The numeri. A new and completely barbarous element was introduced by Hadrian into the Roman army by the organization of the [pg 274]so-called numeri, corps of varying size, recruited from the non-Romanized peoples on the frontiers, who retained their local language, weapons and methods of warfare but were commanded by Roman prefects. The conquered German peoples settled on Roman soil by Marcus Aurelius and his successors supplied contingents of this sort.
The strength of the army. At the death of Augustus the number of the legions was twenty-five; under Vespasian it was thirty; and Severus increased it to thirty-three, totalling over 180,000 men. A corresponding increase had been made in the numbers of the auxiliaries. From about 150,000 in the time of Augustus they had increased to about 220,000 in the second century. The total number of troops in the Roman service at the opening of the third century was therefore about 400,000; one of the largest professional armies the world has ever seen.
The system of frontier defence. A second momentous fact in the military history of the principate was the transformation of the army from a field force into garrison troops. This was the result of the system developed for the defence of the frontiers. Augustus, for the first time in the history of the Roman state endeavored to preclude the possibility of indefinite expansion by attaining a frontier protected by natural barriers beyond which the Roman power should not be extended. Roughly speaking these natural defences of the empire were the ocean on the west, the Rhine and the Danube on the north, and the desert on the east and south. At strategic points behind this frontier Augustus stationed his troops in large fortified camps, in which both legionaries and auxiliaries were quartered. These camps served as bases of operations and from them military roads were constructed to advantageous points on the frontier itself to permit the rapid movement of troops for offensive or defensive purposes. Such roads were called limites or “boundary paths,” a name which subsequently was used in the sense of frontiers. These limites were protected by small forts manned by auxiliary troops.
The fortification of the limites. Although Claudius and Vespasian discarded the maxims of Augustus in favor of an aggressive border policy they adhered to his system for protecting their new acquisitions in Britain and the Agri Decumates. However, these conquests and that of the Wetterau region by Domitian pushed the frontier beyond the line of natural defences and led to the attempt to construct an artificial barrier as a substitute. It was Domitian who took [pg 275]the initial step in this direction by fortifying the limites between the Rhine and Main, and the Main and the Neckar, with a chain of small earthen forts connected by a line of wooden watchtowers. To the rear of this advanced line there were placed larger stone forts, each garrisoned by a corps of auxiliaries, and connected by roads to the posts on the border. While the auxiliary troops were thus distributed along the frontiers in small detachments, the larger legionary cantonments were broken up, and after 89 A. D. no camp regularly contained more than a single legion. Trajan, who also waged his frontier wars offensively, merely improved the system of communication between the border provinces by building military highways along the line of the frontier from the Rhine to the Black Sea, in Arabia, and in Africa.
In the matter of frontier defence, as in so many other spheres, a new epoch begins with Hadrian. He reverted abruptly to the defensive policy of Augustus and began to fortify the limites on a more elaborate scale. The frontier between the Rhine and the Danube was protected by an unbroken line of ditch and palisade, in which stone forts, each large enough for an auxiliary cohort, took the place of the earthen forts of Domitian. At the same time the limes was shortened and straightened, and the secondary line of forts abandoned. In Britain a wall of turf was constructed from the Tyne to the Solway, and in the Dobrudja a similar wall linked the Danube to the Black Sea. The eastern frontier of Dacia was likewise defended by a line of fortifications. Here, as on the other borders, the Roman sphere of influence, and even of military occupation, extended beyond the fortified limes.
Antonius Pius followed Hadrian’s example and ran an earthen rampart with forts at intervals from the Forth to the Clyde in northern Britain. This line of defence was abandoned by Septimius Severus, who rebuilt Hadrian’s rampart in the form of a stone wall with small forts at intervals of a mile and intervening watch towers. In addition seventeen larger forts were constructed along the line of the wall. The limes in Germany was strengthened by the addition of a ditch and earthen wall behind Hadrian’s palisade, but along the so-called Raetian limes, between the Danube and the Main, another stone wall, 110 miles long, took the place of the earlier defences. A similar change was made in the fortifications of the Dobrudja. However, this system was not followed out in the East or in Africa, where the limes was guarded merely by a chain of blockhouses.
The consequences of permanent fortifications. The result of the construction of permanent fortifications along the frontier was the complete immobilization of the auxiliary corps. Stationed continuously as they were for the most part in the same sectors from early in the second century, and recruited, in increasing proportion, from among the children of the camps, it only required the granting to them of frontier lands by Severus Alexander, upon condition of their defending them, to complete their transformation into a border militia (limitanei). At the same time the scattering of the legions along the line of the frontiers made the assembling of any adequate mobile force a matter of considerable time. And the fortifications themselves, while useful in checking predatory raids by isolated bands and in regulating intercourse across the frontiers, proved incapable of preventing the invasion of larger forces. Consequently, when in the third century the barbarians broke through the limites they found no forces capable of checking them until they had penetrated deeply into the heart of the provinces.
The chaos which followed the death of Severus Alexander was the result of a military policy which left the richest and most highly civilized parts of the empire without any means of self-defence; created a huge professional army the rank and file of which had come to lose all contact with the ungarrisoned provinces, all interest in the maintenance of an orderly government and all respect for civil authority; and at the same time rendered the army itself incapable of performing the task for which it was organized.
On the other hand the army had been one of the most influential agents in the spread of the material and cultural aspects of Roman civilization. The great highways of the empire, bridges, fortifications and numerous public works of other sorts were constructed by the soldiers. Every camp was a center for the spread of the Latin language and Roman institutions and the number of Roman citizens was being augmented continuously by the stream of discharged auxiliaries whose term of service had expired. In the canabae, or villages of the civilian hangers-on of the army corps, sprang up organized communities of Roman veterans with all the institutions and material advantages of municipal life. The constant movement of troops from one quarter of the empire to another furnished a ready medium for the exchange of cultural, in particular of religious, ideas. To the ideal of the empire the army remained loyal throughout the [pg 277]principate, although this loyalty came at length to be interpreted in the light of its own particular interests. Not only was the army the support of the power of the princeps; it was also the mainstay of the pax Romana which endured with two brief interruptions from the battle of Actium to the death of Severus Alexander and was the necessary condition for the civilizing mission of Rome.