Late Roman Army during the 5th century

[Nathan Ross]

Ammianus’ use of ‘old’ words was apparently chosen to create a ‘learned’ image of an author who had read his ancient sources.

Yes, he was writing consciously in the 'grand style', as he tells us in his last paragraph. I believe this means he was writing in style of Tacitus, as a deliberate continuator, and so uses the terminology that Tacitus and writers of his day used.

However, while the words might be archaic, I don't think this means that the things referred to didn't exist. All of Ammianus's references to 'auxiliaries' (their higher pay, barbarian origin, exemption from manual labour and hereditary service, etc) are clearly referring to the late Roman auxilia palatina, not the cohorts of the principiate. I think this was the point Adrian was making, actually...

Cohorts that were once formed as auxilia were regular units in the Limitanei border troops, but every auxilia unit that we know from for instance the Notitia Dignitatum belongs to a force of newly raised regiments, all with elite status, belonging to the field army.

Actually, this raises an interesting question - there are a number of cohorts and alae mentioned in the ND as part of the Egyptian garrison that appear to have a barbarian origin - Vandals, Iuthungi, Franks etc. Do we assume that these cohorts were raised on the same principle as the auxilia palatina, perhaps from laeti (and if so, why do they appear to be border troops rather than elite units?), or were they regular cohorts on the old model?