There were many reasons why the Western Roman Empire fell. Many lists are over there, such as one at the History.com. Some are better than others, but here I will debunk one point that sometimes appears on them: “weakening of the Roman Legions”, the eighth point on the previously linked list.
Roman legions didn’t weaken. Their loyalty towards the state did, as the foreign mercenaries which increasingly made up the legions had no loyalty except towards the person who paid them. But legions of the late Empire were, if anything, stronger than those of the Principate when it came to actually fighting battles. They just spent far too much time fighting them against each other.
Strategically, early Empire relied heavily on superior resources to win. If sending a bunch of legions didn’t work out, the Empire would send another bunch of legions. That is precisely what Rome did after the defeat at Teutoburg forest, invading Germania and defeating Arminius in several battles.
Late Empire however had far more limited resources than the early Empire, while its enemies were far more dangerous. Sassanid Persia facing the late Empire was militarily far more powerful than its Parthian predecessor, as well as far more aggressive. Unlike the Parthian Empire, Sassanid Empire had very keen understanding of siege tactics as well as the political will and strategic incentive to try and take chunks of the Roman territory.
On the northern borders, once disunited barbarian tribes had begun to coalesce into large tribal alliances that were in fact proto-states. Militarily, these tribes had adopted much of the Roman equipment and tactics, to the point that any material difference between armies of Rome and those of barbarians had disappeared. Barbarians had also begun to field significant numbers of cavalry – and some, like Sarmatians, fielded high-quality heavy cataphract cavalry. While this alone may not have been enough to threaten Rome – after all, the Roman Republic had conquered Celtic tribes, whose metalworking was superior to Romans of their time – combination of shifting strategic balance of power and tactical capability meant that the Empire had to divert increasing amount of resources to manage this new threat.
This led to significant developments in the art of war. To compensate for the resource mismatch, Late Roman army rapidly developed the complex military art and science, drawing heavily upon Greek intellectual tradition to do so. To defend against large barbarian hordes with fewer resources, equipment of the army was standardized and simplified – mail armour and four-piece helmet were introduced to simplify manufacture and maintenance both.
In fact, late Roman Empire was much better poised to defend itself against major threats than the early Empire. Early Empire relied on intelligence provided by barbarian tribes to intercept the major incursions at or even before they reached the border. But if the enemy managed to get past the border defences, there was nothing standing between them and the Rome itself. Legions would have to be gathered from along the limes itself, weakening other sectors of the border, before going onto a merry chase after the enemy. Said enemy would in the meantime have the enjoyment of the completely undefended interior, and now-defenseless border could be exploited by raiders.
Late Roman Empire did everything that the early Empire did to secure its borders, but it did not labour under assumption that the borders would remain involatile. For this reason, cities and towns were fortified, and a network of forts (burgi) constructed, which were defended by the limitanei and also acted as resupply points for any army. Forts of such type were relatively small, but numerous and well fortified, thus serving well as defense against raiders. These border defences were backed up by central reserves (comitatenses) which would be able to deploy at short notice and destroy any major barbarian incursion that could not be stopped by the border forces note1. This concept had proven itself as late as 4th century, when Flavius Julianus defeated the invading Germanic tribes in 350, and Flavius Stilicho won major victories in early 400s. In 450, Huns were defeated at Catalaunian plains by Flavius Aetius. And in 450s, emperor Majorian nearly restored the Western Roman Empire in just few years.
In fact, late Roman concept of defense in depth had proven itself time and again. Emperor Gallienus used the central reserve to defeat the Alemanni incursion at the Battle of Mediolanum, and Claudius routed a huge Gothic army at the Battle of Naissus. And of course, Middle Byzantine theme system was the concept of defense-in-depth taken to its ultimate conclusion. But the forward defense was not yet abandoned either, as shown by campaigns of Maximinus Thrax (Battle of Hanzhorn) and even emperor Valens, who as late as fourth century AD campaigned north of Danube.
But the greatest enemy of the late Roman Empire was perhaps itself. Roman army spent its strength in civil wars, and the heavily urbanized Empire was also hit by a series of plagues which devastated its population, taxpayer base and the military. None of this however has anything to do with the army itself.
Tactically, armies of the late Empire were far superior to armies of the Principate. Armies of the Principate were in effect heavy infantry, javelin-and-sword armies that relied on foreign auxilliaries to provide capabilities such as cavalry and ranged weapons. Heavy cavalry in particular was essentially nonexistent, and organic legionary cavalry was used only for scouting and carrying messages. During Tiberius, 25 legions will have numbered some 131 000 men (5 240 * 25), but only 3 000 of those will have been cavalry. Majority of cavalry must have been in the auxilliary units, but this will have been relatively light cavalry, and could not have outnumbered auxilliary infantry. Since auxillia will have numbered about the same number of troops as the legiones, cavalry will have numbered some 26% of the army, and army itself will have been around 260 000 men.
Late Imperial armies consisted of heavy infantry, foot archers, heavy cavalry and horse archers all integrated into a cohesive whole. This gave them far superior striking power and flexibility, and armies of Diocletian or Justinian could count on far more diverse array of tactical options. Cavalry in particular had developed due to necessities of warfare against Persia and the Sarmatians. It might not have been proportionately more numerous than the cavalry of the Principate, but in terms of equipment, tactics and flexibility it was head and shoulders above anything fielded by the Principate. Yet the necessities of constant kleinkrieg on the borders meant that the late Roman legions were far smaller, numbering perhaps a thousand men (800 – 1200) for infantry and five hundred (400 – 600) for cavalry. This was in fact the standard size of old vexillationes, which were used far more frequently than full legions, and likely represented merely a recognition of already existing situation. Total number of men during the reign of Diocletian was reported by John the Lydian as being some 390 000 men, similar to numbers attained under Hadrian. These were divided between anywhere from 132 to 180 (Notitia Dignitatum) legions as well as border units. The division between the comitatenses and limitanei is roughly similar to that between the legiones and auxillia under the Principate military. Major difference was in fact strategic, the comitatenses having been pulled towards the interior, serving as a mobile strike force while the limitanei protected the border. Limitanei with time also became much more integrated into the local society, often owning and even working on the land to support themselves. While this made them far less mobile, it also meant they were much cheaper to support and far more effective in home defense than the old auxillia. Originally however limitanei may have been as professional as the comitatenses.
Equipment had also changed. Late Roman heavy infantry now fought in a shield wall, rather than a much looser semi-skirmish formation, and had to content with heavy cavalry far more often than their early Imperial predecessors. For this reason, javelins were abandoned. Where Principate legionary utilized a curved, square scutum shield, a short (46 cm) gladius sword and a couple of javelins, the Dominate legionary would have a round clipeus shield, a long (76 cm) spatha sword and a stabbing spear (spiculum). Ranged offensive power will have been provided by darts (plumbata) and the archers. Overall, Dominate infantry tactics were much more defensive and missile-focused than those of the Principate, reflecting the increased capability of the enemies faced by the Dominate armies and the need to preserve manpower. Late Roman army also abandoned lorica segmentata – which may never have been that widespread in the first place – and relied solely on the mail (lorica hamata) and scale (lorica squamata) armor, which was much easier to produce, maintain and repair. Despite Vegetius’ claims however, late Roman troops were still predominantly armored.
Perhaps the main reason why Late Roman army is seen as weak is the fact that it was active during the time when the Roman Empire fell. But this is again looking from the wrong perspective, and for several reasons. First reason is that the Roman Empire didn’t fall. Western Roman Empire did, but the Eastern Empire survived and even flourished. It would proceed to reconquer much of the West, and only a series of plagues beginning with the Justinian’s Plague would herald its decline.
But performance of the army itself was not the reason why the Empire fell. Early Roman legions suffered massive defeats as well – best known are probably battles at Cannae, Teutoburg Forest and Carrhae. Meanwhile, the late Roman army also had major victories.
If there is any weakness which the late Roman army had, it is in the fact that it was difficult to recruit troops and expensive to pay them. But this had to do with socioeconomic conditions of the Empire. In terms of the military structure itself, the army of the Principate shared the exact same weaknesses. It was only the armies of the early Republic and middle Byzantine Empire, both based around the landowning part-time soldiers, that could field large and effective armies at low cost and had ability to nearly effortlessly bounce back from massive losses.
Yet even this inability to bounce back from losses was only disastrous due to frequent civil wars, which due to nature of Roman warfare frequently produced massive casualties on both sides. Battle of Milvian Bridge left most of Maxentius’ soldiers dead. The bloodbath at Mursa Major wiped out a large portion of Roman Army. And if there was any single battle that destroyed the Western Roman army, it was the Battle of the Frigidus River. This was a battle of one of numerous civil wars, in which Theodosius of the Eastern Roman Empire destroyed the army of usurpers Eugenius and Arbogast – but at massive cost. Large portion of Western Comitatenses had been destroyed, and their ranks never replenished. Stilicho had to withdraw troops from the Rhine frontier to defend Italy, which allowed the barbarians to cross into Gaul unopposed. And yet the civil wars and usurpations continued, sapping resources that could be used to defend against the barbarians. Barbarians didn’t need to defeat Roman legions – just wait for Roman legions to destroy each other.
Undefended provinces were occupied by barbarians, which reduced Western Empire’s revenue, making it more difficult – and eventually impossible – to raise high-quality comitatenses. This increased insecurity in the Empire, which meant that people often sought protection of the barbarian warlords – but with the revenue going now to these warlords, the Empire found it even more difficult to raise new self-destroying comitatenses troops. In order to make up for the lack of Roman troops the Empire recruited barbarians – but while these were decent troops, they were loyal to their own chieftains. And these chieftains often used said troops to carve out their own domains while still remaining in de iure Emperor’s service. Thus the Western Empire never fell – it dissolved, despite its field army never losing a decisive battle against barbarians.
As can be seen, late Roman army was in fact superior to the army of the Principate. But the political and strategic situation was simply too disadvantageous to be compensated for by tactical superiority. This and a several very unlucky breaks led to demise of the Western Roman Empire, and from that fact was born the falsehood of the inferior Late Roman army. There is also the fact that the late Roman army was a defensive force, which is not as glamurous as an offensive, expansionist force – even today, expansionist armies (British Redcoats, US Marines) get far more glamour than defensive armies. But defensive army is not necessarily inferior.
In the end, the late Roman army was created because the Principate army was incapable of handling the new security challenges. That the late army managed to do so for as long as it did is a testament to its strength, not weakness.
note1 There is a debate on whether comitatenses were actually a central reserve to counter the enemy incursions or were there to secure Emperor’s throne from potential usurpers who might use border armies in a bid for the throne. In my view, this debate is superfluous. As can be seen from later Byzantine tagmata as well as the professional armies in the 15th century Europe (e.g. Black Army), a central professional army established for either of these two purposes will inevitably end up fulfilling both of them anyway.