But if the Roman Empire (in the West) went down fighting, why did it collapse? Of course there is no simple answer to that question. The mass migrations of the fourth and fifth century clearly played a very large role, but then the Romans had defeated other such migrations (recall the Cimbri and the Teutones) before. There are strong indicators that other factors, unrelated to our current topic were also at play: the empire had been economically weakened by the Crisis of the Third Century, which may have disrupted a lot of the trade and state functions that created the revenue to fund state activity. At the same time, the Crisis and the more challenging security situation after it meant that Roman armies grew larger and with them the burden of paying and feeding the soldiers which further hurt the economy. Meanwhile, long exposure to Roman armies on the frontiers of the empire had begun to erode the initially quite vast qualitative advantage the Romans enjoyed; the gap between Roman and “barbarian” military capabilities began to shrink (although it never really vanished altogether in this period). But some of the causes do bear on our topic but in quite the other direction from what the Niall Fergusons of the world might assume.

Let’s start with the foederati.

After the Constitutio Antoniniana, there was no longer much need for the auxilia, as all persons in the empire were citizens, and so the structure distinction between the legions and other formations fades away (part of this is also the tendency of the legions in this period to be progressively split up into smaller units called vexillationes, meaning that the unit-sizes wouldn’t have been so different). But during the fourth century, with frontier pressures building, the Romans again looked for ways to utilize the manpower and fighting skill of non-Romans. What is striking here is that whereas in some ways […] the auxilia had represented almost a revival of the attitudes which had informed the system for the socii, the new system that emerged for using foreign troops, called foederati (“treaty men”) did not draw on the previously successful auxilia-system (which, to be clear, by this point had been effectively gone for more than a century). Instead, the Romans signed treaties with Germanic-speaking kings, exchanging chunks of (often depopulated, war-torn frontier) land in exchange for military service. Since these troops were bound by treaty (foedus) they were called foederati. They served in their own units, under their own leaders, up to their kings. Consequently, all of the mechanisms that encouraged the auxilia to adopt Roman practices and identify with the Roman Empire were lost; these men might view Rome as a friendly ally (at times) but they were never encouraged to think of themselves as Roman.

The reason for this different system of recruitment seem to be rooted in financial realities. The Roman army had already been expanded during the Crisis of the Third Century and only grew more under Diocletian and Constantine, probably by this point being between 400,000 and 500,000 men (compared to 300,000-350,000 earlier in the empire). Moreover, Diocletian had opted to reform the empire’s administration with a much more intensive, top-down, bureaucratic approach, which imposed further costs. Taxes had become heavy (although elites were increasingly allowed to dodge them), the economy was weak and revenues were short. The value of the foederati was that the empire didn’t have to pay them; they were handed land (again, in war-torn frontier zones) and expected to use that to pay for their military support. At the time, it must have seemed a brilliant work-around to get more military power out of a dwindling tax-base.

(I feel the need to note that I increasingly regard Diocletian (r. 284-305) as a ruinous emperor, even though he lacked the normal moralizing character flaws of “bad emperors”. While he was active, dedicated and focused, almost all of his reforms turned out to be quite bad ideas in the long run even before one gets to the Great Persecution. His currency reforms were catastrophic, his administrative reforms were top-heavy, his tax plan depended on a regular census which was never regular and the tetrarchy was doomed from its inception. Diocletian was pretty much a living, “Well, You Tried” meme. That said, to be clear, Diocletian wasn’t responsible for the foederati; it’s not quite clear who the first foederati were – they may have been the Franks in 358, which would make Julian (as a “Caesar” or junior-emperor under Constantius) the culprit for this bad idea – he had a surplus of those too.)

The problem, of course, is right there: the status of the foederati made it impossible for them to ever fully integrate into the empire. They had, after all, their own kings, their own local laws and served in their own military formations. While, interestingly, they would eventually adopt Latin from the local population which had already done so (leading to French, Spanish and Italian) they could never become Roman. That wasn’t always their choice, either! As O’Donnell (op. cit.) notes, many of these foederati wanted to be “in” in the Roman Empire; it was more frequently the Romans who were busy saying “no”. It is striking that this occurs in a period where social class in the Roman world was generally calcifying. Whereas citizenship had been an expanding category, after the Constitutio Antoniniana, the legal categories of honestiores and humiliores (lit. “respectable” and “humble” people, but in practice, “wealthy” and “commoners”) largely replaced citizenship as the legal dividing lines of Roman society. These were far less flexible categories, as economic social mobility in the ancient world was never very high. Even there, the tax reforms of Diocletian (with some “patches” under Constantine) began, for tax purposes, to tie tenant farmers (“coloni“) to their land, essentially barring both physical and economic mobility in the name of more efficient tax collection in a system that strongly resembled later medieval serfdom.

Nevertheless, the consequence of this system of organization was that as often as the foederati provided crucial soldiers to Roman armies, they were just as frequently the problem Roman armies were being sent to address. Never fully incorporated into the Roman army and under the command of their own kings, they proved deeply unreliable allies. Pitting one set of foederati against the next could work in the short-term, but in the long term, without any plan to permanently incorporate the foederati into Roman society, fragmentation was inevitable. The Roman abandonment of the successful older systems for managing diverse armies (on account that they were too expensive) turned the foederati from a potential source of vital manpower into the central cause of imperial collapse in the West.

Bret Devereaux, “Collections: The Queen’s Latin or Who Were the Romans, Part V: Saving and Losing and Empire”, A Collection of Unmitigated Pedantry, 2021-07-30.