The Roman Army of Diocletian

[Robert Vermaat]

I’ve been compiling the cavalry numbers in Ammianus and Zosimus and this is convincing me that the organisation of the units throughout Ammianus and Zosimus is derived from the organisation of the cavalry as given by Vegetius in regard to the 6000 man legion. Ammianus (24 5 10) has the emperor reduce the surviving members of the cohort, to the infantry service with loss of rank. Here, like the Vegetius legion, each infantry cohort is assigned a body of cavalry. Zosimus mentions a figure of “about 360 men,” which could be rounded from 330 men and is half the number of cavalry taken from Vegetius if you take away the cavalry assigned to the double cohort.

By following, Ammianus, with the army consisting of cohorts, maniples and centuries, I believe this is derived from the 10 cohort organisation and not the 12 cohort organisation.

Am I right that you say that the strength of each cavalry unit during the time of Ammianus is similar as the number of cavalry attached to a legion during the time of Diocletian? And why 'round off' 360 to 330?

And are you referring to the new style turmae or the old style alae which still existed?

The old style alae numbered 500 and seem to have remained that way. On paper at least. On the basis on the Beatty papyri, Duncan Jones calculated that around 300 AD in the Thebaid (Egypt), a unit of equites was 121 strong, an ala 116 and a cohors 164. These may not have been complete units, or else very much understrength.
Nothing much else is known about cavalry unit strengths. The ala III Assyriorum was organized in old-style 11 turmae, giving it a possible strength of 350 (ChLA XVIII 660). Ammianus mentions that the cataphracti defeated at Strasbourg were 600 strong, which is echoed by Johannes Lydus who says that alae were that number, and turmae 300 but also 500 (De Mag. I.46). Ammianus also mentions two turmae at Amida numbering 700 together (XVIII.8.2). Procopius has various sizes, between 200 and 800 strong (800: Bella VI.5.1, VI.7.25-6). Some units are larger, between 1000 and 1500, but it is unclear if these are units grouped together, or maybe allied forces (1500: Bella V.27.22-3 and VII.34.42). Maurikios mentions cavalry units should be between 300 and 400, but in any case not less than 200 and not above 400; if understrength, they should be combined.