Why did the Eastern Roman Empire survive during the fifth century, when the Western Empire fell?

This is a vast topic, so whilst I will attempt to cover all the important points, do note that there is always more to say on this question.

First of all, we have to note that there were issues beyond any Roman'' control. Michael Kulikowski's recent Imperial Triumph: The Roman World from Hadrian to Constantine (2016), for example, pointed out that the acquisition of Caucasian gold mines would prove to be important to the wealth of the eastern empire in the fifth century. This steady source of gold bound together the eastern elite with the structures of imperial government, whereas the western provincial elite had less incentive to be so closely tied to the centre. This, for Kulikowski, partially explains why it was the west that fractured rather than the east when faced with the geopolitical aftershocks of convulsions in the Eurasian steppes. This is only one factor, but it is a useful reminder that we also have to look beyond the fifth-century Mediterranean for explanations, for events beyond the empire had nonetheless contributed to the transformation of the Roman world.

More traditionally, we can point to the wealth of the more urbanised east, particularly in Egypt and the Levant, neither of which were seriously threatened in the fifth century. The Sassanid Persians, although a geopolitical rival, only engaged in two brief wars with the Eastern Roman Empire (a far cry from the conflict-ridden sixth century), which provided Constantinople with room for manoeuvre when threats emerged elsewhere. The western empire did have its own wealthy provinces, but they were in much greater danger. A crucial cause of its eventual 'fall' is for example frequently identified as the Vandal invasion of North Africa, which took away a safe, dependable tax-base from the western treasury. Little wonder then that imperial forces, both eastern and western, dedicated so much effort to try to take it back, the last attempt as late as 468.

However, if I remember a conference paper last year correctly, then North Africa was also subject to mass internal migrations, quite apart from the Vandal invasion - the so-called 'Völkerwanderung', or the Migration Age, has been frequently used only to refer to European migrations, but we need to remember that late antiquity was a time of fluidity and mobility, and that again, events beyond western Europe ought to be taken into account. Who knows, perhaps North Africa was not as stable as it used to be anyway (problems with the Moors were certainly present under Vandal rule), undermining therefore the western empire even when it had control of it. The same convulsions were not visible in the eastern breadbasket province of Egypt and I can only think of the Isaurians in Anatolia as another example of an internal non-Roman group (or at least, were not seen as Romans by the 'proper' Romans) that caused trouble. Even then, the Isaurians were all about placing themselves into positions of power in Constantinople, to gain control of the court, not to bring the whole institution down. The resulting political instability was not helpful, but imperial legitimacy was not seriously threatened.

Spain and Gaul were of course not worthless, but by the fifth century the empire could not rely on them. By then, the western empire was facing powerful organised groups, not disorganised 'barbarians', an ironic result of the empire's violent treatment of its neighbours. The 'barbarians' were not hellbent on the empire's destruction, but instead sought to find a more privileged place for themselves within the imperial structure. This meant the creation of client kingdoms in order to receive gold and offices from the Roman state, the only difference to the past was that they were now placed within imperial provinces, not beyond the (always nebulous) border. Indeed, the most famous of these 'barbarians', Alaric the Goth (who sacked Rome in 410), ought to be seen as a Roman general gone rogue rather than anything else, no different from any other rebellious governor or usurper from this time.

It is worth expanding on this point, as political fragmentation was more common in the west than in the east. In the eastern empire, there were no rogue generals who carved out a few provinces for themselves or usurpers who succeeded in overthrowing the legitimate emperor (the first successful attempt would be Phocas' coup in 602). The same was not true in the west. Constantine III for instance revolted in 407, ostensibly to protect the Rhine frontier, but in practice created a separate domain for himself, whilst a plague of even shorter-lived rebels also thought that they had a good chance of becoming emperor throughout the fifth century, with one revolt even in North Africa. The same problem can be seen in the fourth century, when the eastern emperor Theodosius I had to attack the west twice to remove its usurpers. Political stability, even through puppet rulers, was seemingly difficult to find in the west, whereas in the east succession was largely peaceful (barring some palace intrigue and political purges in Constantinople of course).

In these circumstances, it was logical for western provincials to turn to local armies to protect them, as opposed to the distant government in Italy (which sometimes they didn't recognise as the legitimate authority anyway, if the person in charge was an usurper). These local bigwigs were sometimes 'barbarians', but since they had a recognised place in the imperial structure and had Roman titles, it wasn't so unusual for the provincials to do this. Over time, these leaders also took on an administrative role, becoming regional governors in all but name. This was not a dramatic change by any means and, as Kulikowski rightfully pointed out, these 'barbarian' kings continued to behave like client-kings until the 460s, with occasional bouts of tension, warfare, and independence when the political situation changed. Then again, the 'proper' Roman generals were doing the exact same thing, so it's hard to really blame this on the 'barbarians'. In any case, over time it would become more normal for the distant court in Ravenna to be ignored - there was certainly no need for a centralised empire that could not protect you, when you have a Roman general around who could do the same job.

This I feel is the crux of the matter, that the western empire collapsed not because it faced more invasions, had less wealth, or simply because of luck. Instead, it was because political fragmentation made it acceptable for local Romans to go their own way. The eastern empire on the other hand did not experience this, as the political centre was always based at Constantinople and its authority was not seriously challenged (nor could it be, due to its wealth). People went to the court to seek the favour of the emperor (or his puppet-master), not to make their own emperors on the fringes. Yet in the west we have pretenders popping up everywhere - why go to Italy when you can have a piece of the action closer to home? Throughout this period, Romanness itself was never threatened, so people never had to defend their way of life against someone genuinely trying to turn the world upside down. Instead, political competition was what mattered and it was the consequences of this infighting between the Roman elite, expressed in different ways, that shaped the different political trajectories of east and west.

It is perhaps a good idea to end by talking about what happened afterwards, which can help us understand the earlier period a little better. This is because well after 476, people in the west continued to recognise imperial suzerainty, even when Constantinople could do very little to interfere with the 'post-Roman' kingdoms. As succinctly summed up in the New Cambridge Medieval History:

The beginning of the century saw Anastasius (491–518) on the imperial throne, ruling an empire that was still thought of as essentially the Roman Empire, coextensive with the world of the Mediterranean, however unrealistic such a view seems to modern historians, who have the benefit of hindsight. Although Anastasius ruled from Constantinople, ‘New Rome’, over what we call the ‘Eastern Empire’, the Western Empire having been carved up into the ‘barbarian kingdoms’, this perspective is ours, not theirs. Through the conferring of titles in the gift of the emperor, and the purchasing of alliances with the wealth of the Empire – wealth that was to dwarf the monetary resources of the West for centuries to come – the barbarian kings could be regarded as client kings, acknowledging the suzerainty of the emperor in New Rome, and indeed the barbarian kings were frequently happy to regard themselves in this light. The discontinuation of the series of emperors in the West, with the deposition of Romulus Augustulus in 476, was regarded by very few contemporaries as a significant event: the notion that East and West should each have its own emperor was barely of a century’s standing, and the reality of barbarian military power in the West, manipulated from Constantinople, continued, unaffected by the loss of an ‘emperor’ based in the West.

The western empire had disappeared, but the Roman world itself did not fall. Client kings ruled on behalf of the emperor as they had always done. For some, such as the supporters of Theoderic the Great in Italy, they could even imagine their ruler, a patrician and ex-consul who had been educated in Constantinople, as a new emperor of a revived western empire, acclaiming him as a man 'born for the good of the res publica', 'propagator of the Roman name', and, most intriguingly of all, as 'semper Augustus' - 'forever Augustus'. When we say that the Western Roman Empire fell, we therefore have to be very careful about what we mean. What collapsed was the centralised infrastructure that held a diverse range of provinces together. Roman institutions, culture, and people on the other hand did not fall at all.