How powerful was the roman army towards the end of the 4th century compared to when it was at its peak?

Part I

This is my first content post, so I hope I’m doing it right. Apologies in advance for the length.

To begin to answer this question, it is important to define “powerful” and “peak.” For this response, I am going to assume that by “powerful” you mean “effective,” as the effectiveness of the 4th century army is a question heavily debated by scholars. I am also going to assume that your “peak” of the army is during the 1st and 2nd centuries A.D. (a period often referred to as the Principate). Our question is thus: how effective was the Roman army of the 4th century compared to the army of the Principate?

It is worth noting that the effectiveness of an army is difficult to determine, as by what metric is “effectiveness” to be measured? The simplest measure is perhaps battlefield performance, though this is complicated by the fact that victory is influenced by many factors, from simple numbers to morale to sheer luck. With this in mind, the 4th century army did win the majority of it’s battles, although Goldsworthy critically notes that quite often these victories were in civil wars against other Romans. Winning battles also does not necessarily mean winning wars; just ask Hannibal. This is a good, but not complete measurement. Another possible metric is how well the army defended the Roman state, as this is the premier goal of any army. On the surface, it would appear that the later army failed miserably, as the Western half of the Empire collapsed in the fifth century. This collapse though is complicated by the survival of the East, which retained the overall structure of the army until the Arab Invasions of the 7th century. This suggests that contemporaries saw little structural issues with the army, and questions the role the army played in the West’s collapse.

These are important considerations to keep in mind. As you can see, it’s hard to produce a universal metric by which effectiveness can be measured. It is therefore better to examine various aspects of the 4th century army, such as discipline, training, and equipment, and see how they compare with their predecessors. This will produce a more holistic and contextualized definition of effectiveness. This is the approach taken by modern scholars, who (spoiler alert) generally view favorably the 4th century army. I will try to keep the descriptions below as concise as possible, but these are complex topics. I am more than happy to expand upon anything.

Organization

Broadly speaking, the army of the Principate was comprised of around 28 legions of around 5,120 men, broken into ten cohorts of 480 men. Whilst the legions (in theory) consisted of Roman citizens, the many regiments of auxilia were recruited from the peregrini, the Empire’s non-citizen subjects. Auxiliary regiments, while supplying the bulk of the Empire’s cavalry, also supplied heavy and light infantry. The vast majority of this army was stationed along the Empire’s frontiers, with few forces stationed in the interior.

In contrast, the 4th century army was divided into two corps, the comitatenses, the troops which comprised the regional field armies stationed in the interior of the Empire, and the limitanei (sometimes referred to as ripenses in imperial legislation), the troops which guarded the frontiers. Differences between legionary and auxilia had disappeared. Aside from lengths of service, physical requirements, and remuneration, there appears to have been few qualitative differences between the comitatenses and the limitanei. They were not, as traditionally assumed, a part-time farmer-soldier militia which declined in quality (Mike Duncan describes them thus in his otherwise excellent History of Rome podcast and it drives me nuts!) but rather a thoroughly professionalized and competent corps.

In both the comitatenses and the limitanei the basic unit was the regiment, which were alternatively described as legiones, cohortes, or alae or vexillationes for cavalry (the exact terminology is vague, the classicizing tradition of history not really caring about such technical details). Evidence for the size of these units is limited, but it appears that infantry regiments were between 1,200 and 800 men and cavalry regiments between 600 and 400. It is likely that these numbers represent paper-strength, with units in reality much smaller. Units would also have a small administrative and medical staff.

Regiments were commanded by either a tribunus or praefectus, though both are often referred to as a praepositus. Limitanei regiments were stationed in frontier regions commanded by an officer known as a dux, who were responsible to the magister militum of their region. This officer commanded the regional field army, assisted by several senior officers known as comites (sing., comes). The field armies consisted of several thousand men, and were stationed in the interior of the Empire.

So, we have two major organization changes in the 4th century. Military units are now much smaller than during the Principate, and many thousands of soldiers are now stationed inside the Empire, rather than on the frontiers. Did these reorganizations have a beneficial or harmful effect?

It should be noted that during the Principate it was common for legionary detachments to be sent on assignments, often throughout the Empire. Normally these detachments (known as vexillationes) returned to their bases, but often they remained detached. It is doubtful that smaller units decreased the efficacy of the individual soldier, as they remained fully integrated into the army’s logistical support network.

From the reign of Gallienus (253-268), frequent military crises (both external threats and challenges from usurpers) facilitated constant imperial campaigning, and the creation of a corps of troops permanently attached to the emperor. Both Diocletian and Constantine personally commanded large escort armies (the comitatus, from which comitatenses is derived). Constantine’s sons divided his field army between them, creating separate field armies which eventually came to be assigned to specific regions. The creation of field armies and their role in imperial defense is a subject disputed by historians. One view, espoused by Elton and Whitby, views the creation of the field armies as a rational change in the Empire’s defenses (although Elton does not believe that the barbarian threat increased during Late Antiquity). Adrian Goldsworthy disputes these claims, believing that the creation of field armies had nothing to do with imperial defense, but rather were designed to protect the emperor from potential usurpers, the security of the Empire taking a backseat to the security of the reigning emperor. Thus, the effect of the strategic reorganizing of the Empire’s armies in the 4th century on the army’s efficiency is disputed.

Continued in Part II