As previously explained in ‘The Imperial Fall Of The Tetrarchy Northern Frontiers,’ it was between 313 and 316, that Constantine and Licinius maintained the cordial neutrality that had allowed them to work together during the last years of the civil wars, but their truce was uneasy and they came to blows in 316. The western Balkans fell to Constantine in this war. He took over Licinius’ residence at Sirmium, dividing his time between that city and Serdica, and leaving his son and Caesar Crispus in Trier to guard the Rhine frontier and campaign against the Franks and Alamanni. Constantine’s eastern ambitions were now clear, as his choice of residence could hardly fail to demonstrate, and he used the old tactic of disciplining the barbarians to provoke a final confrontation with Licinius.
In 323, Constantine campaigned against the Sarmatians on the frontiers of Pannonia, winning one battle, over a king called Rausimod, at Campona in the Pannonian province of Valeria, and a second considerably further downstream at the confluence of the Danube and Morava in Moesia Superior. Coins issued at Trier, Arles, Lyons and Sirmium celebrated the success with the legend Sarmatia devicta (“Sarmatia conquered”) and Constantine took the victory title Sarmaticus. He may also have instituted new celebratory gladiatorial games, as an epigraphic reference to Ludi Sarmatici, Sarmatian games, suggests. Regardless, the campaigns were a provocation of Licinius, into whose territory Constantine had marched while attacking the Sarmatians. Almost certainly intentional, this violation of his fellow Emperor’s sovereignty led to the final break between Constantine and Licinius — the latter supposedly melting down Constantinian gold coins celebrating the victory in order to make the point as publicly as possible.
In the ensuing civil war, both sides made substantial use of barbarian soldiers. Licinius had won a victory over the Goths before 315 and peace terms may have included Gothic service in his army. In the war against Constantine, Goths fought on the side of Licinius, probably under a general named Alica. Constantine had used Frankish auxiliaries in his earlier campaigns and by the time of the war with Licinius, the Frankish General Bonitus had reached a position of rank in Constantine’s army. As we have seen, barbarians had always served in Imperial armies, but there is some reason to think that the buildup to war between Constantine and Licinius represents a new phase in this phenomenon.
For one thing, the early 320s were the first period since the onset of the military crisis in the third century during which rival Emperors had ample leisure to recruit troops for themselves. For another, both Constantine and Licinius were competing for roughly the same pool of manpower, that is to say, barbarians from the middle and lower Danube — Sarmatians and Goths, generically “Scythians” — and such competition almost always increases both supply and demand. This increasing reliance on barbarian recruits is partly hypothetical but is probably confirmed by the testimony of the Caesars, a satire on his predecessors written by Emperor Julian, which is scathing about Constantine’s recruitment and subsidy of barbarians. Certainly, as the fourth century progressed, Emperors made more and more use of barbarians in filling up the ranks of the army. That being the case, it seems likely that the precedent set by Constantine and Licinius in the early 320s was validated by its very success: Constantine routed Licinius.
That victory allowed Constantine a free hand in the Balkans, which he used partly for grandiose construction schemes. The manpower which these projects required is attested by a dramatic increase in the region’s supply of bronze coinage in the late 320s. In the valley of the Porecka near the Iron Gates, a major wall system was put up to control threats from across the river. That was eminently practical, but a more spectacular venture was a new bridge over the Danube from Oescus to Sucidava, which in 328 established a real and a symbolic bridgehead onto what one source now calls the ripa Gothica.
Constantine also continued the tetrarchic program of constructing quadriburgia along the Danube. These small forts, enclosing less than one hectare, were a new development in the early fourth century. They were characterised by a tower at each of their four corners (hence their name) and were built both on the right bank of the river in the Roman provinces of Moesia Secunda and Scythia, and also on the barbarian left bank. Primarily useful for keeping the barbarians under observation, quadriburgia could also serve as advance posts for Roman military action. Although the whole Danube frontier received this sort of Imperial attention, the lower stretch of the river, and hence presumably the Tervingi beyond it, was the main focus. Thus in parallel to the Oescus-Sucidava Bridge, Constantine built a new quadriburgium at Daphne, on the left bank of the Danube across from Transmarisca.
How should we account for this focus on the stretch of the Danube opposite the lands of the Gothic Tervingi? Perhaps the most obvious explanation is the fact that Goths had fought on Licinius’ side in the recent civil war. But the support which the tetrarchs and Licinius seem to have given to the rise of Tervingian power in the region probably also worried Constantine.