Between AD284 and AD337 – the era of emperors Diocletian and Constantine, and the setting of the “Twilight of Empire” novels – the Roman army not only fought a series of bloody civil wars, but defeated every known enemy on the frontier and expanded the empire for the first time in a century. Clearly the legions of the later empire, when properly led, were a formidable force indeed. But this army appears in many ways quite different to the more familiar force of the earlier empire, portrayed on monuments like Trajan’s Column in Rome, and (albeit without a great degree of authenticity!) in films like Gladiator, Centurion and The Eagle. How did this change happen, and why?
As with most large and complex organisations over long periods of time, the development of the Roman army from the 2nd to the 4th centuries shows signs of both evolution and revolution, besides continuity. Nevertheless, by looking back at the army of an earlier era, we can perhaps identify the seeds of future developments.
During the 1st and 2nd centuries AD, the Roman army transformed itself from a strategically and tactically aggressive force of conquest and expansion, into a microcosmic military society based in permanent fortresses on the frontiers, adapted for small-scale local defence and periodic large-scale punitive campaigns. Already, though, emperors like Trajan, Hadrian and Marcus Aurelius were calling on smaller detachments of troops – called vexillations – to make up or reinforce field armies in more wide-ranging campaigns across the empire.
It was during the Marcomannic wars of the later 2nd century, when the empire for the first time was invaded by large numbers of Germanic barbarians, that this use of mobile detachments really seems to have gained primacy. By combining smaller bodies of men from different legions, Roman commanders could create strong and flexible ‘offensive’ field armies without depleting the ‘defensive’ strength of the legions on the borders, maintaining security over a wide stretch of frontier while allowing for rapid reaction against incursions and reprisal strikes into barbarian territory.
But this system would be tested to breaking point in the years that followed. The ‘crisis of the third century’ saw the Roman empire beset by barbarian invasions, civil wars, plague and economic upheaval. At Abrittus in AD251, the emperor Decius was slain and his entire army destroyed by invading Goths. Less than a decade later, the emperor Valerian was defeated and taken captive by the Persians, and his troops annihilated or enslaved. These disasters could have been catastrophic for the security of the empire, and it is a testament to the strength and flexibility of the Roman military system that the army maintained itself through the decades that followed, before emerging in a new and quite different form by the century’s end.
It was the emperor Diocletian (AD284-305) who, building on the reforms of his predecessors, established this new-style army. At its core was the comitatus, or imperial retinue, made up of select bodyguard troops. One of these accompanied each of the four emperors of the Tetrarchic system. The comitatus was reinforced on campaign by detachments from the frontier legions – often drawn from the heavily militarised Danubian provinces of Pannonia and Moesia – and new cavalry units called equites. These regular troops were frequently supported by large bodies of barbarian ‘allies’, serving under their own chiefs or kings, perhaps as part of a treaty arrangement with Rome. Galerius had a Gothic contingent during his Persian campaign, while Constantine would later make use of Frankish and Alamannic auxiliaries.
A papyrus from Oxyrhynchus in Egypt gives details of Diocletian's field force, c.AD298; it included troops from two Moesian legions, IV Flavia and VII Claudia, commanded by an officer (praepositus) called Julianus, together with another paired detachment from legions XI Claudia and I Italica. An inscription from Mauretania, meanwhile, probably dating from the emperor Maximian’s campaign of the same year against a rebel group called the Quinquegentiani, records a dedication to ‘Deo Invicto Mithras’ (the invincible god Mithras) constructed by men of the seventh and tenth cohorts of II Herculia.
Some of these legion detachments must have covered vast distances in the service of their emperors. While the two cohorts of II Herculia were on campaign in north Africa, other men of the same legion are recorded in the Crimea, while others seem to have accompanied the junior emperor Galerius on his Persian campaign of AD298. The inscription of a soldier called Aurelius Gaius records a lengthy military career that spanned the empire, from the Rhine frontier to Mauretania and from Mesopotamia and Egypt to the Danube. Gaius even claims to have served in ‘India’ – probably the region of Berenice on the Red Sea, entrepot for the Indian Ocean trade.
In time, these mobile detachments acquired a semi-independent status, losing their connection with the parent legion. They became referred to as 'legions' in their own right, some of them using variations on the old legion name: II Italica Divitensis, which seems to have formed part of Constantine’s army in Italy in AD312, were probably formed from a detachment of legion II Italica based at Divitia on the Rhine. Tombstones from Aquileia in Italy mention a unit called the Moesiaci, also known as prima italicae moesiacae, clearly formed by men of I Italica from Moesia. By the later fourth century both the ‘Moesiaci’ and 'Divitenses' appear in army lists as regular legions.
The appearance of the Roman soldier, on both imperial monuments and individual tombstones, changes radically during this period. A mobile force would perhaps be a more lightly equipped force, but it was also cut off from the traditional legion armoury at home base. This might explain the shift from segmented armour, for example, which required skilled craftsmen to repair, to the more versatile mail, scale and even musculata (breastplate) type armours. The institution of state-run armouries (fabricae) at centralised locations by Diocletian may have been an attempt to give these mobile new armies the supply of armour and weapons previously provided by the legion fortresses. A different type of armour too, perhaps - the ‘ridge’ helmet may have been more cost-effective to build on a production line; constructed of several plates riveted together, they could also have been stronger and more durable than the traditional helmets, which were hammered from single sheets of metal.
Changed equipment suggests a change in battlefield tactics too - the late Roman 'fulcum', or shield-wall, may have developed from the old testudo (‘tortoise’), but appears to have been largely a defensive formation. Longer swords and round or oval shields may have been a response to the need for a more flexible combat style. Rather than the destructive javelin-volley followed by fast aggressive sword-charge that typifies (perhaps even defines) the military style of the earlier empire, we have a variety of considerably more static tactics, coupled with increased and sustained missile potential - a tactic of endurance and attrition, rather than the delivery of a sudden killer blow, perhaps reflecting both the increased sophistication of Rome's enemies and the increased frequency of civil wars.
But amid all this change, there was continuity, and perhaps more than appears immediately obvious. Equipment probably didn't alter suddenly - the traditional pilum (heavy javelin) was still in use into the late 3rd century and perhaps, under a different name, later. The familiar rectangular shield seems to have survived in some places into the 260s, and even the distinctive ‘lorica segmentata’ may not have died out as rapidly as once thought - fragments found at Leon in Spain and Caerleon in Britain date from the later 3rd, perhaps even 4th centuries, so perhaps it was still in use in the era of the tetrarchs.
All armies are responses to the particular military needs of their era. The Roman army repeatedly proved itself able to adapt and to survive - in doing so, it demonstrated a flexible response to military and political necessity. Just as the Roman empire, and the world surrounding it, changed almost beyond recognition over the centuries, so did its army.
In Part Two of this article, I will go on to discuss the changes to the command structure in the legions of Constantine's era.