In modern scholarship, the "late" period of the Roman army begins with the accession of the Emperor Diocletian in AD 284, and ends in 480 with the death of Julius Nepos, being roughly coterminous with the Dominate. During the period 395–476, the army of the Roman Empire's western half progressively disintegrated, while its counterpart in the East, known as the East Roman army (or the early Byzantine army) remained largely intact in size and structure until the reign of Justinian I (r. AD 527–565).
The Imperial Roman army of the Principate (30 BC – 284 AD) underwent a significant transformation as a result of the chaotic 3rd century. Unlike the army of the Principate, the army of the 4th century was heavily dependent on conscription and its soldiers were paid much less than in the 2nd century. Barbarians from outside the empire probably supplied a much larger proportion of the late army's recruits than in the army of the 1st and 2nd centuries, but there is little evidence that this adversely affected the army's combat performance.
Scholarly estimates of the size of the 4th-century army diverge widely, ranging from ca. 400,000 to over one million effectives (i.e. from roughly the same size as the 2nd-century army to 2 or 3 times larger).Treadgold (1995) 43-60 This is due to fragmentary evidence, unlike the much better-documented 2nd-century army.
Under the Tetrarchy, military commands were separated from administrative governorships for the first time, in contrast to the Principate, where provincial governors were also commanders-in-chief of all military forces deployed in their provinces.
The main change in structure from the 2nd-century army was the establishment of large escort armies (comitatus praesentales), typically containing 20,000–30,000 top-grade palatini troops. These were normally based near the imperial capitals: (Constantinople in the East, Milan in the West), thus far from the empire's borders. These armies' primary function was to deter usurpers, and they usually campaigned under the personal command of their emperors. The legions were split into smaller units comparable in size to the auxiliary regiments of the Principate. Infantry adopted the more protective equipment of the Principate cavalry.
The role of cavalry in the late army does not appear to have been greatly enhanced as compared with the army of the Principate. The evidence is that cavalry was much the same proportion of overall army numbers as in the 2nd century and that its tactical role and prestige remained similar. However, the cavalry of the Late Roman army was endowed with greater numbers of specialised units, such as extra-heavy shock cavalry (cataphractii and clibanarii) and mounted archers.Elton (1994) 106–7 During the later 4th century, the cavalry acquired a reputation for incompetence and cowardice for their role in three major battles. In contrast, the infantry retained its traditional reputation for excellence.
The 3rd and 4th centuries saw the upgrading of many existing border forts to make them more defensible, as well as the construction of new forts with stronger defenses. The interpretation of this trend has fuelled an ongoing debate whether the army adopted a defence-in-depth strategy or continued the same posture of "forward defence" as in the early Principate. Many elements of the late army's defence posture were similar to those associated with forward defence, such as forward location of forts, frequent cross-border operations, and external buffer-zones of allied barbarian tribes. Whatever the defence strategy, it was apparently less successful in preventing barbarian incursions than in the 1st and 2nd centuries. This may have been due to heavier barbarian pressure, or to the practice of keeping large armies of the best troops in the interior, depriving the border forces of sufficient support.
Sources Much of our evidence for 4th century army unit deployments is contained in a single document, the Notitia Dignitatum, compiled c. 395–420, a manual of all late Roman public offices, military and civil. The main deficiency with the Notitia is that it lacks any personnel figures, so as to render estimates of army size impossible. Also, it was compiled at the very end of the 4th century; it is thus difficult to reconstruct the position earlier. However, the Notitia remains the central source on the late Army's structure due to the dearth of other evidence.Lee (1997) 212 The Notitia also suffers from significant lacunae and numerous errors accumulated from centuries of copying.
The main literary sources for the 4th-century army are the Res Gestae (History) of Ammianus Marcellinus, whose surviving books cover the period 353 to 378. Marcellinus, himself a veteran soldier, is regarded by scholars as a reliable and valuable source, but he largely fails to remedy the deficiencies of the Notitia as regards army and unit strength or units in existence, as he is rarely specific about either. The third major source for the late army is the corpus of imperial decrees published in the East Roman empire in the 5th and 6th centuries: the Theodosian code (438) and the Corpus Juris Civilis (528–39). These compilations of Roman laws dating from the 4th century contain numerous imperial decrees relating to all aspects of the regulation and administration of the late army.
De re militari is a treatise on Roman military affairs by Vegetius, a late 4th or early 5th-century writer, and contains considerable information on the late army, although its focus is on the army of the Republic and Principate. However, Vegetius (who wholly lacked military experience) is often unreliable. For example, he stated that the army abandoned armour and helmets in the later 4th century (offering the absurd explanation that this equipment was too heavy), which is contradicted by sculptural and artistic evidence.Elton (1996) 110–5 In general, it is not safe to accept a statement from Vegetius unless it is corroborated by other evidence.
Scholars of the late army have to contend with a dramatic diminution of the epigraphic record in the 3rd and 4th centuries, compared with the 1st and 2nd centuries. Diplomas were no longer issued to retiring auxiliaries after 203 (most likely because almost all were already Roman citizens by then). In addition, there was a huge reduction in the number of tombstones, altars and other dedications by Roman servicemen. Official stamps of military units on building materials (e.g., tiles) are much rarer, but this trend should probably not be seen as indicating a decline in the army's administrative sophistication. Papyrus evidence from Egypt shows that military units continued to keep detailed written records in the 4th century (the vast bulk of which are lost due to organic decomposition). Most likely, the decline in inscriptions is due to changing fashion, in part influenced by the increase in barbarian recruits and the rise of Christianity.Mattingly (2006) 247–8 The dearth of inscriptions leaves major gaps in our understanding of the late army and renders many conclusions tentative.
The seminal modern study of the late army is contained in The Later Roman Empire, 284-602 (LRE) by the "high priest" of late Roman studies, A.H.M. Jones. Because of its wealth of detail and documentary references, this 1964 publication remains an essential tool for all scholars of the period. However, its primary weakness is its age, for a considerable amount of archaeological work and other relevant scholarship has transpired in the decades since its publication.
Evolution of the 4th-century army
Background: the army of the Principate
The regular army of the Principate was established by the founder–emperor Augustus (ruled 30 BC – 14 AD) and survived until the end of the 3rd century. The regular army consisted of two distinct corps, both being made up of mainly volunteer professionals.
The elite legions were large infantry formations, varying between 25 and 33 in number, of c. 5,500 men each (all infantry save a small cavalry arm of 120) which admitted only Roman citizens.Goldsworthy (2003) 50, 78 The auxilia consisted of around 400 much smaller units of c. 500 men each (a minority were up to 1,000 strong), which were divided into approximately 100 cavalry alae, 100 infantry cohortes and 200 mixed cavalry/infantry units or cohortes equitatae.Holder (2003) 120 Some auxilia regiments were designated sagittariorum, meaning that they specialised in archery. The auxilia thus contained almost all the Roman army's cavalry and archers, as well as (from the late 1st century onwards) approximately the same number of foot soldiers as the legions.Goldsworthy (2003) 56–8 The auxilia were mainly recruited from the peregrini: provincial subjects of the empire who did not hold Roman citizenship, but the auxilia also admitted Roman citizens and possibly barbari, the Roman term for peoples living outside the empire's borders.Goldsworthy (2003) 80 At this time both legions and auxilia were almost all based in frontier provinces.Holder (2003) 145 The only substantial military force at the immediate disposal of the emperor was the elite Praetorian Guard of c. 10,000 men which was based in Rome.Goldsworthy (2003) 58
The senior officers of the army were, until the 3rd century, mainly from the Italian aristocracy. This was divided into two orders, the senatorial order (ordo senatorius), consisting of the c. 600 sitting members of the Roman Senate and their sons and grandsons, and the more numerous (several thousand-strong) equites or "knights".
Hereditary senators and equites combined military service with civilian posts, a career path known as the cursus honorum, typically starting with a period of junior administrative posts in Rome, followed by 5–10 years in the military and a final period of senior positions in either the provinces or Rome.Goldsworthy (2003) 60, 66 This tiny, tightly-knit ruling oligarchy of under 10,000 men monopolised political, military and economic power in an empire of c. 80 million inhabitants and achieved a remarkable degree of political stability. During the first 200 years of its existence (30 BC – 180 AD), the empire suffered only one major episode of civil strife (the Civil War of 68–9). Otherwise, usurpation attempts by provincial governors were few and swiftly suppressed.
As regards the military, members of the senatorial order (senatorii) exclusively filled the following posts: (a) legatus Augusti pro praetore (provincial governor of a border province, who was commander-in-chief of the military forces deployed there as well as heading the civil administration) (b) legatus legionis (legion commander) (c) tribunus militum laticlavius (legion deputy commander).Goldsworthy (2003) 60
The equites provided: (a) the governors (procuratores) of Egypt and of a few minor provinces (b) the two praefecti praetorio (commanders of the Praetorian Guard) (c) a legion's praefectus castrorum (3rd-in-command) and its remaining five tribuni militum (senior staff officers) (d) the praefecti (commanders) of the auxiliary regiments.Goldsworthy (2003) 64–5