Late Roman army
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 remained largely intact in size and structure until the reign of Justinian I.
The Imperial Roman army of the Principate 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. 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, typically containing 20,000–30,000 top-grade palatini troops. These were normally based near the imperial capitals:, 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 and mounted archers. 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.
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 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 which admitted only Roman citizens. The auxilia consisted of around 400 much smaller units of c. 500 men each, which were divided into approximately 100 cavalry alae, 100 infantry cohortes and 200 mixed cavalry/infantry units or cohortes equitatae. 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 approximately the same number of foot soldiers as the legions. 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. At this time both legions and auxilia were almost all based in frontier provinces. 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.
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, consisting of the c. 600 sitting members of the Roman Senate and their sons and grandsons, and the more numerous 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. 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, the empire suffered only one major episode of civil strife. Otherwise, usurpation attempts by provincial governors were few and swiftly suppressed.
As regards the military, members of the senatorial order exclusively filled the following posts:
The equites provided:
By the late 1st century, a distinct equestrian group, non-Italian and military in character, became established. This was a result of the established custom whereby the emperor elevated the primuspilus of each legion to equestrian rank on completion of his year in office. This resulted in some 30 career soldiers, mostly non-Italian and risen from the ranks, joining the aristocracy each year. Far less wealthy than their Italian counterparts, many such equites belonged to families that provided career soldiers for generations. Prominent among them were Romanised Illyrians, the descendants of the Illyrian-speaking tribes that inhabited the Roman provinces of Pannonia, Dalmatia and Moesia Superior, together with the neighbouring Thracians of Moesia Inferior and Macedonia provinces. From the time of Domitian, when over half the Roman army was deployed in the Danubian regions, the Illyrian and Thracian provinces became the most important recruiting ground of the auxilia and later the legions.
3rd-century developments
The seminal development for the army in the early 3rd century was the Constitutio Antoniniana of 212, issued by Emperor Caracalla. This granted Roman citizenship to all free inhabitants of the empire, ending the second-class status of the peregrini. This had the effect of breaking down the distinction between the citizen legions and the auxiliary regiments. In the 1st and 2nd centuries, the legions were the symbol of the dominance of the Italian "master nation" over its subject peoples. In the 3rd century, they were no longer socially superior to their auxiliary counterparts and the legions' special armour and equipment was phased out.The traditional alternation between senior civilian and military posts fell into disuse in the 2nd and 3rd centuries, as the Italian hereditary aristocracy was progressively replaced in the senior echelons of the army by the primipilares. In the 3rd century, only 10% of auxiliary prefects whose origins are known were Italian equestrians, compared to the majority in the previous two centuries. At the same time, equestrians increasingly replaced the senatorial order in the top commands. Septimius Severus placed equestrian primipilares in command of the three new legions he raised and Gallienus did the same for all the other legions, giving them the title praefectus pro legato. The rise of the primipilares may have provided the army with more professional leadership, but it increased military rebellions by ambitious generals. The 3rd century saw numerous coup d'état and civil wars. Few 3rd-century emperors enjoyed long reigns or died of natural causes.
Emperors responded to the increased insecurity with a steady build-up of the forces at their immediate disposal. These became known as the comitatus. To the Praetorian Guard's 10,000 men, Septimius Severus added the legion II Parthica. Based at Albano Laziale near Rome, it was the first legion to be stationed in Italy since Augustus. He doubled the size of the imperial escort cavalry, the equites singulares Augusti, to 2,000 by drawing select detachments from alae on the borders. His comitatus thus numbered some 17,000 men, equivalent to 31 infantry cohortes and 11 alae of cavalry. The trend for the emperor to gather round his person ever greater forces reached its peak in the 4th century under Constantine I the Great whose comitatus may have reached 100,000 men, perhaps a quarter of the army's total effective strength.
The rule of Gallienus saw the appointment of a senior officer, with the title of dux, to command all the comitatus cavalry. This force included equites promoti, plus Illyrian light cavalry and allied barbarian cavalry. Under Constantine I, the head of the comitatus cavalry was given the title of magister equitum, which in Republican times had been held by the deputy to a Roman dictator. But neither title implies the existence of an independent "cavalry army", as was suggested by some more dated scholars. The cavalry under both officers were integral to mixed infantry and cavalry comitatus, with the infantry remaining the predominant element.
The 3rd century saw a progressive reduction in the size of the legions and even some auxiliary units. Legions were broken up into smaller units, as evidenced by the shrinkage and eventual abandonment of their traditional large bases, documented for example in Britain. In addition, from the 2nd century onwards, the separation of some detachments from their parent units became permanent in some cases, establishing new unit types, e.g. the vexillatio equitum Illyricorum based in Dacia in the early 2nd century and the equites promoti and numerus Hnaufridi in Britain. This led to the proliferation of unit types in the 4th century, generally of smaller size than those of the Principate. For example, in the 2nd century, a vexillatio was any detachment from a legion or auxiliary regiment, either cavalry or infantry. In the 4th century, it denoted an elite cavalry regiment.
From the 3rd century are the first records of a small number of regular units bearing the names of barbarian tribes. These were foederati converted into regular units, a trend that was to accelerate in the 4th century. The ala I Sarmatarum, based in Britain, was probably composed of some of the 5,500 captured Sarmatian horsemen sent to garrison Hadrian's Wall by emperor Marcus Aurelius in c. 175. There is no evidence of irregular barbarian units becoming part of the regular army of the Principate until the 3rd century.