The barbarian invasions of the third century (212–305) constituted an uninterrupted period of raids within the borders of the Roman Empire, conducted for purposes of plunder and bootyThus, these were not yet mass displacements of entire populations such as those that would occur in later centuries, when the irruption of the Huns into the European stage would induce many Germanic tribes to seek new sites of settlement within the Roman Empire. In the third century numerous hordes of warriors were on the move, mostly leaving behind them, in the territories where they had settled immediately beyond the Limes, the families and camps of the tribes; after a season or two of raiding, they would return to their bases, not caring to establish permanent colonies in Roman territory. by armed peoples belonging to populations gravitating along the northern frontiers: Picts, Caledonians, and Saxons in Britain; the Germanic tribes of Frisii, Saxons, Franks, Alemanni, Burgundians, Marcomanni, Quadi, Lugii, Vandals, Juthungi, Gepids and Goths (Tervingi in the west and Greuthungi in the east),Peter Heather, La migrazione dei Goti: dalla Scandinavia alla Tracia, in Roma e i Barbari, la nascita di un nuovo mondo, p. 239. the Dacian tribes of the Carpi and the Sarmatian tribes of Iazyges, Roxolani and Alans, as well as Bastarnae, Scythians, Borani and Heruli along the Rhine-Danube rivers and the Black Sea.
Since the time of Marcus Aurelius during the Marcomannic Wars (166/167-189), Germanic-Sarmatian tribes had not exerted such strong pressure along the northern borders of the Roman Empire.
The growing danger to the Roman Empire of Germanic peoples and Sarmatians was mainly due to a change from previous centuries in the tribal structure of their society: the population, constantly growing and driven by the eastern peoples, needed new territories to expand, or else the weaker tribes would become extinct. Hence the need to aggregate into large ethnic federations, such as those of the Alemanni, Franks and Goths, in order to better attack the neighboring Empire or to defend themselves against the irruption of other neighboring barbarian populations. For other scholars, however, in addition to the pressure of outside populations, it was also the contact and confrontation with the Roman imperial civilization (its wealth, language, weapons, and organization) that prompted the Germanic peoples to restructure and organize themselves into more robust and permanent social systems, capable of better defending themselves or seriously attacking the Empire.. Rome, for its part, had been trying since the first century A.D. to prevent the penetration of the barbarians by entrenching itself behind the limes, that is, the continuous line of fortifications extended between the Rhine and the Danube and built precisely to contain the pressure of the Germanic peoples..
The breakthrough by the barbarian peoples along the limes was also facilitated by the period of severe internal instability that ran through the Roman Empire during the third century. In Rome, there was a continuous alternation of emperors and usurpers (the so-called military anarchy). Not only did the internal wars unnecessarily consume important resources in the clashes between the various contenders, but – most seriously – they ended up depleting precisely the frontiers subjected to barbarian aggression.
As if this were not enough, along the eastern front of Mesopotamia and Armenia from 224 onward the Persian dynasty of the Parthians had been replaced by that of the Sasanids, which on several occasions severely engaged the Roman Empire, forced to suffer attacks that often joined the less strenuous but nonetheless dangerous invasions carried out along the African front by the Berber tribes of Moors, Baquates, Quinquegentiani, Nobati and Blemmyes. Rome showed that it was in serious difficulty in conducting so many wars at once and almost collapsed two centuries early.
It was also thanks to the subsequent internal and provisional division of the Roman state into three parts (to the west the Empire of Gaul, in the center Italy, Illyricum and African provinces, and to the east the Kingdom of Palmyra) that the Empire managed to save itself from ultimate collapse and dismemberment. However, it was only after the death of Gallienus (268) that a group of emperor-soldiers of Illyrian origin (Claudius Gothicus, Aurelian, and Marcus Aurelius Probus) finally succeeded in reunifying the Empire into a single bloc, even though the civil wars that had been going on for about fifty years and the barbarian invasions had forced the Romans to give up both the region of the Agri decumates (left to the Alemanni in about 260) and the province of Dacia (256-271), which had been subjected to incursions by the Dacian population of the Carpi, the Tervingi Goths, and the Iazigi Sarmatians.
The invasions of the third century, according to tradition, began with the first incursion conducted by the Germanic confederation of the Alemanni in 212 under Emperor Caracalla and ended in 305 at the time of Diocletian's abdication for the benefit of the new Tetrarchy system..
Historical context
After about thirty years of relative quiet along the Rhine-Danubian frontiers, a new crisis broke out along the Germanic-Rhaetian Limes in 212, caused by the first invasion of the Alemanni confederation.
The Germanic world between the 2nd and 3rd centuries
In Central and Eastern Europe, the barbarian world was shaken by strong internal unrest and migratory movements of populations that tended to change the balance with the neighboring Roman Empire. These peoples, seeking new territories in which to settle due to the increasing population growth in ancient Germania, were also attracted by the wealth and affluent life of the Roman world..
Fifty years earlier, on the fringes of the Germanic area, along the Danubian and Carpathian frontier, there had been movement and mixing of peoples, with the advent of a new phenomenon among the Germans, which represented an overcoming of the tribal dimension: entire peoples (such as Marcomanni, Quadi and Naristi, Vandals, Cotini, Iazigi, Buri, etc.), had grouped together in coalitions, mostly military in nature, implementing greater pressure on the nearby Roman limes.
Under Caracalla the phenomenon of aggregation had evolved further, going so far as to establish in the area of the Agri decumates some real ethnic confederations of tribes: the Alemanni, who aggregated Catti, Naristi, Hermunduri and part of the Semnones and positioned themselves on the upper Rhine from Mogontiacum to the Danube near Castra Regina; the Franks, on the lower Rhine from the mouth of the river to Bonn;. and the Saxons, composed of the seafaring peoples between the mouths of the Weser and Elbe rivers...
At the same time the thrust of the East Germans from Scandinavia also increased, such as the Goths (in the various branches of the Ostrogoths, Visigoths, and Heruli), who came from the Vistula: they had been slowly moving southeastward for more than fifty years, and had now come close to the northern shores of the Black Sea. In that region they came into conflict with the Sarmatian peoples of Roxolani and Alans. Also from the Silesian-Vistula region came two other major populations: the Vandals, who had already come into contact with the Roman legions of Pannonia and Dacia Porolissensis at the time of the Marcomannic wars under Marcus Aurelius, and the Burgundians, who were heading westward toward the rivers Elbe and Main..
Prelude to the invasions of the 3rd century: the Marcomannic Wars (166-189)
In 166/167, the first clash occurred along the Pannonian frontiers by a few bands of marauding Lombards and Osii, which, due to the prompt intervention of the border troops, were promptly repelled. The peace stipulated with the neighboring Germanic peoples north of the Danube was handled directly by the emperors themselves, Marcus Aurelius and Lucius Verus, who were now wary of the barbarian aggressors and traveled for these reasons as far as distant Carnuntum (in 168).Historia Augusta - Marcus Aurelius, 14.1-5 The untimely death of Lucius (in 169 not far from Aquileia), and the breaking of pacts by the barbarians (many of whom had been "clients" since the time of Tiberius) led a never-before-seen wave of troops to swarm devastatingly through northern Italy all the way to below the walls of Aquileia, the heart of Venetia. The impression caused was enormous: it was since the time of Marius that a barbarian population had not laid siege to centers in northern Italy.Cassius Dio, Roman History, LXXII, 3.1.
Marcus Aurelius fought a long and exhausting war against the barbarian populations, first repelling them and "cleaning up" the territories of Cisalpine Gaul, Noricum, and Rhaetia (170-171), then counterattacking with a massive offensive in Germanic territory, which took several years of fighting, until 175. These events forced the emperor to reside for several years along the Pannonian front, never returning to Rome. However, the truce signed with these peoples, particularly the Marcomanni, Quadi and Iazyges, lasted only a couple of years. By the end of 178 the emperor Marcus Aurelius was forced to return to the castrum of Brigetio from where, in the following spring of 179, the last campaign was conducted.Cassius Dio, Roman History, LXXII, 20.2. The death of the Roman emperor in 180 soon put an end to Roman expansionist plans and resulted in the abandonment of the occupied territories of Marcomannia and the making of new treaties with the "client" peoples northeast of the Middle Danube.
Forces in the field: along the Rhine and Danube European front Numerous were the legionary and auxiliary forces fielded during this period by the Roman Empire. The numbers are difficult to estimate, since in the course of the century some units were destroyed and replaced with new ones; moreover, when Diocletian's new tetrarchical system took over, it was the overall strategic organization itself that changed.
Romans
In the 3rd century the Roman Empire deployed numerous legions against barbarian invasions: I Adiutrix, I Illyricorum (recruited under Aurelian), I Italica, I Maximiana (under Maximian), I Minervia, I Pontica (under Diocletian), II Adiutrix, II Italica, legio II Parthica, III Italica, IIII Flavia, IIII Italica (under Alexander Severus), V Macedonica, VII Claudia, VIII Augusta, X Gemina, XI Claudia Pia Fidelis, XIII Gemina, Legio XIIII Gemina Martia Victrix, XV Apollinaris, XX Valeria Victrix, XXII Primigenia and XXX Ulpia Victrix.. The total forces fielded by the Roman Empire may have exceeded 200 to 250,000 armed men from the beginning to the end of the third century; of these, one half were legionaries, the remainder auxiliaries..
At the death of Caracalla in 217 out of 33 legions along the entire system of imperial fortifications, as many as 16 were along the Rhenish and Danubian limes (accounting for 48.5% of the total), as well as 2 others in the rear as "strategic reserve" (in Hispania and Italy), as is highlighted below in the summary table on their deployment (in 217):