This page created 14 July 2015, and last modified: 10 October 2015 (legion derivation link added). This page is still "under construction"...
Much has been written about how in the late Roman army, by the time of the compilation of the Notitia Dignitatum, cavalry had become the main strike force, along with the relatively new-fangled auxilia palatina. Nonetheless, the importance of these troops can be overstressed, for it can be seen from an examination of the Notitia that legions were still very important indeed, as a simple tally of unit numbers easily attests:
I say "simple", but of course such a tally is far from simple. I have removed certain duplications from the above tally, but not others, depending on whether I believe the duplications are temporal or not, for example. But these details do not affect the overall impression: the total is somewhere on the order of 250 units or detachments of units, and thus making up a quarter of the units listed in the Notitia. Further, since there is every reason to believe legionary units were, on average, the largest formations in the army in terms of personnel, a simple tally like this actually underestimates their continued importance.
Palatine legionary units: 25 Comitatenses legionary units: 69 Pseudocomitatenses legionary units: 33 Limitanei legionary units and detachments: 100 Naval "legionary" units and detachments: 18
Legionary units in the Notitia are categorised in a number of different ways. Some are to be found in the field armies, either as legiones palatinae, legiones comitatenses, or (legiones) pseudocomitaenses. Other legions are limitanei units, stationed along the empire's frontiers, and, depending on their station's location, might be identified further (or instead), as being milites (although that word was also used of auxiliaries), or as ripenses.
One difficulty in quantifying the importance of legionary units in the Notitia is that for the limitanei units, unlike field army units, it is not units themselves that are listed, but rather officers of units. Thus under the Dux Moesiae secundae the following officers are listed:
76.21 Praefectus legionis primae italicae novasi.e. a prefect of the legion I Italica, at Novae (i.e. Svishtov in Bulgaria); a prefect of the riparian legion I Italica with the 5 "upper" cohorts, at Novae; and a prefect of the riparian legion I Italica with the 5 "lower" cohorts, at Sexaginta Prista (i.e. Ruse in Bulgaria). In this case, it would appear the first prefect is in charge of the HQ, which just happens in this case to be stationed in the same place as 5 of the legion's cohorts, while another five are located downstream. In contrast, under the Dux Moesiae primae, we have the following:
76.22 Praefectus ripae legionis primae italicae cohortis quintae pedaturae superioris novas
76.23 Praefectus ripae legionis primae italicae cohortis quintae pedaturae inferioris sexagintaprista;
78.20 Praefectus legionis quartae flaviae singiduno;i.e. a prefect of the legion IIII Flavia, at Singidunum (i.e. Belgrade in Serbia); and there is no concrete further indication as to whether this might be just an HQ station, an HQ plus some unidentified number of cohorts, or an entire legion of (e.g.) 10 cohorts. One might conclude from the absence of any further units entitled Legio IV Flavia in the Notitia that the entire unit was stationed there (whatever "entire" may have meant), and yet we know from epigraphic evidence that, at least at some point in time, the unit was indeed split into upstream and downstream sections like I Italica was (see Peter Kovacs, The late Roman army in Pannonia (2004), available here). It may have been so even in the time of the Notitia, if none of the sub-units had an officer with enough dignitas to warrant mention in the Notitia Dignitatum - the "Register if Dignitaries".
Further complicating any analysis is that some portion of a large unit may appear in another place in the Notitia under a different name, e.g. as the Honoriani felices Gallicani in the particular case of Legio IIII Flavia. That Legio I Italica has officers listed in charge of 10 cohorts, plus another (presumably HQ) station, would seem to indicate that that legion was up to full strength when the Notitia was compiled, and yet there is good reason to believe the palatine legion the Primani may have also been a detachment of that very unit, as most assuredly the pseudocomitatenses legion the Prima Italica (15.30 ) is, in which case Legio I Italica, plus its offshoot(s), would amount to more than a "full" legion.
Since the Notitia gives no unit strengths, one must resort to combination of external evidence, plus internal inferences, to judge how many men the Notitia legionary units might have amounted to. What is clear is that it is impossible that every legionary unit in the Notitia each had the five-or-so thousand men a legion had in the the days of the Principate, since this would require absurd manpower levels - there were only about 30 to 40 legions at most in existence at any one time during the 1st, 2nd and 3rd centuries, a tiny fraction of the 250-odd units recorded in the Notitia. The evidence would appear to suggest that legions were still this size at the end of the 3rd century, even the newly-raised ones (although here "newly-raised" may actually mean "created by amalgamating former auxiliary units"). However, by the time of the Notitia a hundred years later, most, perhaps all, must have been considerably smaller, since their numbers had grown considerably; the question, of course, is how much smaller? This question has been dissected by Terence Coello (Unit Sizes in the Late Roman Army, BAR 645 (1996)), without a firm conclusion; but epigraphical and papyrological discoveries since then have added further evidence, and things are perhaps starting to become slightly clearer.
Most of the evidence now appears to point to typical late 4th-/early 5th-century legionary units being somewhat under 1200 men in strength. See e.g. the Transtigritani (15.35) for a unit of probably very close to 1200 men, and also the Quinta Macedonica (15.16) for a unit closer to 750 men. These would appear to correspond to two old-style cohorts, full-strength (or even over-strength) in the former case; understrength in the latter. But whether a legionary "cohort" specifically itemised in the Notitia as such (e.g. as in those of Legio I Italica mentioned above) was of the same size as such an old-style cohort is a separate question, however. Note that an inscription from Perge in Pamphylia (Turkey), dating to the later part of the 5th century, apparently gives something like 1600 men on the rolls of what appears to be a palatine legion; this may represent a brigaded pair of legions; see Fatih Onur, in Momumentum Pergense, here, 2nd-last page.
Vegetius in describing his "former legion" is now believed to have been working with a (mostly) 3rd-century model, albeit with many contemporary references as well, which included, e.g., cavalry in addition to the infantry, and also included artillery, whose crew were apparently not extra to the infantry. By the time of the Notitia, the cavalry seem to have been removed (see Equites promoti), and since artillery were now also apparently separated out (see Balistarii), their parent units may have shrunk in size to provide the crews (or else replacements would have to have been recruited to fill the gaps in the ranks, or to provide new crews to man the machines). Shrunken units would have kept the overall size of the army the same, but would likely have been an organisational headache (unless "ghost soldiers" were officially encouraged in the units' books!). Since literary sources such as Lactantius and Zosimus indicate the armies really were larger than before, retaining the old organisation with extra units might comport better with the evidence of unit numbers from the Notitia.
In many cases, it is plain to see that field army legionary units listed in the Notitia are detachments of limitanei units listed elsewhere in the document: e.g. the afore-mentioned Prima Italica (15.30) is instantly recognisable as deriving from Legio I Italica due to its name. In other cases, the coonection is not obvious, at least from interanl evidence, but is attested from a source external to the Notitia, such as the Divitenses seniores (98/9.23) descending from Legio II Italica. In other cases, connections can only be inferred, rather than demonstrated concretely; and in some cases, no connections can be made. This page tracks these connections, both concrete and hypothetical.
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