We saw through history how Diocletian won a victory over one group of Goths, the Tervingi, as the panegyric of 291 attests. We do not know what prompted the campaign that led to that victory, but the decade that followed seems to have witnessed the substantial growth of Tervingian power. Although this Gothic expansion is not attested by positive evidence, it can be inferred from other known events, most importantly the displacement of an older barbarian grouping. Sometime before early 307, Galerius fought a campaign against the Sarmatians, which is to say in the region between the Danube and the Tisza rivers. Then, in the summer of 307, he attacked the Carpi further east, settling a very large number of them in a Roman province south of the Danube as defeated subjects of the Roman Empire.
The willingness of the Carpi — virtually all of them, it seems — to be removed from a territory in which they had dwelt for well over a century is significant. It suggests that the military pressure of a neighbouring barbarian power had become too great for them to sustain and that their attempts to find refuge in the Roman Empire had provoked a punitive Imperial campaign. The Gothic Tervingi are the barbarian group most likely to have affected the Carpi in this way. We seem, in other words, to see an increasingly powerful Tervingian policy near the mouth of the Danube extending its power at the expense of its immediate neighbours, perhaps with the tacit support of the Imperial government.
That support can probably be inferred from the fact that the tetrarchs fought no campaigns against the Tervingi after 291. On the contrary, Goths may have been recruited into the Imperial army and served with Galerius in Persia, though the only evidence comes from Jordanes and is therefore suspect. It is thus quite likely that the tetrarchs were complicit in the build-up of Tervingian power, viewing them as a favoured barbarian group which could help keep other barbarians in check further up the course of the Danube.
There was a real logic to that approach. While the lower Danube was consistently under the firm control of an Emperor resident in the Balkans (first Galerius, then Licinius), the provinces of the middle and upper Danube were the usual setting for confrontations between rivals in the years after 305. Because this Imperial preoccupation with the upper and middle Danube lasted for a full two decades after 305, Imperial support of Tervingian hegemony in this period is quite plausible. It would, moreover, allow us to make sense of two massive ditch-and-rampart wall systems which were built around this time in Bessarabia and Galatz, well beyond the Imperial Frontiers. Like the long east-west wall system known as the Csörsz-árok, built beyond the Pannonian frontier in modern-day Hungary, these fortifications are of a quality and on a scale that could not have been attained without imperial approval.
From the Imperial point of view, it would be useful to have a reliable Gothic ally keeping the lower Danube quiescent. By favouring the Tervingi, allowing fortifications to be built in their lands on such a scale, their strength and security could act as an additional layer of imperial defence, allowing emperors to focus on more immediate threats elsewhere. Imperial support along these lines explains why the Tervingi are so much more powerful when we next meet them in our sources, around the year 320.
In the meantime, however, the tetrarchic experiment had broken down entirely. Diocletian and Maximian abdicated in 305, for reasons that remain extremely controversial. Galerius and Constantius became Augusti, but the choice of new Caesars caused problems. Rather than the sons of Maximian and Constantius, who had long been groomed for the succession, two of Galerius’ close supporters were appointed as Caesars. Before long, however, both the imperial children had seized the purple for themselves. After his father died at York in 306, Constantine was acclaimed Emperor, supposedly at the instigation of the Alamannic king Crocus, a client of the late Constantius and an early example of a barbarian noble holding a high position in the Imperial army.
Maxentius, the son of Maximian, was proclaimed Emperor in Rome in the same year, with the support of the Roman populace. Constantine’s proclamation was soon recognised by the senior Augustus Galerius, but Maxentius was never accepted as a legitimate Emperor. For half a decade between 307 and 313, the Roman empire was wracked with civil wars that gradually eliminated most of the key claimants to the Imperial title. By 313, there were only two emperors left, Constantius’ son Constantine (r. 306–337), now a fervent Christian, in the West, and Licinius (r. 308–324), an old comrade of Galerius, in the East.
Despite their violence, the civil wars of 307–313 demonstrate the basic solidity of the Diocletianic reforms, because the hallmarks of the third-century crisis are entirely absent from the post-tetrarchic conflicts: no provincial general made an opportunistic bid for the throne, no provinces broke away under their own imperial succession, and no barbarian kings exploited the situation to launch a major invasion across the frontiers.
Indeed, a firm hand was kept on the Imperial frontiers despite the active civil war. Even before they had done away with other rivals, Constantine and Licinius between them controlled most of the Rhine-Danube frontier. Both undertook traditional imperial campaigns into the barbaricum, Constantine leading Frankish kings in triumph at Trier, Licinius attacking Sarmatians near the Danube bend.
As always, we cannot know precisely what prompted the individual campaigns, but the perpetual demand for Imperial victories, combined with a need to control barbarian politics while preparing for internal Roman conflict, can explain most of the fighting. A similar calculation probably lies behind the momentous propaganda decision which Constantine took in 310. In the old tetrarchic ideology, Constantius had been the adoptive son of Maximian, and hence took on his adoptive father’s putative descent from the god Hercules, along with the name Herculius that represented it. In 310, however, Constantine repudiated the Herculian name which he had inherited from Constantius. He instead began to claim descent from the Emperor Claudius Gothicus, a fiction first attested on July 25th, 310.
It made sense for Constantine to rid himself of the old Herculian connection after his final break with Maximian and Maxentius in 310, but there may have been more to it than that. Claudius, one of the third century’s great military heroes, won his Gothic victories in the Balkans. Constantine’s claim to a Claudian descent may be the first indication of the Balkan ambitions he was to demonstrate before too long.
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