Late Roman Unit Sizes

[Nathan Ross]

I Always had my doubts about the origins of the auxilia palatina being among the old auxilia cohorts

...even during the life time of Vegetius, the auxilia seems to be still of strong barbarian origin and character...

This is one of these questions that we lack the evidence to answer adequately, but I think I'd take a middle course.

It does seem likely to me that the roots of the various later Roman auxiliae lie in the assorted 'barbarian' contingents that seem to have frequently accompanied imperial expeditions from the later 3rd century onward. From Aurelian and Probus, certain of these contingents may have been given regular status as old-style cohorts and alae (like the Cohors Gotthorum and Ala Iuthungorum in Egypt in the ND), while others may have been used as garrison forces on the frontiers (the limitanei 'auxilia' of the ND).

At some later point, either under Diocletian or Constantine, this mass of semi-irregular 'barbarian' formations, many of whom may have been serving (under their own leaders?) with the imperial forces for many years, were given a more regular status in the Roman army as auxilia numeri, with a Romanised rank structure, state-supplied equipment and training (and presumably some sort of standard size too!).

After this date, while these units may have preserved aspects of their 'barbarian' origins, and may have relied heavily on manpower from across the frontiers and from settled laeti communities, they also took in Roman citizen recruits, perhaps volunteers. In time this 'Romanisation' of the auxilia came to dissolve the boundaries between them and the legions; quite possibly by this point both sorts of unit had come to have a generally similar size anyway.