Cavalry and auxiliary units were either about 500 men (Cohors Quineraria) or larger (Cohors Milliaria) and legions were up to 6,000 prior to Diocletian. After the reforms attributed to Diocletian and Constantine, there is considerable speculation about unit sizes. Legions were split into more permanent and smaller detachments, which may explain the reduction in legion strength. The parent unit and its detachments may have been nominally a total of 6,000. During his campaign in Egypt, Diocletian used vexillationes drawn from 18 legions with each detachment comprising troops from two legions (the use of pairs was common during the period). Some of these vexillationes remained in Egypt to form the garrisons of the reorganised provinces.
Using pre-Diocletian unit strengths as benchmarks, one could assume that a legion could be up to 6,000 or more troops, and auxiliary units were either approximately 500 or 1,000. According to Ammianus Marcellinus and other contemporary historians (and supported by the Notitia Dignitatum), units were often paired (as mentioned above) so that auxiliary units operated in bodies of 1,000 to 2,000.
Detachments could have been formed from sub-units of one or more centuries (about 80 men) or cohorts. According to Barker, legionary detachments may have been based on the ordo (about 500 men), possibly an alternate name for cohort. Post-Diocletian detachments may have been based on a single cohort or equivalent of about 500 or less. By the 6th Century, most arithmoi were about 300-500, which may indicate the end state of the transition from the 2nd Century organisation. Larger units were common, but possibly only temporary groupings for campaigns.
Whatever the intended strength of the units, many were subject to shortfalls because of combat, medical problems, desertion and corruption. The latter was the result of commanding officers inflating the number of men assigned to their units in order to pocket the salaries. This is one of the more infamous practices of military forces and has been recorded throughout the ages. There was also reluctance by some troops to campaign far from their homes. This may have resulted in the retention of the unwilling troops as a garrison to prevent desertion en route to the area of operations. Marcellinus comments on the reluctance of Gallic units to travel to Syria for a campaign against the Persians. These troops had enrolled with the understanding that they would only serve in the west.
The reduction in unit strengths may have been a deliberate policy started under Diocletian to reduce the military forces of would be usurpers. It is possible that the field armies of the tetrarchs outnumbered all other military commanders; however, all field armies were small and thus needed reinforcements in the form of detachments or special recruiting/conscription for major campaigns. If this was official policy, it did not discourage rebellion as evident in the many civil conflicts of the period.
There was possibly long standing consistency in the size and titles of units as many of the 4th Century organisation survived in the Byzantine military, despite the switch from traditional Latin to vulgar Latin, and finally Greek names. The legion, V Macedonia, was recorded in the 6th Century, which means that it existed in various forms for over 600 years. Numerus becomes arithmos (also called tagma and katalogos) in Greek, which still represented about 500 men. Meros of 6,000 and taxies of 1,000 may also be the natural successors of the legion and milliaria cohort respectively. An alternate name for the latter may have been millenarii (used by Victor Vitensis in his description of the census of the Vandals prior to them crossing from Spain to Africa in 429 AD). Procopius in his history of the Byzantine war with the Vandals uses the title chilliarchoi, which is the Greek equivalent of a commander of 1,000 (chillarchy or taxi).
There are indications that legions contained a variety of specialty troops such as artillerymen (ballastarii), skirmishers (exculcatores, lanciarii, velites, expediti, ferentarii, psiloi), scouts (exploratores, speculatores, procursatores) and cavalry (promoti, stablesiani). Again the actual numbers of each troop type are difficult to determine. During the early 4th century, some of these sub-units may have been amalgamated into independent units. The Notitia Dignitatum lists three artillery legiones comitatenses and other artillery units, and units of skirmishers, scouts and legionary cavalry. Alternatively, some or all may be new units created to complement existing units. Mixed auxiliary units (cavalry, infantry and camelry) are known from earlier periods and may have endured into the 4th Century.
While there are many indications of the creation of specialist units, some, particularly the legions, were probably still expected to do a variety of tasks. Sub-units could have been assigned to engineering, scouting, skirmishing and manning artillery.
Historians comment on the size of the field armies suggesting that they were significantly smaller than earlier armies, which is indicative of the difficulty in raising large armies. Conversely, the small size of the opposing forces and logistical requirements may be more important factors in considering army size. Most of the barbarian forces were small and tended to avoid battles. The Romans fielded similar sized forces that could move quickly to deal with barbarians. Larger Roman forces would be both slower in moving and concentrating because of supply requirements.
For example, Elton (pg 237) states that Julian's combat troops needed daily supplies of 30 tons of grain, 13 tons of fodder and 30,000 gallons of water during the 357 AD campaign against the Alamanni. These figures do not include the requirements of the baggage train and ancillary personnel. Since Roman armies endeavoured to minimise their dependence on baggage and supply trains, which slowed the rate of movement, reliance was placed on the soldiers' ability to carry much of their own food (20-30 days worth) supported by a small number of wagons.
Goldsworthy also suggests that the size of Roman units was influenced by the changes in doctrine. Smaller units would be formed into ad hoc formations for a specific operation or campaign. Forces could be tailored to oppose specific enemies and operate in particular terrain.
Despite its size, most of the Roman military was very static. Limitanei were garrison troops and there was a trend amongst the comitatenses to become garrisons for large urban centres (where their families usually lived). It is therefore likely that a quick reaction to any threat larger than a raid would be difficult to organise. Consider the requirements to move the coalition forces to the Middle East for Operations Desert Shield and Storm, and more recently, the NATO concentration of aircraft, and ground forces for Operation Allied Force and KFOR.
Many of the campaigns conducted during this period were defensive or punitive operations. In the European theatres of operations, small mobile forces were probably adequate most of the time to deal with barbarians. These are ideal as tabletop scenarios and campaigns. There were few large-scale invasions (such as those organised by Constantius, Julian and Valens against the Sassanian Persians) planned or conducted. Larger forces were also needed for civil wars and there were plenty of those.
More Late Roman Army
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Late Roman Army: Introduction
Late Roman Army: Weapons
Late Roman Army: Personal Organization
Late Roman Army: Bibliography
Late Roman Army: Sources and Terminology
Late Roman Army: Military Organization
Late Roman Army: Unit Sizes and Types
Late Roman Army: A Model Army
Late Roman Army: Painting Tips
Late Roman Army: Bibliography
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© Copyright 2000 by Terry Gore
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