaticus (from 317).
In the following years, Constantine mostly occupied himself in the central section of the Danubian Limes, mostly fighting against the Sarmatians in Pannonia, residing at Sirmium almost continuously until 324 (when he moved against Licinius once more), making it his capital along with Serdica. At this time Constantine also demonstrated a very active military bent, travelling along the whole of the limites of his newly acquired territory. From 320 he appointed his eldest son, Crispus, Praetorian prefect, with military command of Gaul.
When he learnt that an army of Goths had crossed the Danube to raid Roman territory in Moesia Inferior and Thrace, which belonged to Emperor Licinius, he left his general headquarters in Thessalonica and marched against them (323). The fact that he had trespassed into a part of the empire which was not under his control unleashed the final phase of the Civil wars of the Tetrarchy, which ended with the complete defeat of Licinius and the consecration of Constantine as the sole Roman Emperor.
The final period of Constantine's reign, until his death (337), saw the Christian Emperor consolidate the entire defensive system on the Rhine and Danube, obtaining important military successes and reasserting control over a large part of the territory that the Romans had abandoned under Gallienus and Aurelian: the Agri Decumates from the Alemanni, the area south of the Tisza from the Sarmatians, as well as Oltenia and Wallachia from the Goths.
Historical context With the death of Emperor Numerian in November 284 (who had been entrusted with Eastern Roman empire by his father Carus) and the refusal of the eastern troops to recognise Carus' eldest son Carinus as his successor, a proven general of Illyrian origins, Diocletian, was raised to the purple. At the end of the civil war which followed, Diocletian was victorious and in 285 he named Maximian as his deputy (or Caesar) and then a few months later elevated him to the rank of Augustus (1 April 286), thereby forming a diarchy, in which two emperors divided the government of the empire on geographic lines. This also entailed the division of responsibility for the defence of the northern frontier from Germanic and Sarmatian incursions.Grant, p. 265.Scarre, p. 197-198.
Given the increasing difficulty of containing the internal revolts and those along the borders, a further territorial division was executed in 293 to facilitate military operations: Diocletian named Galerius as his Caesar in the east, while Maximian chose Constantius Chlorus in the west.Cameron, p. 46.
However, this tetrarchy fell into crisis only a year after the abdication of the two Augusti in 305, beginning a new Civil War (306–324), permitting new breaches along the Roman external border, with populations attempting to settle within Roman territory.
It was only with Constantine's accession to the throne, becoming sole Augustus of the West after the Battle of the Milvian Bridge in 312 and later still defeating Licinius and reuniting the Empire under a single emperor (324) that the northern frontiers were adequately defended once more. It is no coincidence that Constantine is attributed the responsibility for perfecting the military reforms of Diocletian and also for the reconquest or vassalisation of all the territory which Trajan had controlled.Julian, De Caesaribus, 329c.
Background: the death of Constantius Chlorus With the death of Constantius Chlorus at Eboracum (York) on 25 July 306,Eutropius, Breviarium ab Urbe condita, 10.1.Zosimus, Storia nuova, 2.9.1. the Tetrarchy entered a crisis: the eldest son of the dead emperor, Constantine was proclaimed AugustusEutropius, Breviarium ab Urbe condita, 10.2. by the Alemanni general Chrocus and the army of Britannia.Panegyrici latini, 7.3.3.Orosius, History against the Pagans, 7.26.1.Aurelius Victor, De Caesaribus, 40.2.Eusebius of Caesarea, Church History, 8.13.14. His election was in accordance with a dynastic principle rather than the meritocratic system of the Tetrarchy created by Diocletian. Only Lactantius maintains that Constantine was named Augustus by his father on his deathbed.Lactantius, De mortibus persecutorum, 24.8. Galerius was displeased by this act and offered the son of his deceased colleague the title of Caesar, which Constantine accepted, allowing Flavius Severus to succeed his father Constantius instead.Barnes, Constantine and Eusebius, pp. 27–28; Barnes, New Empire, p. 5; Lenski, pp. 61–62; Odahl, pp. 78–79. A few months later, Maxentius son of the old Augustus Maximian was acclaimed Emperor by the Praetorian guard with the support of officials like Marcellianus, Marcellus and Lucianus (but not Abellius, vicar of the Praefectus urbi, who was killed), reaffirming the dynastic principle. It was in this period that Constantine began to achieve important military successes against the Alemanni and the Franks, along the stretch of the frontier attributed to him, as is recounted by Eutropius.Eutropius, Breviarium ab Urbe condita, 10.3.
Forces in the field Romans Regarding the Roman forces garrisoned along the whole stretch of the northern limites from Britannia to Moesia, at this time there was a very important reform of the Roman army, a new deployment of the Roman legions along the borders and an increase in the size of the Roman army. With Diocletian's Tetrarchy reforms, the total number of legions was brought to 55 or 56 in the year 300.J. R. González, Historia de las legiones romanas, pp. 709–710. Constantine's accession to the throne and the return of a dynastic monarchy brought about the final increase of the number of Roman legions to 62 or 64 around the year 330.J. R. González, Historia de las legiones romanas, pp. 711–712.
Barbarians
Along the Rhine limes the Franks and Saxons in particular pressed on Gaul and Britannia. The Alemanni also made some incursions in these regions, but the main goal of their attacks at this time was Northern Italy via Pannonia (the western part of the Danubian Limes). The major clashes occurred along the Lower Danube in the Roman provinces of the Balkan region, where the Marcomanni, Quadi, Sarmatians and especially the Goths (divided into the Tervingi and the Greutungi) concentrated their attacks.
Phases of the conflict First phase (306–316): defence of the Rhine limes
306 The twenty-one-year-old Constantine, unable to get permission to visit his ailing father Constantius Chlorus (Augustus of the west) from the Augustus of the East, Galerius, in whose court he had lived since the time of Diocletian, decided to escape in the spring of this year. He found his father at Gesoriacum (Boulogne-sur-Mer) about to cross the English Channel to Britannia and joined him in a successful military campaign against the Picts and Scotti to the north of Hadrian's Wall.E. Horst, Costantino il grande, Milano 1987, pp. 84–85. When his father died during the summer, Constantine was proclaimed Augustus of the West by his father's loyal troops at Eboracum on 25 July. The young tetrarch however needed his election to the Imperial office to be recognised, particularly by Galerius, the most senior of the Augusti. Galerius preferred his friend and comrade in arms, Licinius, to Constantine. Confronted with a fait accompli and in the face of the "secession" of Gaul and Britannia, Galerius appealed to precedent and named the former Caesar, Flavius Severus as the new Augustus with control over Italy, Africa, and Spain and recognised Constantine only as a Caesar.E. Horst, Costantino il grande, Milano 1987, p. 88. Constantine voluntarily accepted this and in Autumn of the same year he returned to Augusta Treverorum (Trier) whence he could more easily monitor the Gallic frontier, which was being menaced by the Franks. He continued to defend this important stretch of the limes for the next six years, transferring his whole imperial court to Trier and transforming it into his capital (with c.80,000 inhabitants), constructing the imposing Aula Palatina in 310.E. Horst, Costantino il grande, Milano 1987, pp. 92–93 e 96. During these years, not only did he reinforce the defences of this region against the continued incursions of the barbarians, but he also strengthened the forces under his control,E. Horst, Costantino il grande, Milano 1987, p. 90. augmenting his forces through the creation of new legions.
307 At the beginning of spring, Constantine planned a new campaign in German territory. He found it necessary to confront the Franks, Chamavi, Bructeri, Cherusci and Alemanni. The young emperor had been educated in the military training ground of the east by Diocletian and Galerius and, despite his youth, conducted the war with the sort of determination and energy which his father had not been able to muster in the preceding years.E. Horst, Costantino il grande, Milano 1987, p. 98. In the course of the military operations, he achieved important successes, managing to heavily wallop the Franks who had invaded the Roman territory east of the Rhine the previous year.Eusebius of Caesarea, Life of Constantine, 1.25.1.Panegyrici latini, 7.6 QUI .Anselmo Baroni, Cronologia della storia romana dal 235 al 476, p. 1026. It is reported that, while the Franks were planning to cross the Lower Rhine, Constantine quickly crossed the river in another location and surprised the enemy with an unexpected attack, which prevented a new invasion. Many of the Franks were killed, captured or enslaved – some of these were employed as gladiators. All their livestock was seized and their villages were burnt to the ground.Panegyrici latini, 7.12–13 QUI . As a result of these successes, Constantine was awarded the cognomen Germanicus Maximus at the end of the year.Y. Le Bohec, Armi e guerrieri di Roma antica. Da Diocleziano alla caduta dell'impero, Roma 2008. pp. 46 & 53. In the course of this campaign and those following it, Constantine may have used the legionary fortress of Castra Vetera as a base and the valley of the Lippe (as had been done in the time of Augustus and again fifty years later under JulianAmmianus Marcellinus, History, 17.1.11; 17.10.4; 20.10.2.) as an invasion route by which to outflank the enemy, who were found to the north of this major river, and catch them from behind after devastating their territory.