Roman Legion in late Antiquity

[Nathan Ross]

as far as I am now aware neither the supposed 'increases of Severus', nor the 'cavalry reforms of Diocletian' have any direct evidence to support them.

You're right - this idea rests on fairly wobbly foundations! It's Gallienus, I think who is often credited with the reform of the cavalry as a separate 'mobile force', although this idea was recently attacked by Duncan Campbell in Ancient Warfare magazine. Diocletian, supposedly, sent the cavalry detachments back to the frontiers! Brigading together the cavalry components of several legions was no new thing though - Josephus and Arrian report it happening in the 1st-2nd centuries.

Severus certainly reformed the army, doubling the size of the Guard, raising new legions (I-III Parthica) and making the social conditions of military service more attractive - this last may in turn have been intended to boost recruitment and build up legionary numbers, perhaps following the losses of the Antonine era with its plagues and wars. Smith's The Army Reforms of Septimius Severus (Historia, 1972) says nothing about increasing the size of legions, or their cavalry component, however. In fact legions of 6000 men are mentioned under Marius, so perhaps this was always the official 'paper strength'.

The enlarged size of the legion cavalry component does seem to have been a product of the third century though, and some writers (e.g. Karl Strobel) date this to Severus, perhaps on the logical assumption that he was a military reformer anyway, and his only reign was long and stable enough to have introduced such changes - besides which, Severus's new II Parthica legion seems to have included (additional?) specialist troops like lanciarii, and perhaps extra cavalry too, to function as a sort of 'army in miniature'. An inscription from the reign of Alexander Severus shows that the equites of III Augusta were still commanded by an optio equitum, however, rather than a praepositus or even a centurion. So perhaps the enlarged cavalry force that we find with II Traiana in the Egyptian papyrii was indeed a product of the later third century?