Unit strength of the remodelled late Roman army is much debated. The numbers of men in each fort may have been considerably reduced compared to the standards of the early Empire. The enormous numbers of men required to keep units up to full strength were probably never reached at any time, and it is likely that in the late period units were permanently under strength. Recruitment in the British provinces is undocumented, largely because soldiers lost the epigraphic habit of recording their origins and careers. The later Roman army relied increasingly on Germanic tribesmen to fill the ranks, and it is probable that this was how some units were kept up to strength in Britain, in addition to local recruitment.

There is no evidence that Germanic troops were brought to Britain as laeti and foederati, federate settlers with an obligation to provide recruits for the army. It has been suggested that the sections of the Notitia that might have listed foederati in Britain have been lost, but there is no proof for this theory (Mann 1977a: 14). There is ample literary evidence for the presence of various tribesmen, Burgundians under Probus, Frankish mercenaries under Allectus, and Alamanni with their leaders Crocus and Fraomar,3 but otherwise they are not attested, so that either they were part of special campaign armies or they were recruited into regular army units. The rise of Germans in the regular army was widespread in the later Empire, and in Britain the dux Fullofaudes and the comes Nectaridus are presumed to be of Germanic origin.

Unit commanders in the late Empire were predominantly tribunes. The title was not standardized, in that it did not denote the same rank with the same pay scales and level of authority, but was commensurate with the grade of troops. The tribunes who commanded cohorts ranked as the lowest in the hierarchy, and the tribunes of the field army as the highest. The cohorts of Hadrian’s Wall were commanded by tribunes. Commanders of alae, vexillations of cavalry, legions and the numeri were generally prefects (praefecti). These officers are listed in the Notitia as commanders of the alae and numeri under the Dux Britanniarum. Most of the units of the Saxon Shore forts are listed under praepositi, a title that had a wide application to all kinds of posts and did not denote a specific rank.

While the Empire was hard pressed during the later third and early fourth centuries, cash payments to the soldiers were commuted into payments in kind, except on those occasions when the emperor distributed cash donatives, usually on his accession and on his birthday. Supplies of food for men (annona) and animals (capita), and the production of arms, armour and clothing was centralized and State-controlled. Differentiation in pay scales was represented in multiples of the standard rations. With regard to supplies of food, Britain could boast a surplus, and shipped grain to the Continent, at least on occasion, which implies that food supplies were sufficient (Ammianus Marcellinus, 18.2.3; Zosimus 3.5.2). Each military unit may have farmed land around its base as well as requisitioning food from the local population, though this has not been proved in Britain.

The military clothing and armament of the late Empire can be reconstructed from sculptural representations and from archaeological finds. The most common finds are metal belt fittings, some of them highly decorated. In the late Empire it was the military belt rather than body armour that distinguished soldiers and state officials from civilians. Soldiers wore long-sleeved tunics, trousers and cloaks, and are often depicted in this mode of dress on sculptures and mosaics. Body armour comprised mail tunics (lorica hamata) or scale tunics (lorica squamata), helmets and shields (Southern and Dixon 1996: 89–126; Bishop and Coulston 1989, 1993).

Soldiers served for twenty years to qualify for honesta missio, or honourable discharge, but to qualify for full privileges as veterans (emerita missio) they had to serve for another four years, and then they were entitled to tax exemptions, and allotments of land or cash payments. These privileges applied to the comitatenses and eventually to the ripenses. The soldiers of the northern frontier, being the lowest ranking, may not have enjoyed the same privileges.

Though it is denied that the limitanei of the northern frontier formed a peasant militia, it is generally considered that they were less efficient than their predecessors, but since it is not really known precisely how the frontiers operated at any time, there is nothing to compare, and no firm evidence that the probably drastically reduced units were not performing their tasks properly. Limitanei were sometimes drafted into the field armies as pseudo-comitatenses, implying that they had not lost all their military skills. A more serious problem, which applies to all the military establishments in Britain, is that very little is known of the nature of the enemy, their origins and intentions. It is not known how the Romans perceived them or how they operated against them, and on these scores buildings and archaeological finds are mute.