While the later 3rd and early 4th centuries saw the development of a newly organised Roman army, the essential building blocks of the Roman legion remained much the same. It was in the structures of military command, however, that the greatest changes took place.
In the earlier empire, legions and other military units were traditionally led by men of the aristocracy. Legions were commanded by legates drawn from the senatorial order, while the lower aristocracy of the equestrian order supplied the cohort and cavalry commanders and the legion tribunes. During the chaos of the mid 3rd century, these aristocratic officers were withdrawn from military positions. Instead of the senatorial legati, legions were now commanded by officers with the title of either praepositus agentes vice legati, or just praefectus legionis (‘Prefect’), initially drawn from the equestrians but later from men promoted from the ranks.
Smaller units of the legion were still commanded by centurions, although by the later 3rd century these officers seem to have acquired the additional title of ordinarius (centurio ordinarius is recorded as a transitional term), and this term later came to supplant the original one. Greek documents from the eastern (‘Byzantine’) empire still record the existence of the centurion (κεντυρίων or kentyrion) and ordinarius (ὀρδινάριος) in the 6th century.
With the demise of the old equestrian legion tribunes, centurions were able to take up a wider range of command positions. The legionary detachments of the new field armies were led by praepositi, once an ad-hoc title for any small-unit commander, but now formalised as a command position for senior centurions in the field armies.
In the earlier legions, the senior centurion ranks were primus pilus (‘first file’, approximately) and praefectus castrorum, but these positions seem to have been phased out in the last quarter of the 3rd century. Instead, we see a greatly expanded centurionate, with a number of different grades in a hierarchy that still remains largely obscure. But while the old primipilate offered veteran centurions the chance to move up to higher command positions, the new army of the tetrarchy developed an alternative means of advancement, the schola protectorum, or Corps of Protectores.
The first Protectores appeared under the emperor Valerian (AD253-260). Initially a title of high honour for senior officers close to the emperor, by the end of Gallienus's reign centurions were holding the title, and by a few decades later the Protectores were apparently organised as a bodyguard corps attached to the imperial retinue, or comitatus (protectores divini lateris, or 'protectors of the sacred flank'). The schola protectorum functioned as a promotional mechanism to move veteran soldiers and centurions into command positions. Admission carried the honorific grade of ducenarius, which signalled an elevation above the centurionate and allowed field command in the imperial comitatus. Newly appointed men would therefore gain this title upon first joining the corps and ‘adoring the purple’: in a rigidly hierarchical empire, this ritual of kissing the hem of the emperor’s robe signified that an officer had moved into the inner circles of power, and was perhaps destined for higher things.
Inscriptions dating to the early 4th century provide some evidence of this new, and rather more meritocratic, system of promotions. Florius Baudio, formerly ordinarius of II Italica, served and died in Italy as a Protector, while Aurelius Firminus, a former Protector, rose to become Prefect of Legion II Adiutrix. The future emperor Galerius reportedly served as a scutarius (guard cavalryman, probably), then Protector and Tribune before attaining the purple. Protectores, it seems, were expected to fight, and often die, in the service of their emperors: the Protector Valerius Valentus ‘fell in civil war in Italy’, while 30-year-old Viatorinus died ‘in barbarico’ during a battle with the Franks.
The subordinate ranks of the legions of Diocletian's day were probably quite similar to those of the earlier empire. We still find optiones (centurion’s deputies) and tesserarii (watch officers) right though later Roman history. One of the senior centurion or veteran positions, drillmaster or campidoctor, seems to have taken a similar role to the old primus pilus or praefectus castrorum. There were sometimes several of these men in a unit, and they also functioned as front line soldiers. Later papyri and inscriptions, plus the writer Vegetius, mention several other ranks or roles that may have appeared around this point, or some time later. Flaviales and Augustales, for example, could have been centurion grades, or (perhaps more likely) senior soldier grades; the former is most probably Constantinian. Better attested positions, perhaps dating from the later 3rd century, would include actuarius (legion quartermaster, approximately), adiutor (clerk or record keeper) and draconarius (the bearer of the ‘windsock’ draco standard).
By the middle decades of the fourth century, new units had developed in the imperial field armies: the auxilia palatina and the scholae. The latter appear to have been elite cavalry formations, perhaps intended to replace the old Praetorian Guard and equites singulares (‘Horse Guard’), which had been disbanded by Constantine after his victory at the Milvian Bridge in AD312. The former comprised a new front-line infantry force, perhaps developing from the irregular units of semi-barbarian troops (called numeri) of earlier centuries. These new auxilia are first attested on a tombstone from Nakolea in Asia, recording a soldier of the numerus Iovi Cornuti Seniores, who would have enlisted c.AD329. Perhaps significantly, the man was a Roman citizen, born in Singidunum on the Danube; the auxilia seem to have lost their ‘barbarian’ origins earlier in the century.
These units appear to have used a new and different structure of ranks, perhaps adopted from the cavalry. Centenarius was the equivalent rank to the old centurion, although perhaps of higher status. Biarchus and circitor also appear as subordinate positions. It would seem that these new ranks were never adopted by the traditional legions, however. One or two crossover posts used by both legions and auxilia - vicarius, for example - probably appeared later in the 4th century. Ducenarius (Greek δουκιναρίους), which began as an honorific title for centurions promoted into the Protectores, was apparently adopted (by at least c.350) by the auxilia and scholae to denote a senior unit commander.
By the end of the fourth century, the structure of the Roman army had once more metamorphosed: the units of the imperial field armies now gained a special exalted status as comitatensis troops, with an elite core of legions and auxilia called palatini. The remnants of the older legions, and the few remaining cohorts of old-style auxiliaries dating from the earlier empire, were left on the frontiers and given the lower grade of limitanei. The Corps of Protectores changed too, becoming a sort of officer training cadre for younger men of the wealthy classes (protectores domestici) and barbarian noblemen, many of whom went on to higher command positions. This later army, recorded in a document from the late 4th or early 5th century called the Notitia Dignitatum, still preserved many of the old titles and organisational structures from the Tetrarchy and the centuries that preceded it, and was to survive until the final collapse of the Roman empire in the west.
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