Roman imperial power rested ultimately on military force. Both the western and the eastern empires maintained professional armies of lower-ranked officers and career soldiers. In the early empire the organization of Roman armies is very fully documented epigraphically, and the dense distribution of inscriptions set up by and for soldiers and veterans along the frontiers and elsewhere is an excellent indication that Rome's military establishment was maintained in reality as well as on paper. The evidence of the later empire, when inscriptions are much rarer, provides no such reassurance. The basis for most reconstructions of Roman military deployment in the earlier part of late antiquity is the Notitia Dignitatum, which accounts for the officials and military units of the eastern empire around 400 and the western empire around 425. Analyzing the data contained in the Notitia, and indeed the other evidence for military deployment during the later empire, is an intricate technical business, but it appears that about half the western regiments had simply vanished from the lists in the period of intense barbarian and civil warfare between 395 and 425, and that although notionally their numbers had been replenished in the latter part of this period, this was largely achieved by regrading less well equipped and trained frontier troops (limitanei) as part of the main field army (comitatenses). Moreover the overall number of field army units had dropped by a quarter.23 Even this estimation seems optimistic. Between 430 and 450 the Roman hold on Gaul was maintained under the leadership of the magister militum Aetius, but his military superiority in the face of Visigoths, Burgundians, Bagaudae, and others depended on his access to Hunnic mercenaries. We may conclude that the units of the west Roman field army were not only reduced in number, but also individually seriously undermanned. By the mid-fifth century, the army was a shadow of its former self.24
We gain precisely this impression in microcosm by examining an episode from a corner of the eastern empire at the beginning of the fifth century, revealed in the letters of Synesius, formerly a philosopher but now bishop of the city of Ptolemais in Tripolitania (modern Libya). Synesius praised a small cavalry band of forty Huns for their role in defending his city's territory from marauding nomads. The Hunnic troops had asked Synesius to champion their cause and to forward a petition to the emperor in Constantinople requesting that their numbers should be made up to a more viable figure of 200, that they should not be absorbed into the ranks of the local native troop units, and that they should receive the mounts, pay, and equipment that were due to them.25As it happens, Libya is the one province of the Roman Empire for which the information of the Notitia Dignitatum is missing, so we do not know the theoretical composition of its garrison. However, other letters of Synesius draw attention to the demoralization, or more precisely the demilitarization of the native troops, the limitanei. He records that one of their commanders,
as if it were the law that the pay of the rank and file belongs to the generals, pocketed what they all used to get and in return gave them immunity from service so that they need not stay in their units, letting them go where each thought he would make his living. (Synesius, ep. 62, trans. A. H. M. Jones)
On another occasion the same commander simply stole the horses of his mounted archers and sold them for his own profit. We can hardly avoid the conclusion that the provincial garrison was understrength, ill equipped, and undermined by corrupt leadership. We should perhaps not be surprised that matters of local security seem a greater concern to the local bishop than to the military commander, and that actual protection was delivered by a modest but estimable band of imported barbarians.26
The ethos of the eastern empire during the first half of the fifth century was not military. One attitude to warfare with the main enemy, the Sassanians, is summed up in the famous mot of Nestorius; when he was made bishop of Constantinople, to Theodosius II, “Give me, King, the earth purged of heretics, and I will give you heaven in return. Aid me in destroying heretics, and I will assist you in vanquishing the Persians” (Socrates HE 7.29.5).27 Constantinople was defended by its massive new fortifications against the unruly Balkans, and two of the eastern empire's five magistri militum with the infantry and cavalry under their command were quartered in the capital. Defense of the capital and the security of the emperor were the twin priorities. During the fifth and sixth centuries the members of the scholae, the emperor's bodyguard, had abandoned any military pretensions and lived a life on their salaries corresponding to that of the Russian imperial officer class depicted in the pages of Tolstoy's Anna Karenina (Procopius, Secret History 24.15–23). Theodosius II made no military attempt against Attila's Huns, preferring to use a sizeable proportion of state income to pay protection money. Likewise peace was maintained along the eastern frontier, not least because substantial subsidies were given to the Sassanian Empire to cover the cost of defending the passes through the Caucasus, where nomadic incursions threatened both empires. Serious military responsibilities in the later fifth century were devolved on ethnic groups: the Ostrogothic tribal forces under Theoderic Strabo and Theoderic the Amal controlled Thrace and the east Balkans, while Isaurians were the dominant force in the capital itself.
The fifth and sixth centuries also witnessed the growing importance of private militias, the bucellarii, who were recruited and fought for individual commanders. In origin these seem to have been soldiers who formed the bodyguards of barbarian commanders in Roman service, numbering from the low hundreds to 7,000 in the case of Belisarius in AD 534. Most groups of bucellarii were identified by their ethnicity: Isaurians, Armenians, Pisidians, Cappadocians, Thracians, Persians, and others.28 The importance of these groups will have been that they were paid and maintained by their private masters, not by the Roman state.
It is notoriously difficult to assess the numerical strength of the late Roman armies. Even the cautious calculations of A. H. M. Jones convey an impression that is closer to voodoo than to scientific estimation. However, it is striking that the high global figures found in Zosimus writing around 500, and in John Lydus in the mid-sixth century, which imply total forces approaching 450,000 men, relate not to their own day but, unverifiably, to the situation at the beginning of the fourth century (see pp. 176–7). Agathias stated that at the death of Justinian in AD 565 total troop strength was around 150,000, less than a quarter of its earlier peak of 645,000.29 None of these figures is trustworthy, but we should surely accept that there had been a very steep numerical decline since the fourth century. This impression is borne out by the numbers of troops actually attested on campaigns. The largest reliably attested field army assembled in late antiquity after AD 350 was the 65,000-strong force with which Julian invaded Mesopotamia in 363 (Zosimus III.13). In AD 503, the Romans assembled an army of 52,000 at Edessa for war with Persia.30 These figures were never subsequently matched. It is astounding to note that when Khusro I invaded Syria in 539–40, Justinian's only immediate response was to send a force of 300 men from Constantinople under the command of his nephew Germanus (Procopius Bell. Pers. II.6.9); effectively the cities of Syria, including Antioch, were left to fend for themselves. In the conflicts with the Persians of 530, 531, and 543, Belisarius and his fellow commanders led forces ranging between 20,000 and 30,000 (Procopius, Bell. Pers. I.13.23; I.18.5; II.24.16). The initial expeditionary forces against Africa and Italy respectively comprised 15,000 and 7,000 regular troops, although 12,000 and later 18,000 troops were finally deployed against the Ostrogoths in 543 and 554 (Procopius, Bell. Vand. III.11.; Bell. Goth. V.5.2–4; VII.3.4; Agathias II.4).31
The sources provide no reliable figures for Roman forces in warfare between Rome and the Sassanians from 602 to 630. Their enemies, the Avars, are said to have brought a vanguard of 30,000 troops to the walls of Constantinople in 626, and during the siege of the city the Persians are said to have attempted but failed to ferry 3,000 or 4,000 troops across the Bosporus. No numbers are supplied for the field armies that Heraclius led into Armenia and Atropatene in 624–5, or during the counter-offensive of 627–8, in which he marched into Mesopotamia, menaced Ctesiphon, ravaged the cities of the Diyala valley, and withdrew north of the Zagros mountains to the region of lake Urmia. However, it is inconceivable that such mobility, often during early spring campaigns when the mountains were barely passable, across a region without Roman roads, could have been achieved by large forces. It is doubtful whether Heraclius commanded more than 10,000 men on these huge forays.32
In fact during the later sixth century Roman forces had been seriously undermined by economic weakness. Reductions or the withholding of military pay, leading to mutinies, are a recurrent theme in the narrative of late Roman warfare. Procopius alleges that pay for the limitanei was running four or five years in arrears, until Justinian simply denied that these units had the status of an army at all (Secret History 24. 12–14).33 The financial strain of maintaining armies in the field was also a major factor during Justinian's wars. The small Roman garrison of Beroea (Aleppo) deserted to Khusro I in 540 as they had not been paid for a long time (Procopius, Bell. Pers. II.7.37). After the reconquest of Africa, a major mutiny of 8,000 Roman troops was led by Stotzas, who stoked the rebellion with a reminder that the troops had not been paid since long in the past, and had furthermore been deprived of the spoils of war, which their efforts had earned them (Procopius, Bell. Vand. IV.15.55). The same complaint was made by subsequent mutineers under Maximinus (Bell. Vand. IV.18.9). In the course of the Gothic wars payment to the Roman troops in Italy was substantially in arrears (Bell. Goth. VII.6.7). A body of Illyrian troops which had been sent to support Belisarius, simply withdrew from their position outside Bononia in 544, sending their excuses to the emperor: “they had returned to their homelands for no other reason than that, after their long service in Italy without receiving the regular payment at all, the state now owed them a large sum of money” (Procopius, Bell. Goth. VII.11.14). During the siege of Rome in 549, a body of Isaurians guarding one of the gates betrayed the city to the Gothic king Totila. They nursed a grievance “because for many years nothing had been paid to them by the emperor” (Bell. Goth. VII.36.7). The problem was endemic, and not addressed until Narses' decisive intervention with very large forces in 552:
He had received from the emperor an exceedingly large sum of money, with which he was, first, to gather a very formidable army and meet the other requirements of the war, and after that, to pay the soldiers in Italy all the money which was due to them from the past; for the emperor had been delinquent in this matter for a long time, since the soldiers were not receiving from the public treasury, as was usual, the pay assigned to them; furthermore he was able to bring pressure on the soldiers who had deserted to Totila, so that they would be rendered tractable by this money and reverse their choice of allegiance. (Procopius, Bell. Goth. VIII.26.6, trans. Dewing)
The problem remained acute in the late sixth century. In 588 there was a major insurrection by troops in Mesopotamia when their general, Priscus, announced a reduction of military pay by a quarter. Priscus was deposed and replaced by a man of the soldiers' own choice, Germanus, who succeeded in assuaging them with spoils from the capture of Martyropolis (Theophylact III.1.1–III.4.5). The nadir came during Maurice's Balkan campaigns against the Avars and Slavs. Regular pay was again cut (Theophylact VII.1.8) and this was compounded in the winter of 602, when the field army of Thrace was ordered to conduct a winter campaign outside Roman territory north of the Danube unsupported by the commissariat. The mutiny that ensued led directly to the fall of the emperor and his replacement, as emperor, by the ex-centurion Phocas (see pp. 447–8).
Despite the huge military convulsion of antiquity's last war, between Heraclius and the Sassanians, we cannot avoid concluding that the military forces of the Romans, and of their adversaries, were much diminished by comparison with the numbers and capacity of their predecessors in the fourth, still less in the earlier centuries of the empire. The reasons were complex and interlocking: tax revenue that was insufficient to pay for large armies; the gradual but systematic demilitarization of the frontier troops; the steady abandonment of a military ethos; the increased proportion of state revenue used to buy peace through diplomacy, rather than by maintaining a credible threat of war.34 It is conventional to explain the astounding rapidity of the Muslim conquests of the Romans and the Sassanians between 632 and 651 by arguing that the two empires had fought each other to a standstill, exhausting their human and financial resources. In fact, between the accession of Justinian and the death of Muhammad the strategic and political potency of the eastern empire had been hollowed out. Its military weakness had been masked by a combination of Roman diplomatic skills, by the mistakes and misjudgments of its adversaries, and by the fact that incapacity during the later fifth and early sixth century was by no means limited to the eastern Roman Empire. However, at the decisive moment, the Islamic conquests, as rapid, sweeping, and extensive as those of Alexander the Great, were achieved at the expense of an empire that was already severely debilitated.