Why Could Ancient Rome Raise Bigger Armies Than Medieval Kingdoms?

The Roman Republic could field an army of 80,000 soldiers at the Battle of Cannae in 216 BC, but more than 1,600 years later, the King of England invaded France with just 8,000 men. So what happened? How did medieval states end up comparatively weaker than the ancient ones that came before them? Well, it helps to start with the collapse of Roman institutions in the West. As the Western Roman Empire gradually disintegrated, its bureaucracy and administrative frameworks collapsed alongside it. The successor kingdoms that emerged after Rome, which were often labeled barbarian states, were simply incapable of sustaining Roman systems. Large standing armies, extensive urban centres, efficient taxation, all of it was beyond their reach. Let's visualise what this collapse actually looked like. In 400 AD, the Western Roman Empire maintained a sophisticated bureaucratic apparatus. Tax collectors operated across Gaul, Spain, North Africa and Italy. The cursus publicus, the imperial postal and transport system, connected cities across thousands of miles. Imperial mints produced standardised coinage. Professional armies stationed along the frontiers received regular pay and supplies through established logistics networks. By 500 AD, barely a century later, almost all of this had vanished. The roads fell into disrepair. Cities shrank dramatically or were abandoned entirely. The tax system collapsed as local strongmen refused to cooperate with distant authorities. Germanic kingdoms like the Visigoths in Spain and the Franks in Gaul inherited the territory, but not the institutional knowledge or administrative capacity to run it. Long-distance trade networks shrank, bureaucratic expertise vanished, and post-Roman and later post-Carolingian governments struggled to effectively govern their territories. Under these conditions, assembling armies numbering in the tens of thousands became an enormous logistical challenge. At the same time, diminished administrative capacity meant that medieval governments had severe difficulty feeding, supplying and paying large armies. In contrast, the Roman state could rely on vast tax revenues and requisition resources from across the Mediterranean world. This allowed Roman authorities not only to assemble large forces but, crucially, to sustain them on a long-term permanent basis. This naturally raises the question of why state capacity declined so dramatically after antiquity. After all, earlier societies, the Gauls, the Britons, Germanic peoples, appear in ancient sources as fielding massive armies, despite lacking Roman-style bureaucracy. While it's true that ancient authors often exaggerated numbers, even accounting for this exaggeration, it seems clear that less centralised societies could still mobilise large portions of their populations. The key distinction lies in the political structure of medieval states, which were characterised by deeply fragmented authority. Take France around the year 1000 as an example. In theory, the king ruled all of France. In practice, his authority barely extended beyond the immediate vicinity of Paris. The king of France technically ruled over approximately 800km from north to south. But his actual domain, the land he directly controlled, was perhaps 100km across. Meanwhile, the Duke of Normandy controlled the entire north-west. The Duke of Aquitaine governed a territory larger than the king's, stretching from the Loire to the Pyrenees. The Count of Flanders, the Count of Toulouse, the Duke of Burgundy, each of these lords functioned as independent rulers with their own courts, their own armies, their own systems of justice. When the king wanted to raise an army, he couldn't simply order troops from Normandy or Aquitaine the way a Roman emperor could requisition legions from distant provinces. He had to negotiate, persuade and often bribe these powerful nobles to contribute forces. And even when they agreed, those forces remained loyal to their local lord, not to the king. This pattern held across much of Western Europe. As Roman urban life and centralised governance declined, local elites filled the vacuum. These figures, later described as feudal nobles, possessed their own interests, rivalries and power bases which made unified state control nearly impossible. Combined with the deterioration of infrastructure, roads, storage facilities, supply networks, these conditions severely limited the ability of medieval states to field large armies. Additional factors compounded the problem. Medieval armies differed fundamentally from those of the Roman or Hellenistic worlds, and contrary to popular imagery, they were not composed primarily of poorly trained peasants. By the High Middle Ages, armies fielded by groups such as the Normans, Anglo-Saxons, Byzantines and Italian city-states were relatively professional. For example, by the late medieval period, European armies had evolved considerably from earlier feudal levies. English forces in the Hundred Years' War combined contracted professional soldiers with yeoman archers serving for wages, while French armies relied on a mix of noble cavalry, mercenary companies and urban militias. Yet these armies remained relatively modest in size, because medieval states lacked the fiscal and administrative infrastructure to sustain large forces. The comparison with Rome is instructive but often misunderstood. Roman legions were themselves expensive professional standing armies requiring regular pay, equipment and supply. However, the Roman state possessed something medieval kingdoms did not, a monetized tax system that efficiently extracted revenue from a vast territory, combined with roads, ports and supply depots that allowed resources to reach armies across enormous distances. A Roman emperor could tap the wealth of Egypt to fund campaigns in Germania. A medieval king of France? He could barely extract reliable taxes from nobles 50 miles away. When kings did hire professional troops or mercenary companies, they faced constant cash flow problems. Campaigns were frequently cut short, not by enemy action, but by the inability to pay soldiers. Philip Augustus of France spent years building up the administrative capacity just to keep troops in the field for a single campaign season. The issue once again circles back to logistics and state capacity. This dynamic mirrors what we see in the modern world. A functional nation-state like France or Germany can mobilize massive resources, draft millions of citizens, redirect industrial production, establish supply chains across continents. But a failed state or a territory controlled by warlords, even if it has a large population, cannot do the same. Afghanistan has roughly 40 million people, but no warlord-controlled region could field an army comparable to what the much smaller Netherlands could mobilize. Because the Netherlands possesses the administrative capacity, infrastructure, and centralized authority that fragmented territories lack. Medieval Europe wasn't a failed state in the modern sense, but it shared this fundamental limitation. Power was too fragmented to enable the kind of resource mobilization that centralized ancient empires had achieved. Despite these constraints, some medieval states did manage to assemble sizable forces on occasion. Anglo-Saxon England, for example, employed the Ferd, a mass levy that could be mobilized in emergencies. However, this force could not be maintained indefinitely as it lacked the logistical support necessary for long-term deployment and would quickly exhaust local resources. In other cases, such as in the Byzantine Empire, Islamic states, or during the Crusades, larger forces were raised. Estimates for the First Crusade suggest that more than 100,000 people participated, though many were non-combatants rather than soldiers. The First Crusade provides perhaps the clearest illustration of medieval logistical dysfunction at scale. In 1096, multiple armies departed from different parts of Europe, taking different routes toward Constantinople and ultimately Jerusalem. There was no unified command, no coordinated supply system, and no central authority managing the campaign. The first wave, led by a charismatic preacher named Peter the Hermit, departed months before the main armies. This force, sometimes called the People's Crusade, consisted largely of poor, untrained civilians with virtually no military equipment or organization. They pillaged their way through the Balkans, alienating the very Byzantine Empire they were supposed to be helping. When they arrived in Constantinople, the Byzantine Emperor Alexios quickly ferried them across to Asia Minor, largely to get rid of them. Within weeks of crossing into Seljuk Turkish territory, they were annihilated. Contemporary sources describe the Turks finding piles of Crusader corpses, with bones scattered across the battlefield for years afterward. Of perhaps 20,000 people, only a few thousand survived. Meanwhile, the main Crusader armies, led by nobles like Godfrey of Bouillon, Raymond of Toulouse, and Bohemond of Taranto, arrived at Constantinople at different times over several months. They couldn't agree on a unified strategy. Each lord maintained his own supply system, which mostly meant looting the countryside. They quarrelled constantly over who should lead, who would get which conquered cities, and what their ultimate objective should be. That they managed to capture Jerusalem in 1099 was due more to Seljuk political fragmentation and Byzantine support than to effective Crusader organization. It certainly wasn't because of superior medieval logistics. Supplying and maintaining large armies has always been difficult, regardless of time period. In the Middle Ages, however, the breakdown of centralized authority greatly intensified these difficulties. Only later, as urbanization revived, bureaucratic institutions re-emerged, and royal power consolidated, did European states begin to rebuild the administrative capacity necessary to sustain armies approaching the size of those fielded by Rome at its height. A supplemental view is that medieval battles tended to be smaller because the forces involved were smaller, and in relative terms that's not wrong. Medieval battles were not dramatically reduced in scale. When this question comes up, it almost always involves a comparison between a vast classical conflict, like the Second Punic War, and a localized medieval struggle, like the Wars of the Roses. Why wasn't a war between two noble families as large as a war between great empires? Simply because the former involved a handful of noble houses, while the latter pitted enormous imperial states against one another. Most medieval warfare played out on the level of individual counties. A duke might clash with a count over disputed land. One Viscount might fight another over the inheritance of a castle. Even the most powerful nobles, figures like the Duke of Aquitaine, who controlled much of southern France, or the Duke of Apulia, Calabria, and Sicily, who ruled Italy south of Rome, commanded resources closer to those of a single Roman province, or a few Greek city-states, than to an empire such as Carthage, Rome, or Persia. At this point, a reasonable objection emerges. If medieval states were so limited by fragmented authority and weak logistics, how did the Mongols manage to conquer half the known world, with armies that sometimes numbered over 100,000 warriors? The Mongol example actually reinforces, rather than contradicts, our thesis about state capacity and logistics. The Mongol Empire succeeded precisely because it possessed several advantages that medieval European states lacked. First, the Mongols operated under a highly centralized command structure. Genghis Khan and his successors exercised direct absolute authority over their armies. There were no independent nobles who could refuse to contribute troops or withdraw their forces mid-campaign. Every warrior owed personal loyalty to the Khan, and the chain of command was clear and unbreakable. Second, Mongol logistics were specifically adapted to their pastoral, nomadic lifestyle. Each warrior brought multiple horses, typically four or five, which served as both transportation and food supply in emergencies. The Mongols could move faster and travel farther than sedentary agricultural armies because they weren't tied to fixed supply depots or slow-moving baggage trains. They essentially carried their logistics with them. Third, and perhaps most importantly, the Mongols were masters of extracting resources from conquered territories. When they captured a city, they systematically looted it, conscripted local craftsmen and siege engineers, and requisitioned food supplies. They used Chinese and Persian administrative expertise to build tax systems in conquered regions, which funded further campaigns. In essence, each conquest made the next one easier by expanding their resource base. Compare this to a medieval European king trying to invade a neighboring kingdom. He couldn't simply loot his own peasants to fund the campaign. They were his primary source of long-term revenue, and destroying them would undermine his own power. He couldn't conscript skilled workers at will because urban craftsmen belonged to guilds and served local lords. And he certainly couldn't exercise the kind of absolute centralized authority that the Mongol khans wielded. The Mongols succeeded by avoiding medieval limitations through a completely different political and military model, one that European feudal states simply couldn't replicate. The smaller size of medieval armies ultimately reflects a broader historical reality. The collapse of Roman administrative systems created a centuries-long period where European states simply lacked the capacity to mobilize resources on the scale that ancient empires had achieved. Medieval people were just as capable and warlike as their ancient predecessors. The difference was political fragmentation, economic decentralization, and infrastructural decay, conditions that made large-scale military organization extraordinarily difficult. Only with the gradual revival of trade, the growth of cities, and the consolidation of royal authority in the late medieval and early modern periods, did European states begin to rebuild the administrative machinery necessary to field truly large armies once again. The lesson here extends beyond military history. State capacity, the ability to tax, organize, and mobilize a population, is the foundation of everything from public works to military power. Ancient Rome had it. Medieval Europe lost it. And it took centuries to rebuild. 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