The Auxilia of the Roman Imperial Army

by George Leonard Cheesman

SECTION I
THE STRENGTH AND ORGANIZATION OF THE AUXILIARY REGIMENTS

From the death of Augustus to the period when the frontier defences first began to collapse under the strain of the barbarian invasions, more than two centuries later, the imperial army presents a picture of military conservatism unrivalled in history. Not only does the original distinction between the legions of cives Romani and the auxilia of peregrini remain throughout the basis of its organization, but even individual corps show a marvellous power of vitality. Dio Cassius, writing at the beginning of the third century, notes that, of the twenty-five legions in existence in 14, eighteen still survived in his own day, and epigraphical evidence shows that scores even of the more easily destructible auxiliary regiments could claim as long a record. In appearance, indeed, the only considerable change introduced into the organization of the auxilia during this period was the addition of the numeri in the second century to the alae and cohortes which had previously been the only units employed. It is true, of course, that this conservatism was in some respects rather superficial, and that, while administrative forms and nomenclature remained unaltered, in more essential matters the army had been deeply affected by the tendencies of the age. It is still, however, possible, while paying due attention to these changes, to treat the two centuries which follow the death of Augustus as a single period in the history of the auxilia; it is only amid the confusion caused by the barbarian invasions of the third century and the subsequent attempts at reorganization that we definitely lose sight of our old landmarks.

Leaving out of account for the moment the numeri, which, as late creations with a special significance, are reserved for future discussion, let us commence with the alae and cohortes, which remained throughout this period the units of auxiliary cavalry and infantry respectively.

Both these terms, although their history is widely different, originate in the military terminology of the Republic. The term cohors had been originally applied to the infantry contingents of the Italian socii, which were not united in legions after the model of the levies of cives Romani, and it was naturally retained after the disappearance of the socii to describe the similar tactical units of provincial auxilia.

The term ala originated as a metaphorical description of the two divisions into which the contingents of socii were formed, which were stationed in the normal republican order of battle on either flank of the legions. After the disappearance of the socii the term was applied in a more restricted sense to the two flanking divisions in which the average Roman general massed all his available cavalry. This use of the word continued down to the last days of the Republic. When, for instance, the author of the De bello Africo writes, ‘Caesar alteram alam mittit qui satagentibus celeriter occurreret,’[34] or Cicero says of his son in the De Officiis, ‘Quo tamen in bello cum te Pompeius alteri alae praefecisset, magnam laudem et a summo viro, et ab exercitu consequebare equitando, iaculando …’[35] the word is clearly being used in this sense, and does not refer to a regiment of any fixed size. In fact the cavalry of the socii never seem to have been organized in larger units than turmae, and the auxiliary levies naturally adopted the same formation. It has already been noticed that some of the Spanish auxiliaries who served in the Social War are officially described as belonging to a turma Salluitana. Occasional phrases in Livy, such as the statement that the Aetolian cavalry contingent, in the campaign of 171 B.C., was alae unius instar, do not seem to prove anything more than that the historian used the technical terms of his own age to make his narrative clearer.[36] This usage, however, shows that Livy was familiar with the restricted meaning of the word—that is to say, the ala must have been a recognized institution in the reign of Augustus, and we may add that Velleius states that fourteen alae were employed in the Pannonian campaigns of 6-9 in which he himself had served.

It is improbable, however, that the ala was a creation of Augustus, although he may have determined its exact size and organization. In Caesar’s account of his Gallic campaigns we find frequent mention of contingents of tribal cavalry serving as independent units under officers bearing the title of praefecti equitum,[37] and these units must have been much larger than turmae. The organization of these regiments, originally of a purely temporary kind, must have been placed on a more permanent basis when many of them were taken out of their own country to serve in the Civil Wars, and it would have been natural that a new term should be used to describe them.[38]

Evidence in support of this conjecture, which is, as we have seen, lacking in the writings of Caesar and his continuators, has been sought for elsewhere. The majority of Caesar’s praefecti equitum seem to have been tribal chiefs; one may cite, for example, the Aeduan Dumnorix,[39] the heroic veteran Vertiscus,[40] and the two treacherous Allobroges, Roucillus and Egus, the sons of Adbucillus.[41] On the other hand, when we meet with an ala Scaevae on an early inscription, it is difficult to avoid agreeing with Mommsen that it was called after Caesar’s well-known officer of that name.[42] Many other cavalry regiments, which are shown by epigraphical evidence to have existed at an early date and to have been Gallic in composition, bear titles similarly formed from personal names.[43] It is suggested that these corps, or at any rate the majority of them, represent tribal contingents embodied by Caesar at the time of the Civil Wars under the title of alae and placed under his veteran officers. Thus during this period the use of the term ala in the restricted sense would be already known, although only the older and wider use appears in literature. How slowly the new expression won favour is shown by the fact that during the reigns of Augustus and Tiberius the officers commanding these regiments were usually described on inscriptions simply as praefectus equitum, and it was not until after this that the title praefectus alae came to be generally adopted.[44]

Curiously enough, we find very few cohorts with titles which suggest a similar history.[45] Probably in this case Augustus’s reorganization was more thorough and the existing regiments had not, like some of Caesar’s corps of Gallic cavalry, a record of individual achievement which might exempt them from its scope.

Size of regiments. In discussing the size of the auxiliary regiments we have two questions to settle, the numbers of the establishment and the actual strength at which the regiments were maintained. As regards the first question, the evidence of Hyginus[46] and the inscriptions shows us that both alae and cohortes were known as miliariae or quingenariae—that is to say, they contained, roughly speaking, 500 or 1,000 men each. The smaller unit seems to have been preferred in the first century, while the larger predominates among the corps raised by Trajan and his successors. The exact theoretical size, both of the regiments themselves and of the centuries and turmae into which they were divided, is more difficult to determine. Hyginus states that an ala quingenaria was divided into sixteen turmae, and an ala miliaria into twenty-four.[47] He does not state the number of men in a turma in either case, and it seems impossible to arrive at any certainty on the basis of figures found elsewhere in his treatise. Turning to epigraphical evidence we find an inscription from Coptos which describes the composition of a vexillatio drawn from three alae and seven cohorts, as: ‘Alarum III: dec(uriones) V, dupl(icarius) I, sesquiplic(arii) IIII, equites CCCCXXIIII. Cohortium VII: centuriones X, eq(uites) LXI, mil(ites) DCCLXXXIIX’.[48] Von Domaszewski suggests that the cavalry in this detachment are to be divided into ten turmae of 42 men, each commanded by a decurio, a duplicarius, or a sesquiplicarius, and that this figure represents the theoretical strength of the turma in an ala miliaria.[49] In an ala quingenaria, on the other hand, the turma probably contained only 30 men.

This seems to be as near certainty as we are likely to arrive in the present state of our evidence, unless indeed we take literally a statement of Arrian that an ala contained 512 men, a total which would presumably give 32 men to the turma.[50] Arrian is, of course, the best authority on the imperial army whom we possess, but the remark in question is a parenthesis inserted into an account of the ideal establishment of a Hellenistic army, and he may have meant no more than that the unit under discussion corresponded roughly with a Roman ala quingenaria. More satisfactory and conclusive evidence will perhaps be found when the barracks of an ala in a frontier fort have been accurately planned.[51]