08-02-2013, 08:50 PM
Quote:what was the size of a cohort during that time, and how many in a legion (if there were only 1200 in a legion)?
There's very little hard evidence to go on, and a great deal of theorising!
It seems that the full strength of a legion in the 4th century was still officially 5-6000 men (Vegetius (Epitoma Rei Militaris I.17) claims that the Herculiani and Ioviani legions (perhaps actually I Iovia and II Herculia) had 6000 men - these seem to have been new formations of the late 3rd century). So this was perhaps the official number for the post-Severan period, even if most legions were very understrength.
However, many legions of this period sent detachments on active service, and these detachments seem to have comprised two cohorts, probably each under a tribune. The Oxyrynchus papyrii give details of Diocletian's field force in Egypt c.298, with these legion detachments listed in pairs brigaded together under a praepositus (perhaps one of the tribunes given a superior command role). If we assume that the cohorts still numbered around 4-600 men, this would make each detachment about 800-1200, and the brigaded pairs around 1600-2400 men.
Several of these detachments could be operating at one time away from their 'parent' legion - an inscription from Mauretania lists two cohorts of II Herculia, probably in the field army of Maximian, while other units of the Herculiani, perhaps from the same legion, were apparently serving in the east under Galerius and Diocletian, and perhaps also as garrison troops in the Crimea.
I suspect that what might have happened was that these mobile detachments in time acquired a semi-independent status as part of the imperial field armies, losing their connection with the parent legion. They became referred to as 'legions' in their own right, some of them using variations on the old legion name, while other new formations of the same size were raised alongside them. Thus we get the comitatensis 'legion' of c1200 men. Whether they maintained the two cohort split (perhaps this is the source of the 'maniple' mentioned by, I think, Ammianus and Vegetius?), or functioned as collections of centuries I don't think we know. Interestingly, the habit of brigading these 'legions' in pairs seems to have continued, and carried over into other units of the comitatus.
The parent legions, left back on the borders, would necessarily have been depleted by the departure of these detached cohorts, and further divided amongst various different garrisons. This might be why later legion fortresses (Troesmis on the Danube, home base of II Herculia, for example, or some of the forts in the east) appear to be smaller than the traditional bases. The legion itself was about the same size (perhaps), but it was spilt up into several smaller parts. These, then, would be the limitanei legions - a number of 3-4000 has been suggested, and seems right (although I read an article recently suggesting that numbers were very much lower... It probably varied over time.)
Alongside the legions, there were new formations altogether - the independent cohorts on the frontiers were surely the descendents of the old auxiliaries, still inferior to the legions it seems although now all citizens. There were also the 'numeri' of the auxilia, new-style auxiliary formations, some of which were taken into the imperial comitatus as auxilia palatina. Numbers for them are pretty much guesswork, I think. 800-1000 seems about right.
Evan mentioned the ducenarius - this rank is still not fully understood, but from what I can gather it was used only in the new-style formations - numeri of auxilia, cavalry equites and the bodyguard protectores.
If you can find it, take a look at Nicasie's Twilight of Empire - he provides about the most agreed-upon figures for later military units. Although I prefer A.D. Lee's War in Late Antiquity as a more readable alternative.
I've probably made quite a number of mistakes with the above, but hopefully others can chip in and expand further.
mile:
Nathan Ross
