The fall of the Western Roman Empire: the CliffsNotes
The most important data has long vanished
The proximate causes of the fall of the Western Roman Empire are straightforward -between 408 and 476 the Western Roman military withdrew first from Britain (408/9), then from Tunisia (439), then from the rest of northwestern Africa (455), then from Spain (461), and, finally, from Gaul (476, with remnants not aligned with the central government collapsing in 486) into its core territories in Italy and Dalmatia, surrendering its territory to various Germanic and less well-organized native chiefdoms. Why it did so is not entirely clear and answering this question would have to depend on information we simply do not possess. In some cases, the withdrawals occured due to strongmen attempting to rule the Empire fighting each other (more on this later). In some cases, the withdrawals clearly happened due to defeats at the hands of the barbarians -by far most notably the Visigoths, originating in a soldier rebellion of Germans of largely Ukrainian origin starting in 395, and the Vandals, a tribe originating from present-day Slovakia and crossing into the Empire in 406. It’s certainly plausible to think up a scenario in which the Empire falls to the barbarians much as it did without any civil disturbances among its leadership. It’s reasonable to suppose the migration of the Visigoths, Radagaisus’s Goths, Vandals, and Sueves was caused by the Huns -Ammianus Marcellinus states it explicitly in the case of the Visigoths, Socrates Scholasticus states it explicitly (though he doesn’t specify the timeframe, except that the raids began prior to 430) in the case of the Burgundians, and the timeline matches closely with the textual and archaeological evidence in the case of the rest. All the peoples who had a hand in destroying the Roman Empire (Turks, Goths, Arabs, etc.) were some variety of seminomadic barbarian. It is clear such peoples must have had certain advantages over established Empires. The Visigoths could defeat the Roman Empire even when there was no internal conflict within it to speak of -consider the battle of Adrianople. The largest combinations of Visigothic and Vandal forces were likely similar in number to, if not larger than, what the Romans could combine to throw at them (we can’t prove this, but this seems likely given the information in the sources). But a barbarian takeover without preexisting and existing problems of military leadership within the Empire is simply not what happened. It is also possible that the military rivalry of Romans attempting to seize the Empire to have devolved into a war of all against all, constantly shrinking the Empire’s tax base and resulting in growing independence of outlying regions. But that’s not what happened, either.
Though the heart of the empire could be subjected to barbarian raids during the best of times (consider the Marcomannic wars), it is abundantly clear that the Roman Empire did not enter into the fifth century at the peak of its strength, but, rather, that it declined before it fell. There was a great divergence in the propensity to civil war between the Roman East and Roman West during the fourth century. During the third century crisis, both sides of the Roman Empire could be expected to yield a similar amount of tyrants. However, starting with the usurpations of Constantine and Maxentius, the West became increasingly likely to bring forth usurpers who used Roman forces to fight against the established government. Why this is so is not clear. It is possible that this was due to the Western Empire, having to defend itself only against Moors, Picts, and weak German barbarians, having less strategic frontiers to defend than the Eastern, which had to defend itself against the more powerful Persians, Huns, and Goths, but this is not likely as tyrants tended to arise even in the most dire external security situations throughout Roman history. Kulikowski attributes it to the difference between the powerful in the Eastern Empire being more dependent on a functioning central government than that in the West, which relied more on private landholdings, making the job of a local usurper easier. This is also possible, but it seems likely that any Eastern usurper could, if they so desired, coerce local villages into providing them funds for their revolt. In any case, neither the armies on the Persian or Egyptian front revolted in the Roman East during the early fifth century, while those on the Gallic and African fronts certainly did revolt in the Roman West.
It is, unlikely, however, that the tense political situation in the West could have destroyed the Empire on its own. Local Romano-Celtic forces were simply much, much weaker than conventional Roman armies. Though the third century crisis did result in the destruction of the cities of classical Gaul, it is abundantly clear that the destruction of the Roman Empire’s key sources of tax revenue by Romans fighting each other was not what happened, whether during the fourth or fifth centuries. Britain recovered quickly after the usurpation of Carausius and the parts of Gaul not raided by barbarians experienced a substantial rural (though not urban) renaissance during the fourth century. Needless to say, as A.H.M. Jones has long ago pointed out, the Empire’s tax revenues during the fourth century came far more from the country than from the city. However, though the wars of the Western usurpers were exceedingly unlikely to destroy the Empire’s key sources of tax revenue, they could and did dramatically weaken the state of its frontiers. The frontier in northern and western Britain shows dramatic deterioration under Magnus Maximus (who was, on the other hand, remembered fondly in Gaul and Hispania by the likes of Orosius and Sidonius -see Halsall’s Barbarian Migrations and the Roman West, 376-568, p. 209). Troubles start even earlier in northern Gaul, where Magnentius’s revolt seems to have been devastating for frontier defenses (see any relevant source on this, e.g., the Oxford Handbook of the Archaeology of Roman Germany, Esmonde-Cleary’s The Roman West, AD 200-500, etc.). After the completion of Theodosius’s wars against Maximus and Eugenius, the evidence for Roman military weakness on both the Rhine and Danube from both the textual and archaeological sources becomes patently obvious. The existing sources continuously testify to a shortage of manpower among both Eastern and Western militaries during the decade leading up to the Great Invasions of the middle of the first decade of the fifth century. Vegetius was wrong. “A sense of security born of long peace” was not the cause of the late fourth century Roman Empire’s military weakness, nor did a restoration of violent conditions reverse its military’s enervation. Similarly, there was simply no way he was right when he claimed “it costs less to train one’s own men in arms than to hire foreign mercenaries”, as the hiring of foreign mercenaries only accelerated when the state’s budget came under stress. At this point, however, neither military was totally impotent -the Eastern Empire dealt with the Huns in Asia, the revolt of Tribigild, and the Huns of Uldin, while the Western Empire solved the revolt of Gildo, some barbarian raids on the upper Danube (a strategically inessential front), and, most importantly, the invasions of Alaric in 402 and of Radagaisus in 406. Needless to say, the East progressively solved this shortage over the course of the fifth century, while it became increasingly dire in the West.
I attempted a brief account of the Germanic conquest of the Western Empire here, but it inevitably grew to too great a length due to the many historical events involved. Instead, consult the map above. The Visigothic invasion of Italy in 408 came about due to Constantine III’s takeover of Gaul in early 407, the Great Invasion of Gaul by the Vandals, Alans, and Sueves on the last day of 406, and the killing of the de facto ruler of the Western Empire Stilicho in 408, which resulted in the disintegration of the Italian army. It is not clear when or why the Burgundians moved across the Rhine, but it was likely in 408. The Vandal, Alan, and Sueve move into Hispania in late 409 came about due to the instigation of the British general Gerontius, who rebelled against Constantine III by setting up Maximus in Hispania and attempted to seize Gaul. The Visigothic move into Gaul came about due to an alliance between the Visigoths and the Italian government to defeat the Gallic usurper Jovinus, who succeeded Constantine III in 411 and was overthrown in 413, resulting in the restoration of Gaul to Italian control. The Visigoths were settled in the region of Toulouse in 418 as a reward for their campaigns against the Alans and Vandals in Hispania. The Vandal crossing from Spain to Morocco coincided with a period of poor relations between the Italian government and the Visigoths between 425 and 439 as well as a conflict between Felix in Italy, Aetius in Gaul, and Bonifatius in Africa between 427 and 433 and the aftermath of the civil war between Bonifatius and the Eastern government on one side and John and Aetius on the other in 424 and 425. Their takeover of Carthage took place in the aftermath of a Roman war against the Visigoths and Burgundians. An Eastern Roman armada meant to take back Carthage for the West was recalled to the East in 441 due to Attila’s wars in the Balkans. The Vandals expanded their kingdom by taking the last Western Roman possessions on the African continent with the killing of the Emperor Valentinian III in 455 (which resulted in the disintegration of the Italian army), while the Visigoths expanded theirs into Hispania and Narbonne and the Roman army in northern Gaul rebelled in the aftermath of the killing of Majorian in 461 as a result of a failed attempt to recapture Tunisia from the Vandals in 460. The Visigoths continued to expand into Southern Gaul beginning in 471, after the failure of the combined East-West offensive against the Vandals in 468 and the outbreak in 470 of a conflict between the armies of the emperor Anthemius and the general Ricimer. The Italian government agreed to give up Clermont during the rule of Julius Nepos in Italy and the Visigoths under Euric finally took Arles and Marseille during the rule of Orestes in Italy. The best books on the matter are Guy Halsall’s Barbarian Migrations and the Roman West, 376-568 (2007) (soon to be in its second edition) and Michael Kulikowski’s Imperial Tragedy (2019), so I recommend to read those for the full picture.
The simple fact is that the ultimate cause of the fall of the Western Empire is, in the absence of a uniquely compelling and extremely unlikely to be written argument, something we will never know. The most important sources relating to the fall of the Western Empire -the Empire’s budgetary figures and troop numbers- are not available to us. The most useful primary source extracts relating to the fall of the Western Empire are probably parts of the Theodosian Code and Novels, especially those relating to the provisioning of troops and adjustments to tax payments. The second most useful are probably the speeches of Claudian, which, though do not extend to the most important part of the relevant period, are still useful enough to tell us what the Western government was thinking in the leadup to the fifth century crisis. The closest thing to a summary of the numbers of troops available to the Empire are the historical sources (e.g., Zosimus) and the Notitia Dignitatum, the first of which are too sparse to be useful and the latter of which, while useful for the East, is an anachronistic mess for the West.
And for those who haven’t yet…