That frankly unusual structure for a multi-ethnic imperial army [the non-citizen auxilia numbering about half the total “Roman” army] brought three principal benefits for the Roman army and consequently for the Roman empire itself.

The most obvious of these is manpower. Especially with a long-service professional army, capable and qualified recruits are in limited supply. The size of the Roman army during the imperial period ranged from around 300,000 to around 500,000, but in 14 AD (the year of Augustus‘ death) there were only 4,937,000 Roman citizens (Res Gestae 8.11), a figure which probably (a word I am using to gloss over one of the most technical and complex arguments in the field) includes women and children. Needless to say, keeping something close to a fifth of the adult male citizen population under arms continually, forever was simply never going to be feasible. After his victory in 31 BC at Actium, Octavian (soon to be Augustus) had acted quickly to pare down the legions, disbanding some, merging others, until he reached a strength of just 28 (25 after the three legions lost in 9 AD were not replaced). It was a necessary move, as the massive armies that had been raised during the fever-pitch climax of the civil wars simply could not be kept under arms indefinitely, nor could a short-term service conscript army be expected to garrison the hundreds of miles of Roman limes (“frontier, border”) in perpetuity.

Harnessing the manpower of the provinces was simply the necessary solution – so necessary that almost every empire does it. By their very nature, empires consist of a core which rules over a much larger subject region, typically with far greater population; securing all of that territory almost always requires larger forces than the core’s population is able or willing to provide, leading to the recruitment of auxiliaries of all kinds. But whereas many imperial auxiliaries, as noted above, turn out to be potential dangers or weaknesses, Rome’s auxilia seem to have been fairly robustly “bought in” on the system, allowing Rome to access motivated, loyal, cohesive and highly effective manpower, quite literally doubling the amount of military force at their disposal. Which in turn mattered a great deal because the combat role of the auxilia was significant, in stark contrast to many other imperial armies which might use auxiliaries only in subsidiary roles.

The auxilia also served to supply many of the combat arms the Romans themselves weren’t particularly good at. The Romans had always performed very well as heavy infantry and combat engineers, but only passably as light infantry and truly poorly as shock cavalry; they generally hadn’t deployed meaningful numbers of their own missile cavalry or archers at all. We’ve already talked a lot about how social institutions and civilian culture can be important foundational elements for certain kinds of warfare, and this is no less true with the Romans. But by recruiting from subject peoples whose societies did value and practice the kinds of warfare the Romans were, frankly, bad at, the Roman skill-set could be diversified. And early on, this is exactly what we see the auxilia being used for (along with also providing supplemental heavy infantry), with sagitarii (archers), funditores (slingers), exploratores (scouts) and cavalry (light, heavy and missile), giving the Romans access to a combined arms fighting force with considerable flexibility. And the system clearly works – even accounting for exaggerated victories, it is clear that Roman armies, stretched over so long a frontier, were both routinely outnumbered but also routinely victorious anyway.

As Ian Haynes notes, the ethnic distinctiveness of various auxilia units does not seem to have lasted forever, though in some cases distinctive dress, equipment and fighting styles lasted longer. Most auxilia were posted far from their regions of origin and their units couldn’t rely on access to recruits from their “homeland” to sustain their numbers over the long haul (although some number of recruits would almost certainly come from the military families of veterans settled near the forts). But that didn’t mean the loss of the expertise and distinctive fighting styles of the auxilia. Rather skills, weapons and systems which worked tended to get diffused through the Roman army (particularly in the auxilia, but it is hard not to notice that eventually the spatha replaces the gladius as the sword of the legions). As Ovid quips, Fas est et ab hoste doceri, “It is right to learn, even from the enemy” (Met. 4.428); the Romans do that a lot. The long-service professional nature of these units presumably made a lot of this possible, with individual cohortes and alae becoming their own pockets of living tradition in the practice of various kinds of fighting and acclimating new recruits to it. Consequently, not only did the Roman army get access to these fighting-styles, because the auxilia were actually integrated into the military system rather than merely attached to it, they also got the opportunity to adopt or imitate the elements of the fighting styles that worked.

Finally, the auxilia system also minted new Romans. We’ve already mentioned that auxilia veterans received Roman citizenship on retirement, but that wasn’t the extent of it. We can see in inscriptions that the degree of cultural fluency that soldiers in the auxilia gained with Roman culture was high; they often adopted Roman or Romanized names and seem to have basically always learned Latin (presumably because their Roman officers wouldn’t have spoken their language). While some units of the auxilia kept distinctive national dress as a sort of uniform, most of the auxilia seem to have adopted a style of dress that, while distinct from the legions, was generally in keeping with the Roman tradition of military dress (which was not quite the same as Roman civilian dress). They also partook of the Roman military diet (Roman soldiers kept a similar diet all over the empire, even if that meant shipping thousands of amphora of olive-oil and sour wine to northern England) which would have given them a diet in common with many work-a-day Romans too. Once retired, auxilia soldiers tended to settle where they served (rather than returning to their “home” provinces), which meant settling in frontier provinces where their citizenship set them apart as distinctively Roman, wherever they may have come from.

Exactly how many auxilia would have retired like this requires a degree of number crunching. Given a 20-year tour of service and zero mortality, we might expect around 7,500 men to pass through the auxilia each year. But of course, mortality wasn’t zero and so we have to expect that of our c. 20-year-old recruits, some number are going to die before retirement. Using some model life tables (following B. Frier, “Demography” in CAH^2 XI (2000)), we should figure that very roughly one third of our recruits will have died before reaching discharge. We then we need to adjust our recruitment figures to retain the same total strength and we get something like 9,000 new recruits each year to keep a strength of c. 150,000 with mortality counted for and 20 year tours. That gives us roughly 6,000 auxilia living to retirement each year. That may seem a small number, but that gradual accretion matters when it runs for decades and centuries and the newly enfranchised family units (recall that the citizenship grant covers children and sort-of-kind-of his spouse¹) tend to settle on the frontiers, which is a really handy place to have communities of citizens. If we assume that these new citizen families mostly reproduced themselves (or more correctly that they went extinct or split with multiple children at roughly the same rate with no natural population growth), then we’d expect this process to produce perhaps something like 1.5 million new citizen households up until the Constitutio Antoniniana. Being very back of the envelope then, we might – once we account for women and children descendants of those soldiers – assume that on the eve of the general grant of citizenship in 212, there were perhaps 4 million Romans whose citizenship status was a product of service in the auxilia somewhere in their history; perhaps representing something like 7% of the entire population (including non-free persons). Were we to assume larger households (which seems wise, given that retired auxiliaries are probably more likely than average to be in an economic position to have a larger family), that figure would be even higher.

That is a very meaningful number of new Romans. And those figures don’t account for some of the other ways Roman citizenship tended to expand through communities both through manumission but also the political networks citizenship created (your Latin-speaking former-auxiliary citizen neighbors are a lot more likely to be able to help intercede to get you citizenship or get your community recognized as a municipia with that attendant citizenship grant). And not only are those new Romans by legal status, but new Romans who have, by dint of military training and discipline, absorbed quite a lot of Roman culture. As best we can tell, they tended to view the Roman Empire as their polity, rather than as a foreign or oppressive entity. They were “bought in” as it were. Again, this does not seem to have been the Roman intent, but rather an opportunistic, self-serving response to the need to maintain the loyalty of these troops; citizenship was, after all, a free benefit the emperor might bestow at no cost to the treasury (since citizens who lived outside of Italy still owed taxes) or himself.

Of course that fits the auxilia in to a later pattern in the provinces which becomes perhaps most apparent as the Roman Empire begins to collapse …

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.

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1. Note on the coverage of the spouse. The grant of citizenship covered any biological children of the discharged auxiliary but did not extend citizenship to his wife. It did however, give an auxiliary the right to contract a lawful marriage with effectively any free woman, including non-citizens and the children resulting from such a union would be citizens themselves. Consequently, it extended one of the core privileges of citizenship to the non-citizen wife of a discharged auxiliary: the right to bear citizen children. Since the wife would be part of the retired auxiliary’s household (and then later, if he predeceased her, potentially in the household of her male citizen children) she’d be legally covered in many cases because a legal action against her would generally be an action against her husband/child. Given that a number of the rights of citizens simply didn’t apply to women in the Roman world (e.g. office holding), this system left the wife of a retired auxiliary with many, but not all, of the privileges of citizenship, so long as her husband and her marriage survived. That said, the legal status remained vested in her husband or her children, which made it more than a little precarious. One of these days, we can talk more about the structure of the Roman familia.