There were several significant changes made to the structure of the Roman army during the third and fourth centuries. While the overall number of troops under arms had probably increased, the size of individual units, especially the legions, dwindled. By the end of the third century there were more than sixty legions in existence that seem to have had a theoretical strength of about a thousand men, 20 per cent of their earlier size. In 212 Caracalla had extended Roman citizenship to the greater part of the Empire’s population, removing much of the distinction between citizen and non-citizen troops. The new units of cavalry titled vexillationes and the old alae and infantry cohorts seem to have mustered around five to six hundred men apiece. The equipment and training of legions and auxiliary infantry seems to have been virtually identical, fighting as close-order troops in the battle line. In the third century the rectangular scutum and the heavy pilum became less common and were replaced by oval or round shields and lighter spears such as the lancea. Some units also carried as many as five lead-weighted darts (plumbatae or mattiobarbuli) slotted into the hollows of their shields. Reconstructions have suggested that these had a range of about 30-65 metres (99—215 feet), more than double that of a pilum and significantly more than most javelins. Most units seem to have worn scale or mail armour and iron helmets if these were available. The tactics of later Roman infantry were less aggressive than those of the early Principate and in some ways more akin to those of their Polybian predecessors. Ammianus describes Roman infantry raising the barritus, a Germanic war cry which steadily increased in volume, in order to build up their own confidence before they entered hand-to-hand combat. The single volley of pila thrown just before contact was replaced by a much longer barrage of darts, javelins and frequently arrows fired by archers in close support. The charge of barbarian infantry was sometimes met at the halt, sacrificing the moral lift given in an advance to ensure that the stationary Roman line remained in good order and delivered the greatest possible number of missiles. However, faced with the numerous archers of the Persian armies, it was more common to sacrifice order and advance at a run, minimizing the time spent in an exchange of missiles in which the enemy would probably prove superior.
It is possible that the average unit in the later army did not have the discipline required to advance slowly and in silence, waiting until very close range before delivering a single volley of pila and charging into contact. The shock tactics of the early Principate were probably a more effective way of producing a decisive result in an infantry clash and winning a pitched battle. However, as we shall see, the fighting of pitched battles had ceased to be the main concern of the Roman army by the fourth century. The 5,000-strong legion had been designed for big battles, its commander controlling a sizeable section of the line and supplying units for each of the multiple supporting lines of the army’s formation. The later army maintained the traditional Roman emphasis on the use of reserves, deploying into more than one line as a matter of course. The smaller size of its units meant that, while each formed an effective part of any line, they had little experience of operating in mutual support and lacked a command structure to facilitate the use of reserves. In the defeat at Adrianople one unit of Batavi placed in reserve could not be found when it was needed.
The emperor Caracalla was the eldest son of Septimius Severus. In 212 he extended Roman citizenship to most of the population of the Empire. On campaign he lived the life of a simple soldier, allegedly grinding his own ration of grain into flour with a handmill. He was stabbed to death by one of his own cavalry bodyguard, or singulares.
This detail of the Arch of Galerius at Salonika shows a battle scene front his campaign against the Persians at the end of the third century. The Roman troops are depicted wearing scale armour and carrying round or oval shields. They carry vcxilla flags and the snake-like draco standards, where a multi-coloured fabric tube streamed out behind a bronze animal head, like a modern windsock. On Trajan’s Column these standards were carried by the enemy, but we hear of their use by the Roman cavalry as early as Hadrian's reign.
It has often been claimed that cavalry assumed a greater importance in the later army, although it has proved difficult to trace any specific changes. In part, this view derived from a misunderstanding of the great importance of the auxiliary horsemen of the Principate. Although a slight change is possible, there is no good evidence for a significant increase in the proportion of horsemen, and the ideal as in earlier periods was to have a balanced army composed of both foot and horse, the latter always in a minority. The number of heavily armoured units, cataphractoi and clibanarii, did increase, especially in the eastern army which often had to face the equally heavily armoured cavalry of the Persians. However, the vast majority of Roman cavalry continued to be trained to fight with missiles or mount a charge, and the specialist shock cavalry were not more numerous than the units of specialist light horsemen.
One of the most significant changes in the structure of the later army was the division between the troops of the field armies, the comitatenses, and the troops assigned to the border areas, the limitanei. This development occurred gradually and was not completed or perhaps officially sanctioned until Constantine. For most of the Principate the Roman army had been distributed mainly in the frontier provinces, with few troops being stationed in the interior of the Empire. Severus stationed one of his newly raised legions, Legio II Parthica, near Rome which, combined with his enlarged Praetorian Guard, provided a force of about fifteen thousand infantry and two thousand cavalry at his immediate disposal. Gallienus seems to have concentrated legionary vexillations and elite cavalry units in the approaches to northern Italy. Diocletian and Maximian gathered high- quality units into their comitatenses, including some elite Pannonian legions, although the size of these forces was not as great as the fourth-century field armies. Traditionally it was assumed that this trend towards troop concentrations within the Empire supplied a need for strategic reserves, allowing the emperor to cope with hostile incursions which could no longer be stopped by the troops deployed as a cordon around the frontier. This view is not supported by the evidence. Severus came to power after a long civil war, and his need for personal security was probably the main reason for the creation of an army in Italy. Gallienus had lost most of the eastern and western provinces of the Empire, hence the need for loyal troops to protect the areas which he did control. The tetrarchs similarly needed to maintain their rule by military force, although they returned the vast majority of troops to the frontiers. Constantine, victor in a civil war and insecure for at least the first half of his reign, created the large field armies. The comitatenses provided the emperor’s ultimate guarantee of power, protecting him against political rivals. His control of such an army increased the likelihood of his support in provinces eager to be protected by concentrated forces positioned to deal with their local problems. All these forces, from Severus’ Italian troops to the fully developed field armies, did provide the basis for campaigning armies whenever the emperor decided to wage a foreign war or had to retrieve a major disaster somewhere in the Empire. The field armies gave the emperor personal security and the ability to wage war when required, but proved very much a two-edged sword. An emperor could not afford to leave a concentration of troops where it might be suborned by a rival. He needed to keep close control over the comitatenses, but this close contact with the troops made it harder to ensure their loyalty. The field armies were responsible for the murder of many emperors and the elevation of their replacements.
Hunting scenes like these mosaics front the Imperial Villa at Piazza Armerina in Sicily are a common feature of the art of Late Antiquity. They frequently show soldiers without armour and bareheaded, hut carrying their shields. Hunting was popular with army officers from the early Empire onwards.
The limitanei were the troops allocated to the command of the officers (duces liniitis) who controlled the regions into which the Empire was now divided. They were most certainly not a local militia of farmer soldiers as has often been claimed, but units of the regular Roman army. The limitanei carried out duties ranging from internal security, the policing of roads, defence against banditry and raiding, as well as support for officials such as tax collectors and magistrates. The commanders of the limitanei were powerful men in the day-to-day life of the provinces. They fulfilled all the duties which had tended to devolve on any Roman unit stationed in one locality for any length of time since the beginning of the Principate.
The decorated tunics worn by these men, white or off- white with darker colour circular patches and borders, were standard military dress in Late Antiquity. Poor evidence for the colour of Roman uniforms suggests that white or off-white was the most likely shade, with officers probably wearing brighter white clothing, or possibly red.
The Arch of Constantine was erected in Rome in AD 315, but made use of many sculptures from earlier monuments. These scenes show Marcus Aurelius making a speech to a parade of soldiers and, in the right hand frame, performing a sacrifice on the army's behalf. Much of the equipment depicted would not have been familiar to Constantine’s soldiers, for instance the segmented armour which had fallen out of use in the early third century. It is unclear whether the reuse of so much material on Constantine’s monument was simply a measure of economy or a desire to legitimize his rule by association with the military glories of the past.
Usually the unit occupied several posts within the region, and frequently was broken up to provide many small detachments. The limitanei were used to oppose small- scale enemy threats, and might also be added to a field army operating in the area; they seem to have performed well in both roles. Units detached to a field army for a long period assumed the grade of pseudocomitatenses.
The field armies were mobile in the sense that, unlike the limitanei, they were not tied to a particular frontier region which would suffer in their absence. Their removal from the frontiers and concentration, usually in or near cities, in theory meant that they were not called upon to perform everyday policing and administrative duties.
Their mobility and availability as strategic reserves should not be exaggerated. The speed of a field army was never faster than that of a marching infantryman. An even greater restriction on their actions was the need to supply a force en route to and in the campaigning area. Major foreign expeditions took at least a year’s preparation before they could be launched. Constantine appointed two senior subordinates, the magister equitum and the magister peditum, and under these were counts (comites) who might command smaller detachments. In 350 there were three major field armies, in Gaul, Illyricum and the east, but by the end of the century smaller forces had been created in Africa, Britain, Spain, on the Upper and Lower Danube and Thrace. This was in part a result of the inability of the larger field armies to deal with problems arising simultaneously in different provinces, but was also a reflection of the trend towards decentralization of authority in the late Empire.
The soldiers of the later army remained long-service professionals, although various forms of conscription became more common methods of recruitment than voluntary enlistment. Sons of soldiers were forced to serve in the army and local landlords were obliged to supply a set quota of men. Military service was not always popular and there were frequent attempts to avoid it, not least by landlords who were reluctant to see their labour force reduced. However, once in the army men seem to have adapted to a military career. Our sources make frequent mention of desertion, but this had always been a problem for the professional army and it is impossible to gauge whether or not the situation had worsened. Under the Principate the supply of recruits seems to have been adequate to maintain the professional army, which only grew slowly from the size set by Augustus. Levies often occurred before major w'ars to bring existing units up to strength, but it was difficult to raise large numbers of whole units at short notice. During the Pannonian and German crises Augustus had recourse to recruiting units of freed slaves, while Marcus Aurelius employed units of former gladiators in his Danubian campaigns. The professional army simply did not possess the great reserves of manpower of the old citizen militia. The expansion of the army in the fourth century, combined with a probable decline in the Empire’s population made it even harder for later emperors to raise large numbers of troops quickly.
The Roman fort at Qasr Bsheir in Jordan dates to the reign of Diocletian. Its desolate location prevented the theft of its stone for reuse in more recent buildings and ensured its remarkable preservation. Even the inscription recording its construction by the governor of Arabia in ad 292-305 remains in place over its gateway. Like most Late Roman bases it is very small, covering only 0.3 hectares and providing stabling for sixty-nine mounts, but has formidable defences. Walls are higher than in the early Empire and the corner towers now project in front of the walls, to provide enfilade fire against any attackers. The term fort is far more appropriate for these sites than the large barracks of the earlier period.
The units of the field armies were graded as either palatini or comitatenses, the former being the more senior. There was also a complex system of seniority between individual units, as well as units titled seniores and iuniores, presumably the result of the division of some units at an unknown date The units of the auxilia palatina gained some prominence and a high reputation for effectiveness in the fourth century. Raised under Diocletian and Constantine, these units do not seem to have differed that significantly in their recruitment or training from the auxilia of the Principate. Some of these units and other parts of the army were recruited wholly or in part from barbarians from outside the Empire. Many recruits came from the laeti, groups of barbarians settled on land within the Empire, but it seems that most of these did not serve in distinct units. By the latter half of the fourth century increasing numbers of senior officers appear with ‘barbarian’, frequently Germanic, names. One German king, Vadomarius, ruler of the Brisiavi, an Alamannic people, was arrested by the Romans after leading raids into the Empire. He later became a dux in the eastern army, holding several responsible commands and fighting with some distinction for Rome. The use of such foreign troops had a long and distinguished history in the Roman army. The third century had seen the officer corps dominated by members of the aristocracy of the Danubian provinces, who either were, or became, as a result of their military service, Roman citizens and equestrians. Usually both officers and men were absorbed into the pattern of service normal to the Roman army. There is little evidence to suggest that the quality of the army was affected by the recruitment of barbarians. Officers in particular were usually employed away from their place of origin, but most of the foreign recruits seem to have been happy to fight for Rome, even against their own people. The adoption of the Germanic war cry, the barritus, by at least some Roman infantry may have been a result of the influx of Germanic recruits. Equally it may have been adopted by the Romans because they were aware that the Germans found it intimidating. Until large contingents of barbarians began to serve in distinct units under their own leaders there does not appear to have been any reduction in the army’s fighting ability as a result of its recruitment patterns.
These scenes from the Arch of Constantine show the mixture of reused and new material employed in its construction. In the middle is a long scene showing fourth-century soldiers attacking a walled city. They carry large oval shields and wield spears. In Late Antiquity the old taboo against showing Roman dead on monuments was abandoned and Constantine was happy to represent the enemy rout in his victory over the Roman army of his rival Maxentius at the Milvian bridge in 312.
The battle of Strasbourg
In 357 the Caesar Julian led an army of 13,000 men against a confederation of the Alamanni, allegedly numbering 35,000 men, under the leadership of two kings, Chnodomar and Serapio. As in the past, the Roman army naturally deployed in more than one line and fed in reserves as the situation required. Its discipline and command structure gave the Roman army great advantages over the Germanic tribal forces, hut even so the battle was hard fought.