Since the first century BC the Roman army had been composed of two elements. The first of these was formed by the legions, each consisting of 5,000 foot soldiers and 120 horse. These men were all Roman citizens who normally joined up at the age of eighteen and served for twenty-five years. The second main component of the army was constituted by the auxilia, recruited from provincials who were not Roman citizens. These were organised in cavalry and infantry units either 500 or 1,000 strong. The emperor was also protected by his praetorian guard of some 5,000 picked citizen troops. During the reign of Augustus the number of legions was stabilised at twenty-eight, which the disaster of Varus in Germany (AD 9) diminished to twenty-five. After further fluctuations, Marcus Aurelius’ creation of two new legions to reinforce the upper Danube frontier raised the total to thirty; and Septimius added three more. Throughout this period the auxiliary soldiers numbered about as many as the legionaries. Under Augustus, the entire strength of the army probably amounted to about 260,000. Under Septimius it reached a figure of more than 300,000.
Officers were now on the crest of the wave. Septimius gave them numerous privileges, and their military jobs provided openings to many careers. This was a new military aristocracy, a special caste which provided the empire with most of its senior administrators and indeed most of its emperors. And this élite was not static but always changing, since the new sort of army officers mostly rose from the ranks; it was Septimius’ policy to promote more and more N.C.O’s to be officers, many of them only slightly Romanised and with little education. Excavations at Dura show that the officers garrisoned there lived well, better than almost anyone else in the town. Nor is that surprising, since Septimius gave them more than fifty times the pay of a legionary.
At the beginning of the imperial epoch, many auxiliaries had originated from the northern parts of Gaul and Spain, while recruits for the legions came from the more Romanised southern regions of the same countries.
Other legionaries, like most of the praetorian guardsmen, were from Italy, and particularly from its northern areas. But already before AD 100, although officers were still of Italian stock, provincials in the legions had come to outnumber Italians by four or five to one.
Originally legionaries and auxiliaries alike had been posted far from their homes. But there was a growing tendency for men of both categories to be recruited on the spot, and by the time of Marcus Aurelius most soldiers were serving in the countries of their origin. Men fought external enemies with increased determination when their own homes were at stake, and in any case it was only by local recruitment that a sufficient number could be found who would not desert. This decentralisation created a greater danger of revolts and usurpations, but since enough soldiers could not be raised in any other way the risk of such usurpations had to be accepted.
Local recruitment had the further effect of gradually making soldiering hereditary. Egyptian lists of early imperial date show only two soldiers’ children out of thirty-six new recruits, whereas in AD 168 there were twenty out of thirty-seven. Within the next century compulsion was employed to make sons of soldiers follow their fathers’ careers.47 Indeed, despite the incentive provided by local postings, almost all recruitment became compulsory. A document describing a levy in Asia Minor dates from the dynasty of Septimius,48 but probably the system of coercion went back considerably further.
Moreover, by c. 200 the old differentiation of recruitment between legionaries and auxiliaries had disappeared or become nominal. When the whole Roman world was given the franchise by Caracalla the distinction was dead; its removal provided a useful, egalitarian uniformity, but made it harder to attract ambitious people into the military profession.
Since there were numerous legions in the east – eight on the frontier and four in other provinces49 – local recruiting meant that a considerable proportion of all Roman legionaries were of eastern origin. But the Danube garrison was larger still; under the earlier emperors the Rhine and the Danube had been allotted seven legions each, but under Septimius their respective totals were four and twelve. Consequently, the predominant element in the Roman army came from the warlike populations of the Danubian provinces (p. 16).
External threats under Marcus had restored the army’s sense of importance, and this applied to the key Danube legions most of all. Their massive concentration and racial unity made them a force which became the bulwark of the empire and the maker of its emperors. Three legions recruited by Septimius for service against Parthia are also likely, for the most part, to have consisted of men from the Danube countries. One of these legions, by a significant precedent, was transferred, when the Parthian war was over, to Albanum in the region of Rome itself. There had never before been a permanent legionary garrison in Italy, which was losing its superiority over the provinces. By the same token, Septimius had already made the traditionally Italian praetorian guard into a body drawn mainly from Danubian legionaries.
These policies were linked with a growing tendency to replace the earlier idea of an indeterminate, variable, invisible frontier protection zone by the doctrine of fixed and fortified barriers. Like the formidable walls which from the later second and particularly in the later third century were built round towns (including Rome itself),50 these frontier defences were studded with numerous forts. The troops that manned them drew assistance, when necessary, from legions marching up from larger reserve camps in the second line. Forward and rear sectors alike were served by large, permanent and varied army workshops and arms factories and production centres.51
One result of this static arrangement was that military units were lodged within existing towns. Their quarters against the city-walls of Dura (c. 165), for example, were several times reconstructed and expanded. A second tendency was for fortresses and camps to develop into completely new towns, many of which still survive as cities. Originally such settlements were divided into a military section and a camp town for the civilian populations which collected to work and trade with the army, and sometimes for other native communities as well. Both the military and civilian accommodation was often improved and remodelled by imperial initiative. In a further attempt to make the army more attractive, Septimius granted legal recognition of the soldiers’ associations with native women, who were often their compatriots.52 Although regulations had not allowed men to marry while on active service, these unions had inevitably existed on a large scale; a good deal of earlier evidence is dramatically supplemented by the heaps of women’s and children’s shoes at the second-century Bar Hill fort on the Antonine Wall. Now, however, the concubines gained some sort of legal status, and so did their sons who were due to follow their fathers into the army.
To encourage this hereditary self-sufficiency, the garrisons were assigned land, in allotments large enough to help materially with supplies. This policy was also intended to reduce the army’s costs, and at the same time, by turning its men into tillers of land, to remedy the dearth of agricultural labour. Consequently, soldiers took to farming as their principal peace-time activity.53 They had other peaceful occupations as well – for instance, Marcus and other emperors placed them at the disposal of civilian builders. But it was Septimius Severus, above all, who made the military into peasants and farmers and owners of property. Papyri tell us the subjects about which they wrote home: their letters are about properties and purchases and servants and rents. Moreover, previous instructions forbidding enlisted men to engage in commerce had become obsolete, and there was a good deal of trade and speculation in the sale of army stores and workshop products.
While these soldiers, immobilised in the defensive zones, were being transformed into peasants, the reverse process was also taking place. That is to say, the local civilian populations were simultaneously being enrolled in new frontier units called limitanei. The contrast between making peasants into soldiers and giving soldiers civilian work may not seem a very real one, since both were recruited in the neighbourhood and both formed part of the army. But the limitanei were not much better than a half-trained, occasional militia, not unlike the medieval soldier-peasantry. Their value, like that of more expert local recruits, lay in the supposition that men would be ready to defend their own soil – and the main armed forces were not large enough to do so without their help.
Septimius and his successors took the initiative in developing this semi-militarised peasantry. However, until the time of Diocletian at least, the pattern of defence varied according to the nature of the country. In north Africa, for example, there was collaboration between the army, the ex-soldiers settled in citizen colonies, and the manpower of imperial estates. The agricultural population of such estates was concentrated within fortified settlements disposed along the frontier in two sectors of depth. There in Africa, and behind the northern and eastern borders as well, the tenants of these castella were the backbone of defence.54
But the conscription of peasants crippled agricultural production, and so recourse was also had to filling the ranks of the limitanei from barbarian tribesmen living in frontier areas. Since Trajan’s time there had been German and Sarmatian irregulars in the Rhine and Danube zones, and by 270, if not already by 245, whole groups of such Germans were promoted to form part of the regular army.55 In Britain, too, entire tribes were pressed into the same task. The extensive use of barbarians as soldiers diluted the army’s Roman character; but they fought well, and again their aid was indispensable.
Finally, and not least important, the Romans continued to make use of mercenaries hired from border kingdoms and puppet states. Conspicuous among these troops, especially in the third century AD, were the mounted archers from Osrhoene and Palmyra. Many of these mercenary bowmen were utilised not only in their own regions but in theatres of war away from their homelands. The specialised local techniques of national units were valued highly.56 Septimius extended their employment, and Caracalla multiplied light troops of this kind along the northern frontier.
Such mercenaries were among the few rapidly transferable elements in the Roman army of the day. But military disasters at the hands of the Germans under Marcus Aurelius, heralding even worse things to come, showed that the empire’s static system of defence was inadequate to deal with recurrent emergencies. If a serious break-through occurred, and worse still if there were several break-throughs simultaneously, the protective zone was of inadequate depth. For, other than the small praetorian guard associated with the emperor’s person, there was no central reserve that could come to the rescue. This had already been a failing in the parsimoniously financed army of Augustus, and now that the frontier situation was so much graver the deficiency was perilous. Marcus Aurelius formed temporary mobile field-forces comprising small detachments from several legions and auxiliary units. Septimius too, when he stationed a legion near Rome, may well have had in mind the idea of a reserve which could be dispatched to whatever sector of the defence line was in difficulties.
But a legion possessed much less mobility than mounted troops, and as the concept of a strategic reserve developed it was of cavalry that this formation came mainly to consist. Archers and javelin-men on horse-back were useful, but what was really needed was heavy cavalry able to move against the similar units prominent in the armies of Parthians and Persians and Sarmatians. For the military strategies of those Iranian peoples had long been based on mailed horsemen (cataphracts) with metal helmets and long heavy lances and swords; and the idea was spreading to the east Germans who came under Sarmatian influence.57 In the third century, the institutions of the Parthians taken over and improved by the Persian monarchy included their heavy cavalry, which was henceforward recruited from the lesser feudal nobles directly dependent upon the King of Kings (p. 24). The Romans had already employed a few cataphracts of their own as early as Hadrian (d. 138), but when they encountered the formidable Persian cavalry (c. 232–3) it was becoming clear that they, too, urgently needed to have more armoured horsemen.58 Gallienus, threatened not only by all manner of external invaders but by the dissident western empire of Postumus, took the important step of creating a field army of cavalry (264–8), which was intended to serve simultaneously as a reserve and a mobile striking force. Its principal base was Mediolanum (Milan), located at a convenient distance from the frontiers and Rome alike. This strategic centre, rapidly becoming even more important than the capital, was joined to Aquileia, Verona and Ticinum (Pavia) in a new system of north Italian defence necessitated by the loss of the upper Rhine-upper Danube area (p. 32). But the new plans differed from the old static protection because they were conceived in terms not only of fortresses but of the newly created cavalry army. This élite force consisted of squadrons (we do not know how many) which were mostly five hundred men strong. They included heavy Persian-style cavalry, looking like knights of the Middle Ages in their conical Iranian helmets, which the Germans later inherited; and an almost medieval concept of knighthood was to be seen in the hereditary gold ring granted to the sons of its centurions. Other elements in this army were Osrhoenian and Palmyrene mercenary archers on horseback, javelin-throwing Mauretanian riders, and a novel and valuable corps of mounted Dalmatians whose Illyrian origin guaranteed loyalty to Rome and leavened the exoticism of the other contingents.
This new arm of the services was celebrated by coins of Gallienus displaying the winged horse Pegasus, to whom a dedication is offered as the spirit of alertness (ALACRITATI). Other slogans speak of the courage of the cavalry (VIRTVS EQVITVM); and there are appeals to their loyalty (FIDEI EQVITVM). But this last point was where the arrangement proved vulnerable. Drawing upon the Greek models he favoured, Gallienus seems to have associated the high officers of the corps, who were mainly of Danubian origin, with certain other officers in a select club or staff college(protectores) stationed in the imperial camp and attached to his own person.59 Yet the commanders of this formidable cavalry army, necessarily men of ability, were under great temptation to revolt. Indeed this temptation proved too strong for the very first commanders whom Gallienus appointed (p. 17). The first of them, Aureolus, before turning traitor, had demonstrated the worth of the new corps against a rebellion near the middle Danube.60 The second commander, Aurelian, when he became the next emperor but one (270), employed his expert knowledge to operate light horse successfully against the massive mailed cavalry of Zenobia. Nevertheless, he also strengthened his own heavy cavalry on a large scale. A relief of a few decades later depicts these men in their scale-armour and Iranian head-gear, carrying Iranian standards emblazoned with gaping, hissing serpents.61
Diocletian (284–305) proceeded to a military reorganisation of farreaching variety and scope. Pursuing his predecessors’ concern with mobile formations, he not only created a new barbarian mounted bodyguard (scholae), but made the field force into one of the two major parts into which the entire army was now divided. This comitatus, as the force was called, contained infantry, but it still remained particularly strong in cavalry. Diocletian is believed to have introduced some numerical reductions in the field army. Yet this is unlikely to have been less costly than before, since it was divided into four parts, all of which required their own supply units. For not only Diocletian himself but each of his three partners in the Tetrarchy of rulers was accompanied by a field corps of his own.
In addition to his innovations with regard to the mobile field army, Diocletian reverted to earlier preoccupations with frontier defence. He strengthened the fortifications along the frontiers, and their elaborate barriers were reconstructed on a uniform system throughout the empire. Uniform, too, were the state factories under military administration which made arms and materials for the defenders of these zones, and indeed for the entire Roman forces. The troops guarding the frontier areas were now constituted into the second main division of Diocletian’s reorganised army, and the name of limitanei (p. 38) was extended in meaning to describe this whole arm. Except in the Danube sector, they were not usually of the same calibre as the field force. Yet any deficiency in quality was henceforward to be compensated by numbers, for Diocletian, perhaps influenced by massive Persian mobilisations, decided that the principal solution to the problems of the day was that the Roman armed services should become enormously larger than they had ever been before. Whereas Septimius’ army had totalled between 300,000 and 400,000 men, Diocletian’s consisted of 500,000 or even more.62 This huge quantity of soldiers was raised by more intensive methods of recruitment. Although Diocletian, like other emperors, was prepared to commute military service for payment, he seems to have been the first to institute regular, annual conscription, assessed on the same schedule as the land tax (p. 53). This was the main source of his citizen recruits. But he was also particularly eager to make use of the warlike tastes and varying specialist skills of barbarian tribesmen. The soldiers mobilised from this almost inexhaustible source of supply included upland Anatolians, who were to be the backbone of Byzantine armies; and many Germans.63
This German element in the army was greatly increased by Constantine. He appreciated their particular qualifications to deal with their hostile compatriots the other side of the border. High status was conferred upon German units, and corresponding favour accorded to German generals. Abolishing the time-honoured praetorian guard (which had fought on the enemy side when he captured Rome), Constantine replaced this by the largely German personal guard started by Diocletian. Many German soldiers were also incorporated into new units of cavalry and infantry. These, combined with detachments drawn from the frontiers, were drafted into the field army, which now constituted a central striking force and strategic reserve substantially larger than any that had existed before.64 In Diocletian’s time the praetorian prefects had often commanded in the field, but Constantine, in order to steer clear of the temptations to which powerful single commanders were liable, placed his field army under a pair of new officers, a Master of the Horse and a Master of the Foot. The frontier forces were less well paid and respected than the central mobile army, and this aroused criticism, but on the Danube border and elsewhere Constantine carried out a large-scale reorganisation and reinforcement of garrisons. (He and Licinius, preparing for hostilities against one another, also strengthened their fleets, and it was at sea that the decisive clash came in 324.)
By the end of the fourth century the army was nearly twice as large as it had been two hundred years earlier, and more than twice as expensive because of the increase in horses, since their feed cost as much as a man’s rations. While privileges were extended to the sons of exsoldier frontiersmen, recruitment was enforced by severe penalties which caused widespread terror, especially in the more civilised provinces. The physical quality of soldiers improved – in earlier times their minimum height had been five feet or four feet eleven inches, whereas in AD 367 this requirement had risen to five feet five. Officers, too, being mostly professionals, were better than they had ever been before.
Since the crises of the third century there had been a gradual tendency to make greater use of mounted troops; yet events were to show that this had still not gone far enough. For the shattering defeat of Valens by the Visigoths at Hadrianopolis (378) was due to the superiority of the barbarian riders. Since reservoirs of manpower within the western empire were now failing, 40,000 barbarian confederates, who had been given lands within the empire, were mobilised by Theodosius to serve as Roman cavalry. This necessity was ominous in its political implications for the west, where confederate mercenaries of this kind soon became the only recorded units. From the middle years of the fifth century, the provincial forces were gradually disbanded, and disintegrated altogether; the western Roman armies had ceased to exist. In the east, on the other hand, the army and the state held together for another millennium. Cavalry was still the most important Byzantine arm, and remained the queen of battles.
The survival of this Byzantine state was due to the military achievements of the period between Marcus Aurelius and Constantine. During those hundred and seventy years the Roman forces had been obliged to face shocks and enemies and problems more lethal than any that had confronted earlier emperors. These menaces had made it necessary to reorganise the army completely and to expand it on a gigantic scale, and so it was able to overcome the crises of the mid-third century, and ward off the imminent dissolution of the empire until a still distant future. Already the army had lived through more than half a millennium of victorious activity; but these years of triumph over supreme emergency must be reckoned as the culmination of the Roman military achievement.