The main source describing the demise of the Hunnic Empire is (the freagmentary remainder of) Jordanes' Getica. Jordanes, a Byzantine writing in the mid 6th century, claimed to have drawn his narrative from the Goth Cassiodorus' history from earlier in the century. This is likely true, but it is the source of a main concern in using evidence from the Getica: it is too centered on the Amal-led Goths (who were later Ostrogoths) and biased in favor of the ruling dynasty of those Goths. Heather concludes from reading between the lines of the Getica and pulling in other fragmentary evidence that the Huns suffered a slow bleed of subject peoples from the time of Attila's death, with peoples asserting their independence as they settled on internal leadership. The Gepids seem to have been the earliest to break away (and they confirmed their independence with victory at the battle of Nedao); the Amal-led Goths apparently didn't secede until Valamer had attained internal supremacy in a series of battles. As the Huns lost tribute and military manpower from more and more subject peoples, their position deteriorated to the point where they fled the Hungarian plain altogether for refuge in the Eastern Empire south of the Danube.
Heather argues that the Hunnic coalition was unstable from the beginning. The subject peoples were enrolled in the Hunnic Empire against their will and often treated harshly, but administered largely though their own native leadership. The Huns depended on intimidation and the distribution of the tribute from predation on the Roman Empire to keep the subject rulers in line. The Huns seem to have run into the limit of their ability to extract money from the Romans, however, and the defeat of successive invasions diminished perception of their power and probably reduced their revenue as well. This left the Attila's sons without the wherewithal to keep their vast empire intact.
Heather points out three significant consequences of the Hunnic collapse for the Roman Empire. First, it complicated the situation on the Danube frontier. The Romans now had to manage many frequently conflicting German tribes. Both the victors, who could be strong enough to exact tribute, and the losers, who often invaded or sought refuge in the Roman Empire, created challenges for Roman policy.
Second, the end of the Hunnic threat put Aetius in a precarious position. Power brokers in Roman politics judged that they could now do without his generalship. In fact, the emperor Valentinian assassinated Aetius in 454, only to be struck down in turn by co-conspirator Petronius Maximus the next year. (Petronius lasted for even less time.)
Third, the Huns could no longer be used as a mercenary counterweight to the barbarian groups already established within the Empire. Since the Western Empire itself no longer had the resources to contain them, either, barbarian groups had to be bargained with. From now on, they would play a leading role in the politics of the empire and the imperial succession. Thus Avitus, with the backing of the Visigoths, succeeded Petronius.
Here Heather has some fun taking apart the propaganda of Avitus' son-in-law Sidonius. Sidonius did his best to convince the Romans in Italy that Avitus was in control of the Visigoths rather than the other way around, and that the Visigoths were fine fellows anyway. The senators weren't buying, however, and Avitus was soon deposed by the Italian generals Majorian and Ricimer. From this, Heather draws the further conclusion that there were now too many factions to satisfy in the Western Empire: with the barbarians directly involved, no stable regime could be established
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