Late Roman Legion size based on the Perge Inscription

[Nathan Ross]

Anyone willing to provide evidence that Vegetius’ legion of 6,000 infantry is not historical?

I think you've addressed that yourself! :

Vegetius’ mistakes... Vegetius getting confused about... Vegetius has incorrectly added... Vegetius has also made a mistake... Vegetius has erroneously allocated...

Seriously though... Vegetius is describing the 'ancient legion', so it's supposed to be 'historical' and not the organisation of his own time (whenever that was!). It's possible that what he describes is close to the genuine legion organisation of some point in the second or early third century, maybe. But, as you've confirmed yourself, there do seem to be mistakes in the picture he presents. 

It looks very much like he's reorganised the 6-century cohort into a 5-century cohort, on the model of the old enlarged first cohort, which for some reason does seem to have had only five double-strength centuries rather than six. As we know, six centurion positions within the normal cohort are known from the early third century, and so this is probably wrong, and so the legion size and structure is distorted accordingly.

As for the 12 cohorts... I'm sure we've discussed this before but I still cannot imagine why any army would restructure itself in order to make camp. Why would they do that? What purpose would it serve? Would it not cause total chaos, every evening and then every morning while reorganising to march out of camp? What madness would have compelled the Romans to do this, and (perhaps more importantly) why did nobody ever notice or report them engaging in this strange activity?

As for the cavalry unit itself, it may be that the smaller size of the late Roman cavalry regiments (ie Equites, Comites, Cuneus) allowed for the legions themselves to incorporate a greater amount of cavalry than the 120 Equites Legionis previously available to them.

Hmm, maybe - although the evidence that we have (both literary and, for example, fort sizes) suggests that all units were getting smaller, not just the cavalry. 

But I think it's quite likely that the legions of the 2nd/3rd century had more than 120 cavalry. If you take a Flavian-era legion of 9 normal cohorts (each comprising 6 centuries of 80 soldiers, plus optio, signifer, centurion = 83 men) and one 'double sized' first cohort (comprising 5 centuries of 160 soldiers, plus optio, signifer, centurion = 163 men), and add a 'Josephan' 120-man cavalry complement you get 5417 men. But if you increase the cavalry by 2.5 to 300 men, the total is 5597; add the aquilifer and praefectus castrorum, and we're close to Suetonius's 5600. So perhaps even by his day the legions had more cavalry?

(alternatively, of course, Suetonius was just describing a legion with a larger century. Century size is one of the greater imponderables of all these calculations.)

I still don't think we can be certain about identifying the 'veredarii' with cavalry, of course. I'm also perplexed about the apparent clear division of their number into 9 subunits of 25 and one double unit of 50... Surely nothing like a 'Vegetian' milliary first unit could have survived into the army of the late 5th century? Or could it...?