After taking control of the western part of the empire in 365, Valentinian relied largely on diplomacy rather than outright warfare to maintain the frontier along the Rhine and the upper Danube, which had been established by Julian's campaigns. Ammianus noted that “even his harshest critic cannot find fault with his unfailing shrewdness in matters of state, especially if he bears in mind that it was a greater service to keep the barbarians in check by frontier barriers than to defeat them in battle” (Ammianus 29.4). In pursuit of this policy Roman forces were brought up to strength by energetic levies of barbarians and provincials in Gaul (Zosimus 4.12), and he constructed or refurbished the entire chain of forts and watch posts, and consolidated strategic bridgeheads along the river frontier.50 The fruits of this policy are illustrated by the treaty which was agreed with the Alemannic king Macrianus in 374:
A courteous invitation to come to the vicinity of Mainz was sent to the aforesaid king, who was himself clearly inclined to come to terms.…On the day appointed for the conference (Macrianus) stood majestically erect on the very bank of the Rhine, while his countrymen clashed their shields around him. From the other side the emperor, also attended by a host of officers of various ranks amid a display of gleaming standards, embarked in some river boats and came within a safe distance of the shore.…A pact of friendship was concluded with solemn oaths. The king who had been a source of such trouble withdrew mollified. From that time on he was to prove our ally. (Ammianus 30.3.4–6, trans. Hamilton)
Such ritualized diplomacy was typical of frontier relations with barbarian tribes at this date. Valens had dealt with the Gothic leaders on the Danube in an almost identical fashion in 369. For a moment it created the illusion that the wholly disproportionate relationship between the Roman Empire and minor Germanic tribal leaders was a transaction between equal sovereign partners. The principle was embedded in the Roman idea of a foedus, a treaty of alliance between nominally equal forces, in which the two parties swore to abstain from hostilities and to help one another against their enemies.
In the early 370s attention switched to the middle Danubian region and Valentinian's home province of Pannonia, where the defense of the frontier was in the hands of the magister militum Equitius, who had dowsed the last flickering embers of the revolt of Procopius, and had been responsible for building forts along the Danube, as part of the wider aim of securing the northern riverine frontier:51
By the imperial order of our masters Valentinian, Valens and Gratian, greatest of emperors, and by the instruction of the illustrious man, commander of the cavalry and infantry, the count Equitius, Toscanus, the commander of legio I Martiorum, together with the soldiers entrusted to his command, constructed this fort, whose name is Commerce (for which purpose it had been built) from its foundations and brought it to its final completion in forty-eight days during the consulship of our master Gratian Augustus and the most distinguished man Probus. (ILS 775 [near Gran in Pannonia], dated to 371)
This inscription reveals the double nature of such positions. They were both military strongpoints and locations for commercial dealings with the barbarians. The economic advantages of being able to trade with neighboring provinces explains the readiness of tribes living near the frontier to prefer stable peace to resistance to Rome, and the Quadi and Marcomanni of modern Moravia and Slovakia had a long history as friendly barbarians.52 We may compare the situation on the lower Danube frontier, where relations with Gothic tribes were on exactly the same footing. There is evidence for cross-border trade in the pattern of finds of Roman coins found in Gothic-held territory in the mid-fourth century.53
In 375 Valentinian spent three months at Carnuntum, and then led a punitive raid from Aquincum against barbarian villages across the river. After he returned to Brigetio representatives of the Quadi approached him seeking an amnesty and were admitted on the urging of Equitius. Their pleas involved shifting the blame for anti-Roman behavior onto foreign brigands who had infiltrated their territory near the river, but they included a reference to the affront which Roman fort-building across the Danube posed to them. This drove the irascible Valentinian into a fit of apoplectic anger, and he died of a seizure (Ammianus 30.6). Peace was made nevertheless, for the emperor's leading generals ordered that the bridge across the Danube, which had been built to enable an invasion of barbarian territory, should be demolished (Ammianus 30.10.3–6).