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.

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.

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).

In the 3rd century, they were no longer socially superior to their auxiliary counterparts (although they may have retained their elite status in military terms) and the legions' special armour and equipment (e.g. the lorica segmentata) was phased out.

[36] At the same time, the Roman army was struggling with the effects of a devastating pandemic, now thought to have been smallpox, the Plague of Cyprian which began in 251 and was still raging in 270, when it claimed the life of Emperor Claudius II Gothicus (r. 268–270).

[39] The armies and, by extension, the frontier provinces where they were based (and mainly recruited), would likely have suffered deaths at the top end of the range, due to their close concentration of individuals and frequent movements across the empire.

[49] They and their successors Probus (276–82) and Diocletian (ruled 284–305) and his colleagues in the Tetrarchy formed a sort of self-perpetuating military junta of Danubian officers who were born in the same provinces (several in the same city, Sirmium, a major legionary base in Moesia Superior) and/or had served in the same regiments.

[17] The Junta reversed the military disasters of 251–71 with a string of victories, most notably the defeat at Naissus of a vast Gothic army by Claudius II, which was so crushing that the Goths did not seriously threaten the empire again until a century later at Adrianople (378).

[52][55] Although the precise terms under which these people were settled in the empire are unknown (and may have varied), the common feature was the grant of land in return for an obligation of military service much heavier than the normal conscription quota.

The policy had the triple benefit, from the Roman government's point of view, of weakening the hostile tribe, repopulating the plague-ravaged frontier provinces (and bringing their abandoned fields back into cultivation) and providing a pool of first-rate recruits for the army.

[68] In parallel with restoring the size of the army, Diocletian's efforts and resources were focused on a massive upgrading of the defensive infrastructure along all the empire's borders, including new forts and strategic military roads.

A traditional view sees the comitatus as a strategic reserve which could be deployed against major barbarian invasions that succeeded in penetrating deep into the empire or as the core of large expeditionary forces sent across the borders.

[1][80] At this point, therefore, the parallel military/civil administrative structure may be summarised as follows: The evolution of regional comitatus was a partial reversal of Constantine's policy and, in effect, a vindication of Zosimus' critique that the limitanei had been left with insufficient support.

At this time, according to the Notitia, in the East there were 2 imperial escort armies (comitatus praesentales), each commanded by a magister militum praesentalis, the highest military rank, who reported direct to the emperor.

These terms appear to have become purely honorific, although they may originally have denoted special equipment or ethnic composition (gentiles were barbarian tribesmen admitted to the empire on a condition of military service).

[121] The majority of Roman cavalry regiments in the comitatus (61%) remained of the traditional semi-armoured type, similar in equipment and tactical role to the alae of the Principate and suitable for mêlée combat.

Bucellarii (the Latin plural of bucellarius; literally "biscuit–eater",[164] Greek: βουκελλάριοι) is a term for professional soldiers in the late Roman and Byzantine Empire, who were not supported directly by the state but rather by an individual, though they also took an oath of obedience to the reigning emperor.

[166] Such forces were employed by the Romans throughout imperial history e.g. the battle scenes from Trajan's Column in Rome show that foederati troops played an important part in the Dacian Wars (101–6).

[181] Recruitment difficulties are confirmed in the legal code evidence: there are measures to deal with cases of self-mutilation to avoid military service (such as cutting off a thumb), including an extreme decree of 386 requiring such persons to be burnt alive.

[184] Barbari ("barbarians") was the generic term used by the Romans to denote peoples resident beyond the borders of the empire, and best translates as "foreigners" (it is derived from a Greek word meaning "to babble": a reference to their incomprehensible languages).

Although protectores were supposed to be soldiers who had risen through the ranks by meritorious service, it became a widespread practice to admit to the corps young men from outside the army (often the sons of senior officers).

[224] Late Roman clothing was often highly decorated, with woven or embroidered strips, clavi, circular roundels, orbiculi, or square panels, tabulae, added to tunics and cloaks.

[244][245] Despite the apparent cheapness of manufacture of their basic components, many surviving examples of Late Roman helmets, including the Intercisa type, show evidence of expensive decoration in the form of silver or silver-gilt sheathing.

He in turn controlled a hierarchy of civilian authorities (diocesan vicarii and provincial governors), whose agents collected, stored and delivered supplies to the troops directly or to predetermined fortified points.

The typical rectangular "playing-card" shape, the long, thin and low walls and shallow ditch and the unfortified gates were not defensible features and their purpose was delimitation and keeping out individual intruders.

[274] This view is too extreme, as all the evidence suggests that such forts, even the more rudimentary earlier type based on the design of marching-camps (ditch, earth rampart and wooden palisade), afforded a significant level of protection.

Late Roman forts were not always located on defensible sites, such as hilltops and they were not designed as independent logistic facilities where the garrison could survive for years on internal supplies (water in cisterns or from wells and stored food).

[285] According to Luttwak, the forward defence system was always vulnerable to unusually large barbarian concentrations of forces, as the Roman army was too thinly spread along the enormous borders to deal with such threats.

[301] This was especially unlikely in the case of these regions, as the Illyrian emperors and officer class that dominated the late army would hardly relish seeing their native provinces reduced to combat zones.

After that, legionaries were trained to engage in aggressive hand-to-hand combat, using the gladius short-sword to execute quick thrusts at the abdomen of their enemies, in a similar manner to more recent bayonet drill.