The Roman Army of 450 AD
Beginning in the year 268, under the emperor Gallenius, the first field army was created out of Legionary Cavalry and Infantry detachments. Although at that time a temporary force stationed in Illyria, it would soon enough become permanent, and the Late Roman Army was, in effect, born. The next major development can be accredited to Constantine the Great in 312, who took the Gallic and British Legions and formally established the Gallic Field Army, which numbered somewhere around 100,000 men at that time, and marched on Italy. In order to leave the empire defended, however, he took the Legionary and old Auxilia regiments on the Rhine frontier and created the Riparienses, the first formal Limitanei. By 324, the Empire had several field armies, primarily in Gaul and in Syria, and Constantine's changes had been formalized and applied across the empire. The next twenty-five years or so would see several civil wars which would reduce the strength of some of these field armies, and create new ones, and transfer units inbetween others. By 386 AD, the classification of the border garrisons as "Limitanei" is first formally established in Roman law. By 395, the changes had become permanent and formal, resulting in the Notitia Dignitatum: a register of dignitaries, administrative posts, military commands, and Late Roman units. It was updated until the year 419, or possibly even as late as 428, which gives us a snapshot of the army of the Roman empire at the turn of the 5th century.
As mentioned above, the Roman army was divided into two stations: the frontier troops called Limitanei, and the field armies called Comitatenses. Limitanei are often considered second class soldiers, which is somewhat true as they had fewer privileges and tax breaks than the Comitatenses. However, they were certainly not inferior, poorly-equipped, or farmer militia, as some are led to believe. Their primary mission and purpose was to stop barbarian raids or invasions at the frontier, making them the Roman empire's high-priority line of defense until the early 5th century. The Limitanei had four grades: Burgarii, who maintained fortlets and watchtowers, Castellani, who may have garrisoned forts or towns, Riparienses and Ripenses, who manned the Danube and particularly the Rhine frontier. Along with these, there were also Muscularii, who manned the fleets on the middle and lower Danube, Classarii, who were sailors in the Roman navy, and the Vigilies, or town watches. The difference between them mostly revolved around unit size, and to some extent equipment as well. Limitanei infantry could be organized into Cohortes, Numeri, or Legiones, while cavalry could be Alae, Vexillationes, or Cunei, and some units were classified as the non-descript Milites and Equites. By the year 450, the Limitanei were becoming increasingly tied to their stations, providing support against the barbarians whenever the battles happened to be within a short range, but unwilling to transfer to the field armies or fight on the fronts that were far from their homes. Unlike the field armies, which increasingly had to recruit from barbarians, the Limitanei remained mostly comprised of local Roman citizens, although on the Rhine frontier they also used Alemannic recruits. In the Eastern Empire, the Limitanei on the Sassanid frontier would remain neglected but intact until the great war of 602 AD, and the subsequent Arab invasions, while the Limitanei on the Danube were annihilated by Attila the Hun in 446 AD, never to fully recover.
The second classification of troops, the Comitatenses, were mobile field armies stationed in Gaul, Italy, Dalmatia, Moesia, Thrace, Asia Minor, Syria, Spain, Britain, Egypt, and North Africa. Their job was to counter usurpations or fight Sassanid armies, but would soon have to take on the role of fighting the barbarian super-confederations of the mid-4th and 5th centuries. Within the Comitatenses were a special classification of troops called Palatina, divided into either Auxilia or Legio Palatina units. It is unknown what the difference was between them and the Comitatenses, other than that they might have been for more specialized roles, while the Comitatenses served as general heavy infantry. The Comitatenses were also different from the Limitanei in that they had more privileges and tax breaks. These field armies did not all exist at the same time: the British field army was created before 405 and absorbed into the Gallic army in that last year. The Spanish field army was destroyed prior to the last updating of the Notitia Dignitatum, and the army in Dalmatia, created in 402, had taken severe losses in 409. In 432 AD, part of the African field army was destroyed and probably used to replenish the Italic field army, and in 439 AD, another 60,000 troops, which would be the entire Gallic field army, had to be cut due to the loss of Carthage. Between 441-447 AD, the Praesentalis Prima, Praesentalis Secunda, Thracian, and Illyrian field armies in the Balkans had all been destroyed by Attila the Hun: the equivalent of two to three battles of Adrianople. Elements of these three armies, as well as the Syrian and Egyptian armies, would all survive into the 6th and early 7th century. The last of the Roman field armies finally were destroyed by the Sassanids in the great war of 602, or at the Battle of Yarmouk in 638, resulting in the transformation of the army and the establishment of the Theme system.
There was also a third and final classification of troops: the Foederati. The majority of barbarians recruited into the Roman army were dispersed amongst Roman units to be trained and fielded as professional Roman soldiers. However, beginning in the 380's AD, this began to change, as barbarian leaders began to retain a degree of independence and control over the peoples coming into the empire that were supposed to be Romanized and absorbed. By 430 AD, Aetius' Gallic army was heavily supported by independent complements of Franks, Heruls, Huns, Goths, Burgundians, and Alans. Although in some cases, like with the Alemanni and Alans, these troops continued to serve as Romans in Roman units, the majority were under their own commanders who were a part of the administrative structure of the lands they were settled on. Gradually, as the Roman forces got smaller between 430-460, the barbarian forces got more powerful, more independent, and more dangerous. There was no "barbarization" of the Roman army, instead there was a complete replacement of Roman troops by barbarian federates, who became the puppet masters of Rome.
Bucellarii were similar to Foederati in that they were small armies recruited from barbarians, but differed in that they were commanded by, served under, and held an undying loyalty to their commander. Aetius is reported to have had large numbers of Huns and Aquitanian Goths serving him, thanks to his connections with the court at Tolouse and the court of the Huns. The Placidi Valentiniani were Bucellarii that were converted into an Auxilia Palatina unit, as they originally had been the Gothic bodyguard of Galla Placidia and were re-organized into a Roman regiment. Bucellarii were very well paid, very well trained, and far better equipped than their normal Roman or barbarian counterparts, and these elite troops were the key to victory in several battles, most notably Dara in 530 AD. Bucellarii played an important role in the fall of the western Roman empire, killing emperors and supporting usurpers, while at the same time they helped form the basis of the eastern Roman theme system.
The condition of the Roman army under Aetius is also open for debate, but there is some availability of source material on it. The Vita Sancti Severini takes place after Aetius' death in the 450's through 470's AD. In it he records the status of the Norican and Raetian Limes on the upper Danube, which at the beginning of his work were in good condition, although somewhat neglected, and now that Aetius was dead were beginning to be completely overrun by the Germanics. Archaeological finds support the continued occupation of the upper and middle Rhine until around 450 AD, with both Roman garrisons and Alemannic or Frankish Foederati. It seems therefore, that under Aetius' time the Limitanei and Foederati units that manned the Danube and the Rhine were in decent shape, were funded, and supplied with adequate arms and manpower to keep them operating with some degree of efficiency. The status of the Comitatenses is also well known, thanks to the work of Sidonius Apollinaris, who records the both the Exercitus in Praesentis and the Scholae Palatinae were still intact in 454 AD in Italy. Most of the units under Aetius were veterans, who had won many battles under his leadership, and many went on to fight and survive the Battle of the Catalaunian Fields. Most of the Fabricae were still under Roman control, which meant that arms and armor were probably more than abundant considering how small the army had become, and this would explain why Tonatius Ferreolus was able to supply the Foederati with equipment in 451. As an all in all, although the Army was battered and significantly smaller, it was just as experienced and strong as ever, thanks to the leadership of Aetius and his policy, as he clearly intended to prevent neglect and corruption in the army.
Unit Sizes and Organization in the Late Roman Army
The Late Roman army was organized into several different sized units, and this was due to the fact that the 3rd through 7th centuries were a period of continuous transition for the army. The basic units for infantry were as follows: the Legio, Numerus, Cohortis, and then "Milites" which were probably Numeri. Cavalry were organized into the Vexillatio, Ala, Cuneus, and then Equites, the latter of which were probably Vexillationes. Determining unit sizes and their organization in the late Roman army had been a matter of debate for decades, but the most modern theory will be presented here, going through the individual unit sizes and their subsequent organization.
The Legio was the old Roman legion, but far smaller and more versatile, with a different organization. The Late Roman Legion seemed to number about 1000 men, based on the works of Claudian and Orosius in the early 5th century, and deductions from the Beatty Papyri from the late 3rd and early 4th centuries. The Legio was likely organized into 12 centuries, possibly divided into two cohorts, totalling 960 men plus its staff officers. Legions were almost always commanded by a Praefectus, but sometimes by a Praepositus in the Limitanei regiments. Underneath the Praefectus was the Primicerius, or Lieutenant Commander. Just beneath the Praefectus Legionis were several Vicarii as well, who were temporary officers or staff officers that could also replace the Praefectus or Primicerius if they were killed or not present to command. Underneath the Praefectus and the Primicerius was the Chief of Staff, the Domesticus, and underneath him was the drillmaster and herald, the Campidoctor and Senator. Next come the Centenarii: the Centurions, and underneath the centurion was the Biarchus, who probably commanded half a century like the Pentakontarchos in the later Strategikon. Finally each century was organized into ten Contubernia, each one probably commanded by a Decanus, like the Strategikon's Dekakontarchos.
By the mid-5th century, the vast majority of cohort sized units had been replaced entirely by Numeri. The Numerus numbered about 640 men, organized into 8 centuries, plus staff and officers, which is consistent with the 508 men given by the Columbia Papyri in the 550's AD, and Sozomen's Arithmoi which numbered about 660 men each. The Numerus was commanded by a Tribunus, who had a Primicerius and several Vicarii under his command, just like the Legio, as well as a Domesticus, Campidoctor, and Senator. However, its unit structure was different: a Numerus was subdivided into three Manipules, each commanded by a Ducenarius, who commanded two Centenarii. Under each Centenarius was a Biarchus, just like the Legion, commanding a half century, and each Contubernium had a Decanus as well. The Placidi Valentiniani Felices were a Numerus.
Cohortes are a tricky subject in this era, as many Limitanei cohorts were still intact as of 419, but were being phased out. The Cohort numbered about 480 men, organized into 6 centuries, plus staff officers. The Cohors was commanded by a Tribunus, with a Primicerius underneath him, several Vicarii, a Domesticus, Campidoctor, and Senator as well. Each century was commanded by an Ordinarius, who was like a Centenarius but did not have the privileges of a Centenarius as the Limitanei were not of the same status as the Comitatenses. Underneath the Ordinarius was the Biarchus, who commanded a half century, and ten Decani commanding the Contubernia. Most Cohortes seem to be classified as Limitanei Castellanii.
A Vexillatio was a detachment from a Legion: for example Legio Septima Gemina had several detachments across the empire which totaled about 2400 men, ranging in status from Limitanei to Comitatenses. However, it was also often used to refer to a cavalry unit, with about the same terminology as the word Numerus, where although initially vague it came to define a set unit size and organization. Cavalry Vexillationes are recorded from being as big as in the thousands, to as small as 20 men!
The Ala in this period, according to Zozimus, numbered 600 men, while Ammianus and Julian both mention Alae at 350 men. It is therefore difficult to discern the size, but the older Quingenary Alae seem to have been fairly close to Zozimus' number of 600 men, as they numbered about 480 plus officers. The Ala was commanded by a Tribunus, and divided into Turmae of 32 men, each led by a Decurio. This system hadn't changed since the principate really, but would be phased out by the end of the 5th century in favor of the newer Cunei.
Equites means cavalry in Latin, but they were also a size of unit, created usually from old Legionary cavalry Vexillationes. The Beatty Papyrii record several sizes for Equites units, including a legionary Equites detachment of II Traiana at 120 men, and a unit of Sagitarii at 360 men. Ammianus mentions two units combined together made an Ala of 700 men, so it is also possible that the Milliary Alae were divided in half into Equites units. Either way, it is known these Equites units were commanded by Tribuni and most likely were simply a label, not a unit size.
The Cuneus was the newest unit in the Late Roman army, and although far older, it seems to have been influenced by the Huns, who also used a term similar to the word Cuneus to describe their clan-based military organization. However, it was not adopted from the Huns or Sarmatians, and it seems to have been a development of what Arrian records as the Tarantiarchia: a 256-man half-Ala. The only example of a Roman Cuneus seems to have numbered around 240 men, according to Symmachus, to which a unit of Huns was adapted to, and divided into units of 40 men. Therefore, it is possible that the Cuneus is the same as the Tarantiarchia, commanded by a Tribunus and divided into 32-man Turmae each commanded by a Decurio. It is likely the Cuneus developed into the cavalry units of the Strategikon.