Late Roman Army - Army Size

Army Size

The traditional view of scholars is that the 4th-century army was much larger than the 2nd century army, in the region of double the size. The late 6th-century writer Agathias, gives a global total of 645,000 effectives for the army "in the old days", presumed to mean at its peak under Constantine I. This figure probably includes fleets, giving a total of ca. 600,000 for the army alone. A.H.M. Jones' Later Roman Empire (1964), which contains the fundamental study of the late Roman army, calculated a similar total of 600,000 (exc. fleets) by applying his own estimates of unit strength to the units listed in the Notitia Dignitatum.

But the Agathias-Jones view has fallen out of favour with some historians in more recent times. Agathias' figure, if it has any validity at all, may represent the official, as opposed to actual, strength of the Constantinian army. In reality, the slim evidence is that late units were often severely under-strength, perhaps only about two-thirds of official. Thus Agathias' 600,000 on paper may have been no more than ca. 400,000 in reality. The latter figure accords well with the other global figure from ancient sources, by the 6th century writer John Lydus, of 389,704 (excluding fleets) for the army of Diocletian. Lydus' figure is accorded greater credibility than Agathias' by scholars because of its precision (implying that it was found in an official document) and the fact that it is ascribed to a specific time period.

Jones' figure of 600,000 is based on assumptions about limitanei unit strengths which may be too high. Jones calculated unit strengths in Egypt under Diocletian using papyrus evidence of unit payrolls. But a rigorous reassessment of the evidence by R. Duncan-Jones concluded that Jones had overestimated unit sizes by 2–6 times. For example, Jones estimated legions on the frontiers at ca. 3,000 men and other units at ca. 500. But Duncan-Jones' revisions found frontier legions of around 500 men, an ala of just 160 and an equites unit of 80. Even allowing for the possibility that some of these units were detachments from larger units, it is likely that Diocletianic unit strengths were far lower than earlier.

Duncan-Jones' figures receive support from a substantial corpus of excavation evidence from all the imperial borders which suggests that late forts were designed to accommodate much smaller garrisons than their Principate predecessors. Where such sites can be identified with forts listed in the Notitia, the implication is that the resident units were also smaller. Examples include the Legio II Herculia, created by Diocletian, which occupied a fort just one-seventh the size of a typical Principate legionary base, implying a strength of ca. 750 men. At Abusina on the Rhine, the Cohors III Brittonum was housed in a fort only 10% the size of its old Trajanic fort, suggesting that it numbered only around 50 men. The evidence must be treated with caution as identification of archaeological sites with Notitia placenames is often tentative and again, the units in question may be detachments (the Notitia frequently shows the same unit in two or three different locations simultaneously). Nevertheless, the weight of the archaeological evidence favours small sizes for frontier units.

At the same time, more recent work has suggested that the regular army of the 2nd century was considerably larger than the ca. 300,000 traditionally assumed. This is because the 2nd century auxilia were not just equal in numbers to the legions as in the early 1st century, but some 50% larger. The Principate army probably reached a peak of nearly 450,000 (excluding fleets and foederati) at the end of the 2nd century. Furthermore, the evidence is that the actual strength of 2nd century units was typically much closer to official (ca. 85%). In any case, estimates of army strength for the Principate are based on much firmer evidence than those for the later period, which are highly speculative, as the table below shows.