[3] 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.

However, it was only after the death of Gallienus (268) that a group of emperor-soldiers of Illyrian origin (Claudius the Gothic, 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.

[5] 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 tetrarchic system.

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.

[9][10] 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.

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.

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).

[17] At the death of Caracalla 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): A little less than a century later, during Diocletian's Tetrarchic period, the number of legions placed along the northern front (Rhine and Danube) was increased to 24, in addition to the 3 positioned to guard the Alps (legio Iulia Alpina, legio II Iulia Alpina, and legio III Iulia Alpina),[18] out of a combined total of 56 (or 48.2%), as highlighted below:[19] Concerning the massive forces that the barbarians were able to field in the course of the invasions of the 3rd century, it is briefly summarized below as follows: Caracalla defeated the Germans north of the Limes and obtained legality and friendship from them, so much so that he dressed his auxiliary troops like them and created with them his own personal bodyguard with selected men of great strength and good looks.[...]

Immediate recourse to force was necessary, deploying tactically superior armies capable of intercepting every possible avenue of invasion by the barbarians as quickly as possible; however, the strategy was made difficult by having to garrison immense stretches of frontier with mostly sparse military contingents.

[9] Many of the emperors who were gradually proclaimed by the legions over a period of twenty-five years did not even manage to set foot in Rome, let alone, during their very short reigns, undertake internal reforms, as they were permanently occupied with defending the imperial throne from other claimants and the territory from external enemies.

Numerous farms and crops had been destroyed, if not by barbarians, then by bands of brigands and by Roman armies seeking sustenance during military campaigns fought against both external and internal enemies (usurpers to the imperial purple).

[Gallienus] had as a concubine a girl named Pipa, whom he received when part of the province of Upper Pannonia was granted under a treaty to her father, king of the Marcomanni, given to him as a wedding gift.From 260 onward, and until about 274, the Roman Empire suffered the secession of two vast territorial areas, but these allowed its survival.

In the west, the usurpers of the Gallic Empire, such as Postumus (260-268),[132] Laelian (268), Marcus Aurelius Marius (268-269), Victorinus (269-271), Domitian II (271), and Tetricus (271-274), succeeded in defending the borders of the provinces of Britain, Gaul, and Hispania.

[133] These emperors not only formed their own senate at their major center of Augusta Treverorum and gave the classical titles of consul, pontifex maximus or tribune of the plebs to their magistrates in the name of Roma Aeterna,[134] but also assumed the normal imperial titling, minting coins at the mint of Lugdunum, aspiring to unity with Rome and, more importantly, never thinking of marching against the so-called "legitimate" emperors (such as Gallienus, Claudius the Gothic, Quintillus or Aurelian) who ruled over Rome (i.e., those who ruled Italy, the West African provinces up to Tripolitania, the Danubian provinces and the Balkan area).

[135] In the East, on the other hand, it was the Kingdom of Palmyra that took over from Rome the government of the provinces of Asia Minor, Syria and Egypt, defending them from Persian attacks, first with Odaenathus (260-267), appointed "Corrector Orientis" by Gallienus, and then with his secessionist widow, Zenobia (267-271).

The Goths were also beaten in a naval battle by General Venerian, and he himself died during the fight.And so the different tribes of Scythia, such as Peucini, Grutungi, Ostrogoths, Tervingi, Visigoths, Gepids, Celts and Heruli, attracted by the hope of looting, came to Roman soil and wreaked great devastation there, while Claudius was engaged in other actions [against the Alamanni, ed.]

The main task awaiting this last emperor was to have to reunite the two "trunks" that had been formed during the reign of Gallienus, namely, the Gallic Empire in the West and the kingdom of Queen Zenobia's Palmyra in the East.

[231] Having obtained power, in November 285 Diocletian appointed as his deputy (caesar) a valiant officer, Marcus Aurelius Valerius Maximianus, whom a few months later he elevated to the rank of augustus (April 1, 286): he thus formed a diarchy, in which the two emperors divided on a geographical basis the government of the empire and the responsibility for the defense of the frontiers and the fight against usurpers.

[232][233] Given the increasing difficulty in containing the numerous revolts within and along the borders, a further territorial division was made in 293 in order to facilitate military operations: Diocletian appointed Galerius as his Caesar for the East, while Maximian did the same with Constantius Chlorus for the West.

Because the barbarians were advancing fast, he was forced to enter the city, and because of the necessity of closing the gates so quickly, he was hoisted on the walls with ropes, but in only five hours the army arrived and tore to pieces about sixty thousand Alemanni.Also along the fourth and final frontier sector of the Roman Empire, the southern one, were numerous and continuous incursions by semi-nomadic African peoples, beginning in the mid-3rd century.

In the crisis that the Empire experienced in the third century, barbarian invasions undoubtedly constituted an extremely important element in the political, social, and economic evolution that led to the new Diocletian and Constantinian state.

In the face of external threats, military commanders endowed with ever-increasing powers had an easy time being acclaimed augusti by an army aware of its decisive role in choosing princes; if before the praetorian cohorts had their decisive weight in this regard, now the new situation created along the borders of the Empire undoubtedly privileged the limitanean legions,[276] revolutionizing a balance that had seen, in the first two centuries of the Empire, the praetorian cohorts represent, in their own ranks, a good part of the municipal Italic elites and of the provinces of ancient Romanization.

From the advent of Maximinus the Thracian, there was instead a gradual but ineluctable change of direction; figures who were expressions of the army, the viri militares, often of modest origin and raised in the ranks of the legions of the limes, obtained roles and powers that had previously been reserved for members of senatorial, Italic or provincial families.

Inevitably, the growing burden of the army, accompanied by political instability and the struggle for power, caused a monetary decline due, moreover, to the long-standing lack of liquidity from the conquests of previous centuries.

Silver, which was along with the denarii the most widespread currency, slowly declined, so much so that it caused the inflation that was at the center of every emperor's thoughts in the third century,[280] and which Diocletian tried in vain to save with his Edictum De Pretiis Rerum Venalium.

It was clear that any effort to maintain the status quo would not produce results within the institutional framework created in its time by Augustus: a new era was coming, and although the barbarian invasions had not by themselves caused the crisis of the third century, they accelerated the process of disintegration and estrangement between West and East that would be the basis of Late Antiquity.