For years, the unstoppable Attila sacked city after city until a Germanic-Roman alliance halted the Huns in A.D. 451. The victory underlined a hard truth for the tottering empire: The barbarian threat could only be held at bay with the help of other barbarians.

Relations between the later Roman Empire and the barbarian tribes that massed on its northern border have been commonly portrayed as a straightforward, mutual hostility. In reality, the complex relationship between Rome and its neighbors grew more interconnected through the third and fourth centuries A.D.

A healthy Roman respect for Germanic tribes dates back at least to the time of Julius Caesar, who admired the rugged courage of his opponents in Gaul. Border skirmishes continued throughout the early empire, but the barbarian threat started to erode imperial authority itself during a series of disastrous reigns in the third century. During this time, severe economic crises weakened central Roman rule. Strapped for cash, successive emperors debased the currency to gain short-term financial relief, but instead sparked hyper inflation that disrupted trade and caused more economic turmoil.

Capitalizing on the chaos, Goths and other Germanic tribes began attacking Roman borders. To help repel these attacks, Rome began forming complex alliances and counter-alliances with the barbarians. More and more, the empire relied on German mercenaries to serve in its army. These men often found themselves defending the frontier against their own people.

Although order was restored by the accession of Diocletian in 284, the empire never recovered its former economic strength. The relationships formed with Rome’s northern neighbors lingered.

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A brilliant soldier and statesman, Aetius effectively directed Emperor Valentinian’s reign after becoming supreme commander of the Western Roman Empire in 432. Aetius had spent time as a hostage of the Huns, which gave him an insider’s knowledge of their culture. His experience in captivity had led him to establish valuable personal relationships with key Hun leaders. Aetius’s own rise to power was due to his shrewd employment of Hun mercenaries in the service of the empire. With their help, he had launched a series of military campaigns aimed at keeping the majority of barbarians settled in Gaul under control. Despite the abilities of its military leader, nothing better illustrates how much power the Western Roman Empire had lost than the need to cobble together such an alliance at all.

Estimates suggest that just 50 years before, the number of Roman soldiers in Gaul exceeded 50,000, but 50 years of civil skirmishes and neglect had depleted its ranks. By 451 there were probably only a few thousand Roman soldiers left in the province.