Since I’m talking about the Fall of Rome, was the military responsible for the fall? Did they fail to adopt new tactics, or fail to foresee new technologies? I do not believe so. What is remarkable is the superiority of the Roman legionaries throughout the collapse of the Empire. There just was not enough of them.

Adrian Goldsworthy provides a good synthesis of Roman military and its transformations from a city-state militia to the late imperial legions. It’s a good introduction.

Bryan Ward Perkins summarizes the military failure:

“The story of the loss of the West is not a story of great set-piece battles, like Hadrianopolis, heroically lost by the Romans in the field. … The West was lost mainly through failure to engage the invading forces successfully and drive them back. This caution in the face of the enemy, and the ultimate failure to drive him out, are best explained by the severe problems that there were in putting together armies large enough to feel confident of victory. Avoiding battle led to a slow attrition of the Roman position, but engaging the enemy on a large scale would have risked immediate disaster on the throw of a single dice.”

The deterioration of the Roman legionaries was a slow process, but they help up – as individual units – very well. The difficulty was the lack of a militia, defense-in-depth, and the inability to assemble large enough armies to counter multiple invasions. The Roman professional military could, and frequently did, defeat one or two invasions at a time. The 4th and 5th centuries saw wars on multiple fronts along the frontiers.

The Legionaries were a professional force of some 600,000 men total at their height. Roughly half of these were “legionaries” in the traditional sense – the heavy infantryman wearing lorica segmentata or chain mail. The other half consisted of Auxiliaries and Client State Allies, who provided many of the light infantry, missile troops, and cavalry for the Romans. Auxiliaries were often equipped like the Roman legionaries, but sometimes they used special regional equipment or tactics. The Romans of Western Europe specialized in heavy infantry given the poor land for horses and the lack of materials for Eastern style composite bows.

The Legionaries underwent a number of transformations across the Empire’s history. Citizenship was extended, first to all Italians, then all imperial subjects. This increased the recruiting pool to encompass non-Italians. Regional equipment and styles of fighting differed to respond to regional threats. So the Western Romans shaped their army to fight off the Germans, and the Eastern Romans to fight the Iranians.

The Romans did not allow their civilians to keep arms, possibly to prevent insurrection by the Celts and other conquered peoples. This left security matters entirely in the hands of the Roman professional military. Civilians could not resist if the limes were breached. This played a critical role in explaining the rapid collapse of the Western Empire. The Eastern Empire adapted better, particularly in Greece and Anatolia, by encouraging militias and local defenses.

The composition of the Legion changed by the 4th and 5th centuries. During the Republican times, the Legion consisted of 4,000 men and up to 6,000. By the Late Imperial period, the many legions only had 1,000 soldiers – typically just three cohorts and a headquarters.

In brief, by the 5th century, the Western Roman Military had not faced a major threat in centuries. It provided defenses along the Rhine and Danube, where most combat involved fewer than 1,000 participants. The Western Romans rarely engaged in massive battles with 50,000 or more troops as the Eastern Empire did. The Western Romans became experts at guerrilla warfare, counter-raiding and other forms of small wars. They decentralized the legions to favor the smaller and more flexible cohort.

The Cohort was roughly 300 men who were stationed at frontier outposts and forts along the limes on the Rhine and Danube Rivers. They built wooden palisades and roads parallel to the rivers to patrol, and maintained watchtowers and signal fires. These cohorts intercepted Germanic raiders and occasionally crossed the rivers to retaliate. If there was a threat larger than a warband, the cohorts would sent signals back to the Legionary Garrison, with some 4,000 to 6,000 men, who would then rush forward to eliminate the larger threat. Larger threats took longer to mobilize, so the Romans often had advanced warning that a new German Confederation was forming with the intent of crossing the limes. Multiple legions could be assembled to defeat this threat.

The Roman military can broadly be divided into two informal groups – the limitanei (the defenders of the limes) and the mobile field armies. Most Romans policed the roads and cities, protected political authorities, eliminated bandits and pirates, built engineering projects, intercepted raiders, and sometimes retaliated against enemy lands beyond the limes. The Field Armies were more of a political instrument half the time, but their real use was to intercept larger invading armies, or to be redeployed across the empire as needed.

As Byran Ward Perkins explained, this system was better designed to counter periodic raiding by fragmented German Tribes. It was overwhelmed at all points by the sheer numbers of invaders during the late 4th and 5th centuries. The Roman military had to decide between saving one region by letting another fall.

The Roman military was well adapted to face their threats in combat. The problems were strategic, moreso than anything else. The Romans were superior to the Germans at guerrilla tactics. The Romans were better at tracking, ambushing, scouting, forest fighting, night fighting and hit and run tactics. This is a curious fact that is not often recognized. This seems counterintuitive – many seem to believe that the Germans would be better at small-scale fighting, but the Germans were untrained warbands fighting against professional soldiers trained to beat them at exactly that kind of warfare. The Roman military clearly adapted to the tactics of its enemy and outperformed them.

Equipment and styles of fighting changed since the Julio-Claudian era.

The Western Roman soldiers looked nothing like their classical image from the Republican and early Augustan Imperial era. In the 4th and 5th century, they were typically equipped with large wooden round shields instead of the classical rectangular scutum. More soldiers fought as spearmen using a medium length spear instead of short swords. The classical lorica segmentata was still used in some units, but many wore hardened leather armor or iron mail and iron helms, and there were far more skirmishers and missile troops than in part armies.

The heavy pilum was discarded in favor of lighter javelins. In the past, legionaries carried two pilums, with a maximum range of 25 meters, but more likely thrown within 10 meters. Late Roman soldiers carried five or six lighter javelins and darts, like the lancea and plumbatae, with a longer range of 30-60 meters. This was necessary given the forest fighting against larger numbers of German Warbands who relied heavily on javelin and missile combat.

The style of fighting was different. There were more German and Celtic troops in the Late Roman legion and they brought their cultural concept of infantry fighting with them. Roman legionaries advanced noisily, banging their shields and screaming Germanic and Celtic war cries – a sharp difference from the silent and orderly advance of the Augustan era legionaries.

The old Augustan era legionary advanced at a slow orderly pace. They halted and reformed their lines within javelin range of the enemy. They threw their two pilums and made the shock charge of heavy infantry. The Late Roman military fought much differently. The formations were looser and a long barrage of javelins, arrows, and darts preceeded the melee. To engage in melee combat, Late Roman troops charged at a dead run, sacrificing order to minimize exposure to missile fire.

The Eastern Roman Empire underwent a different evolution to counter the Persian threats. Eastern Roman troops used far more cavalry and archers than the West. Horse archers and heavily armored Cataphracts appeared in the East, apparently imitations of their Iranian counterparts. Eastern Roman infantry formed a wall of armored spearmen to stop Persian Cavalry charges. Behind the line, archers and slingers unleashed barrages of missiles. Roman cavalry was kept in reserve and used strategically against the much larger numbers of Persian cavalry. There were larger numbers of Asians in the Eastern army, so the Romans had greater access to the composite bow than the Western Empire.

The Romans in the East stuck to urban fighting and sieges where they had an advantage over the Persians, and tried to mitigate the open-field advantage of the Persian armies. The Persians would often harass the rear supply lines of Roman armies instead of fighting direct battles – so fighting was often spread out as a series of skirmishes.

The Roman tactics evolved to use newer technology and tactics, and they retained battlefield supremacy over the Germans in one on one fights.

The Battle of Strasbourg in 357 was one of the major battles fought. 12,000 Romans led by Caesar Julian, fought against the Alamanni Confederacy which brought over 35,000 infantry and cavalry to battle. The initial cavalry battle went to the larger German force. Then Roman legions crushed the Germans warbands – less than 250 Romans died, compared to over 6,000 dead Germans. The disorganized German rout caused many to drown in the Rhine.

The Romans military, as a whole, were definitely superior in large battles and in guerrilla warfare to any German or Gothic tribe. Most of the fighting during the fall of Rome was done on a very small scale, more like chronic raiding than major battles. Yet they were overwhelmed.

The problem was two fold:
First, the Roman Legions were highly dependent on the agricultural tax base.

Second, the Legions were unable to gather sufficient forces to feel confident of major victories against every invader.

These two problems reinforced each other. Every invasion left a smaller tax base, which left a smaller military, which allowed for more invasions.

Byran Ward-Perkins:

“In my opinion, key internal element in Rome’s success or failure was the economic well-being of its taxpayers. This was because the empire relied for its security on a professional army, which in turn relied on adequate funding.”

“… [The] chaos of first decade of 5th century will have caused a sudden and dramatic fall in imperial tax revenues, and hence in military spending and capability. Some of the lost territories were temporarily recovered in the second decade of the century; but much (whole of Britain, large part of Gaul and Spain) was never regained, and even reconquered provinces took many years to get back to full fiscal health”

“Military expenditure was by far the largest item in the imperial budget, and there were no massive departments of state … whose spending could be cut when necessary in order to protect “Defence”; nor did the credit mechanisms exist in Antiquity that would have allowed the empire to borrow substantial sums of money in an emergency. Military capability relied on immediate access to taxable wealth.”

The Roman Limes provided the internal security for massive economic flows in a complex economy. Once the limes were breached by barbarians, there was a rapid breakdown in law and order. German bandits and raiders overran highways, pirates spread on the seas, towns and cities were put under regular siege and sometimes sacked. Agricultural production stagnated. The Roman government could not withstand this strain. It drew less and less money from its tax base and often could not deliver this money to the troops on the frontiers. The Roman legions steadily disintegrated along with the economy they needed to protect.

Nor did the Roman economy have “surge” potential to rapidly increase the size of the legions. Maximum agricultural production was more or less fixed prior to industrialization and the Romans lacked advanced financial institutions. They could not increase production in some areas to meet deficiencies in threatened regions. This led to a constant cash-flow shortage.

I agree with Ward-Perkins that the failure was at the strategic level. And, I cannot think of what else the Romans could have done. The exogenous shock was so sudden and the collapse so rapid – just 70 years – that few structural reforms would have helped. The Romans did everything imaginable. They tried to raise more legionaries. They attempted to wall their interior cities and form armed militias. Celtic noblemen in Gaul, Iberia, and Britain took charge of their own defenses. The Romans tried to keep their navy intact to protect overseas territory in Sicily and North Africa. They tried diplomacy to divide the Germans Confederacies or keep them fighting the Huns.

The one thing that may have saved the Western Empire – or at least given it more resilience – would have been an armed civilian population that could form organized militias. This would have provided the defense-in-depth to stop Germanic migrations and conquests.

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