The principal features in which the military establishment of the fourth century differed from that of the Principate were the existence of a mobile field army, the organisation of the cavalry in bodies independent of the infantry, and the smaller size of the legionary units.

Diocletian had created, and Constantine had developed, a field army in which the Emperor could move to any part of his dominion that happened to be threatened, while at the same time all the frontiers were defended by troops permanently stationed in the frontier provinces. The military forces, therefore, consisted of two main classes: the mobile troops or comitatenses, which accompanied the Emperor in his movements and formed a “sacred retinue” (comitatus); and the frontier troops or limitanei.

The strength of the old Roman legion was 6000 men. The legion of this type was retained in the case of the limitanei; but it is broken up into detachments of about 1000 (corresponding to the old cohort), which are stationed in different quarters, sometimes in different provinces. And these detachments are no longer associated with a number of foot-cohorts and squadrons of horse, as of old, when the legatus of a legion commanded a body of about 10,000 men. The cavalry and the cohorts are under separate commanders.

The field army consisted of two classes of troops, the simple comitatenses and the palatini. The palatini, who took the place of the old Praetorian guards, were a privileged section of the comitatenses and retained the special character of Imperial guards, in so far as most of them were stationed in the neighbourhood of Constantinople or in Italy. The infantry of the field army was composed of small legions of 1000, and bodies of light infantry known as auxilia which were now mainly recruited from Gauls, and from Franks and other Germans. The cavalry, under a separate command, consisted of squadrons, called vexillationes, 500 strong.

Each of these units,- the legion, the auxilium, the vexillatio of the comitatenses, the legionary detachment, the cohort of the limitanei,- was as a rule under the command of a tribune, in some cases of a praepositus. The tribune corresponded roughly to the modern colonel.

All these armies were under the supreme command of Masters of Soldiers, magistri militum. The organisation of this command in the east, as it was finally ordered by Theodosius I, differed fundamentally from that in the west. In the east there were five Masters of Horse and Foot. Two of these, distinguished as Masters in Presence (in praesenti, in immediate attendance on the Emperor), resided at Constantinople, and each of them commanded half of the Palatine troops. The three others exercised independent authority over the armies stationed in three large districts, the West, Thrace, and Illyricum.

It was otherwise in the west. Here instead of five co-ordinate commanders we find two masters in praesenti, one of infantry and one of cavalry. The Master of Foot was the immediate commander of the infantry in Italy and had superior authority over all the infantry of the field army in all the dioceses, and also over the commanders of the limitanei. In the dioceses the commanders of the comitatenses had the title of military counts.

According to this scheme the Master of Horse in praesenti was co-ordinate with the Master of Foot. But this arrangement was modified by investing the Master of Foot with authority over both cavalry and infantry; he was then called Master of Horse and Foot, or Master of Both Services, magister utriusque militiae, and had a superior authority over the Master of Horse. In the last years of Theodosius the command of the western armies was thus centralised in the hands of Stilicho, and throughout the fifth century this centralisation, giving enormous power and responsibility to one man, was, as we shall see, the rule.

The limitanei were under the command of dukes, the successors of the old legati pro praetore of the Augustan system. In the west the duke was subordinate to the Master of Foot; in the east to the Master of Soldiers in the military district to which his province belonged.

The Palatine legions were the successors of the old Praetorian guards, but Constantine or one of his predecessors organised guard troops who were more closely attached to the Imperial person. These were the Scholae, destined to have a long history. We associate the name of School with the ancient Greek philosophers, who gave leisurely instruction to their schools of disciples in Athenian porticoes. It was applied to Constantine’s guards because a portico was assigned to them in the Palace where they could spend idle hours waiting for Imperial orders. The Scholarians were all picked men, and till the middle of the fifth century chiefly Germans; mounted, better equipped and better paid than the ordinary cavalry of the army. There were seven schools at Constantinople, each 500 strong and commanded by a tribune who was generally a count of the first rank. We have already seen that the whole guard was under the control of the Master of Offices. Closely associated with the Scholarians was a special body of guards, called candidati from the white uniforms which they wore.

While the Scholarians and Candidates were in a strict sense bodyguards of the Imperial person and never left the Court except to accompany the Emperor, there was another body of guards, the Domestici, consisting both of horse and foot, who as a rule were stationed at the Imperial Court, but might be sent elsewhere for special purposes. They were under the command of Counts (comites domesticorum) who were independent of the Master of Soldiers. It will be observed that most of the new military creations of the third and fourth centuries had names indicating their close relation to the autocrat, comitatenses, soldiers of the retinue; palatines, soldiers of the palace; domestics, soldiers of the household.

The army of this age had a large admixture of men of foreign birth, and for the historian this perhaps is its most important feature. In the early Empire the foreigner was excluded from military service; the legions were composed of Roman citizens, the auxilia of Roman subjects. Every able-bodied citizen and subject was liable to serve. Under the autocracy both these principles were reversed. The auxilia were largely recruited from the barbarians outside the Roman borders; new troops were formed, designated by foreign names; and the less civilised these soldiers were the more they were prized. Some customs and words illustrate the influence which the Germans exercised in the military world. The old German battle-noise, the barritus, was adopted as the cry of the Imperial troops when they went into battle. The custom of elevating a newly proclaimed Emperor on a shield was introduced by German troops in the fourth century. It would be interesting to know how many Germans there were in the army. The fact that most of the soldiers whom we know to have held the highest posts of command in the last quarter of the fourth century were of German origin speaks for itself.

The legions continued to be formed from Roman citizens; but the distinction between citizens and subjects had disappeared since the citizenship had been bestowed, early in the third century, upon all the provincials, and it was from the least civilised districts of the Empire, from the highlands of Illyricum, Thrace, and Isauria, from Galatia and Batavia, that the mass of the citizen soldiers were drawn. From a military point of view highly civilised provinces like Italy and Greece no longer counted. The legions and citizen cavalry ceased to have a privileged position. For instance, the auxilia on the Danube frontier, who were chiefly of barbarian race, were superior in rank to the legionary troops under the same command.

It was a natural consequence of this new policy, in which military considerations triumphed over the political principle of excluding foreigners, that the other political principle of universal liability to service should also be relinquished. It was allowed to drop. In the fifth century it had become a dead letter, and Valentinian III expressly enacted that “no Roman citizen should be compelled to serve,” except for the defence of his town in case of danger.

A third ancient principle of the Roman State, that only freemen could serve in the army, was theoretically maintained, and though it was often practically evaded and occasionally in a crisis suspended, it is probable that there were never many slaves enrolled.

If we examine the means by which the army was kept up, we find that the recruits may be divided into four classes. (1) There were the numerous poor adventurers, Roman or foreign, who voluntarily offered themselves to the recruiting officer and received from him the pulveraticum (“dust-money,” or travelling expenses), the equivalent of the King’s shilling. (2) There were the recruits supplied by landed proprietors from among their serf-tenants. This was a State burden, but it fell only on the estates in certain provinces. (3) The son of a soldier was bound to follow his father’s profession. But this hereditary military service fell into abeyance before the time of Justinian. (4) The settlements of foreign barbarians within the Empire were another source of supply. These foreigners (gentiles), incorporated in the Empire but not enjoying the personal rights of a Roman, were chiefly Germans and Sarmatians, and they were organised in communities under the control of Roman officers. They are found in Gaul, where they had the special name of laeti, and in the Alpine districts of Italy.

The Imperial army was democratic in the sense that the humblest soldier, whatever his birth might be, might attain to the highest commands by sheer talent and capacity. The first step was promotion to the posts of centenarius and ducenarius, who discharged the duties of the old centurions and our non-commissioned officers. Having served in these ranks the soldier could look forward to becoming a tribune, with the command of a military unit, and the efficient tribune would in due course receive the rank of comes.

In order to follow the history of the fifth century intelligently and understand the difficulties of the Imperial government in dealing with the barbarian invaders it would be of particular importance to know precisely the strength of the military forces at the death of Theodosius.

The strength of the Roman military establishment at the beginning of the third century seems to have been about 300,000. It was greatly increased under Diocletian; and considerable additions were made in the course of the fourth century. The data of the Notitia dignitatum would lead to the conclusion that about A.D. 428 the total strength considerably exceeded 600,000. We have, however, to reckon with the probability that the legions and other military units enumerated in the Notitia were not maintained at their normal strength and in some cases may have merely existed on paper. We may conjecture that if the army once actually reached the number of 650,000 it was not after the death of Theodosius, but before the rebellions of Maximus and Eugenius, in which the losses on both sides must have considerably reduced the strength of the legions. But if we confine ourselves to the consideration of the field army, there seems no reason to doubt that in A.D. 428 it was nearly 200,000 strong. It was unequally divided between east and west, the troops assigned to the west being more numerous. In Italy there were about 24,500 infantry and 3500 cavalry.

The military organisation of Rome, as it existed at the end of the fourth century, was to be completely changed throughout the following hundred years. We have no material for tracing the steps in the transformation; of the battles which were fought in this period not a single description has come down to us. But we shall see, when we come to the sixth century, for which we have very full information, that the military forces of the Empire were then of a different character and organised on a different system from those which were led to victory by Theodosius the Great. These changes partly depended on a change in military theory. The conquests of Rome had always been due to her infantry, the cavalry had always been subsidiary, and, down to the second half of the fourth century and the successful campaigns of Julian on the Rhine, experience had consistently confirmed the theory that battles were won by infantry and that squadrons of horse were only a useful accessory arm. The battle of Hadrianople, in which the East German horsemen rode down the legions, shook this view, and the same horsemen who had defeated Valens showed afterwards in the battles which they helped Theodosius to win, how effective might be large bodies of heavy cavalry, armed with lance and sword. The lesson was not lost on the Romans, who during the following generations had to defend their provinces against the inroads of East German horsemen, and the leading feature of the transformation of the Imperial army was the gradual degradation of the infantry until it became more or less subsidiary to the cavalry on which the generals depended more and more to win their victories. In the sixth century we shall see that the battles are often fought and won by cavalry only. It is obvious that this revolution in tactic must have reacted on the organisation and carried with it a gradual modification of the legionary system. Another tactical change was the increased importance of archery, brought about by the warfare on the eastern frontier.

Rome did not depend only on her own regular armies to protect her frontiers. She relied also on the aid of the small Federate States which lay beyond her provincial boundaries but within her sphere of influence and under her control. The system of client states goes back to the time of the Republic. The princes of these peoples were bound by a definite treaty of alliance – foedus, whence they were called foederati – to defend themselves and thereby the Empire against an external foe, and in return they received protection and were dispensed from paying tribute. In the later period with which we are concerned the treaty generally took a new form. The client prince received from the Emperor a fixed yearly sum, supposed to be the pay of the soldiers whom he was prepared to bring into the field. We shall meet many of these federates, such as the Abasgians and Lazi of the Caucasus, the Saracens on the Euphrates, the Ethiopians on the frontier of Egypt. It was on the basis of a contract of this kind that the Visigoths were settled south of the Danube by Theodosius the Great, and it was by similar contracts that most of the German peoples who were to dismember the western provinces would establish, in the guise of Federates, a footing on Imperial soil.

It may be added that “federation” was extended so as to facilitate and regulate the practice of purchasing immunity from foreign foes, such as the Huns and Persians, a device to which the rulers of the Empire as its strength declined were often obliged to resort. The tribute which was paid for this purpose was designated by the same name (annonae) as the subsidies which were allowed to the client princes.

While the Federate system was continued and developed, a new class of troops began to be formed in the fifth century to whom the name Federates was also applied, and who must be carefully distinguished. These troops were drawn indifferently from foreign peoples; they were paid by the government, were commanded by Roman officers, and formed a distinct section of the military establishment. We shall see that, in the course of the sixth century, these mixed Federate troops had come to be the most important and probably the most efficient soldiers in the Imperial army.

The origin of another class of fighting men who were to play a considerable part in the wars of the sixth century goes back to much the same time as that of the Federates. These were the Bucellarians, or private retainers. It became the practice of powerful generals, and sometimes even civilians, to form an armed retinue or private bodyguard. These soldiers were called bucellarii, from bucella, the military biscuit. Such private armed forces were strictly illegal, but notwithstanding Imperial prohibitions the practice increased, the number of retainers was limited only by the wealth of their master, and officers of subordinate rank had their private armed followers. In the sixth century Belisarius had a retinue of 7000 horse, and these private troops formed a substantial fraction of the fighting strength of the Empire. When they entered the service of their master they took an oath of loyalty to the Emperor.

If the expense of maintaining the army formed a large item in the annual budget the navy cost little. It would be almost true to say that the Empire at the period had no naval armaments. There were indeed fleets at the old naval stations which Augustus had established at Misenum and Ravenna, and another squadron (classis Venetum) was maintained at Aquileia. But it is significant that the prefects of these fleets, which were probably very small, were under the control of the Master of Soldiers in Italy. There was no independent naval command. In the east we find no mention of fleets or naval stations with the exception of the small flotillas which patrolled the Lower Danube under the direction of military commanders on that frontier. For centuries the Mediterranean had been a Roman lake, and it was natural that the navy should come to be held as an almost negligible instrument of war. In the third century it had been neglected so far as even to be inadequate to the duty of policing the waters and protecting the coasts against piracy. An amazing episode in the reign of Probus illustrates its inefficiency. A party of Franks, settled on the shores of the Black Sea, seized some vessels, sailed through the Propontis, plundered Carthage, Syracuse, and other cities, and then passing into the Atlantic safely reached the mouths of the Rhine. Yet in the contest between Constantine and Licinius navies played a decisive part, and the two adversaries seemed to have found many useful vessels in the ports of Greece, Syria, Egypt, and Asia Minor. The fleet of Licinius numbered 350 ships and that of Constantine 200, some of which he built for the occasion. It is not clear what the status of these ships was.

In the fifth century the Empire was to feel the want of an efficient navy, when the Mediterranean ceased to be an entirely Roman sea and a new German power in Africa contested the supremacy of its waters. But the failures and defeats which marked the struggle with the vandals did not impress the government of Constantinople with the need of building up a strong navy. The sea forces continued to be regarded as subsidiary, and in overseas expeditions the fleets which convoyed the transports were never placed under an independent naval command. Not until the seventh century, when the Empire had to fight for its very existence with an enemy more formidable than the Vandals, was a naval establishment effectively organised and an independent Ministry of Marine created.