Roman army

• 1. Introduction

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  In 1091, the Roman army that had once numbered nearly half a million men could muster only 500 professional soldiers. Emperor Alexios I Komnenos led that remnant into battle against the Pechenegs at Levounion. He won, with the help of a large force of allied Cumans. The Roman army had by then been in continuous existence for over a thousand years. Known in Latin as the Exercitus Romanus, it would survive for another three and a half centuries after that low point. From the seasonal levies of the Roman Kingdom to the fall of Constantinople in 1453, the Exercitus Romanus endured for approximately 2,206 years. How a force that began as clan-based war-bands became capable of such longevity is worth asking. What structural advantages did it possess? And how did it find its way back, time and again, from the edge of collapse?

• 2. The Citizen Levy

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  Around 550 BC, during the reign of king Servius Tullius, Rome created its first universal levy of eligible adult male citizens. The new levy produced a standard force of approximately 9,000 men: 6,000 heavily armed infantry, probably modelled on Greek hoplites; 2,400 lighter infantry; and 600 cavalry. When the kings were replaced by two annually elected magistrates called praetores around 500 BC, each praetor commanded one legion of 4,500 men. Service was a seasonal civic obligation. There were no standing forces. The army assembled at the start of each campaigning season and disbanded when it was over.

  In 493 BC, shortly after the Roman Republic was established, Rome signed the Foedus Cassianum with the other Latin city-states. The treaty created a perpetual military alliance, with each party contributing equal forces for joint campaigns under unified command. That compact held until 358 BC. In the day-to-day skirmishing that made up most of the army's work, the basic tactical unit was the centuria, a formation of 100 men. The phalanx was reserved for large set-piece battles, which were comparatively rare. The levy doubled to four legions during the Samnite Wars. This structural expansion set the stage for a more fundamental transformation in how Rome actually fought.

• 3. The Manipular Revolution

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  Around 300 BC, the Roman army abandoned the old phalanx in favour of something entirely new. Instead of a single mass of soldiers, commanders now deployed three lines of small units called maniples, each containing 120 men. The maniples were arranged in a staggered, chessboard pattern. This gave the battle-line far greater tactical flexibility when crossing uneven ground or adapting to sudden threats. Each legion was now routinely accompanied by a non-citizen formation of roughly equal size, the ala, recruited from Rome's Italian allies. Those allies numbered approximately 150 autonomous states, each bound by a treaty of perpetual military alliance. Their sole obligation was to supply fully equipped troops, up to a specified maximum each year. Together, the combined forces brought the normal levy to around 40,000 men, organized as two consular armies of about 20,000 each.

  The Second Punic War, fought between 218 and 201 BC, added a third element to this structure. Numidian light cavalry, Cretan archers, and Balearic slingers joined the army as specialist mercenary units, providing skills the legions and alae lacked. From that point forward, these non-Italian units always accompanied Roman armies. The war also revealed a critical vulnerability. Rome's cavalry had previously prevailed against both Gallic and Greek horsemen. Against the forces of the Carthaginian general Hannibal, it was decisively beaten. The decisive edge was Hannibal's Numidian light cavalry and his greater operational flexibility. By the time the general Marius took command in 107 BC, the maniple had been largely superseded by the larger cohort formation, borrowed from the allied alae, signalling a new phase in Roman military organization.

• 4. The Social War and Citizenship

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  The Social War of 91 to 88 BC was the direct consequence of accumulated resentment among Rome's Italian allies, the socii. Those allies had fought Rome's wars for generations. As non-citizens, they were excluded from the redistribution of public lands that Roman citizens received. They revolted. Rome could not easily suppress them, precisely because it had trained and equipped them so thoroughly. The settlement that ended the Social War granted Roman citizenship to all Italians. With that, the alae lost their legal basis and the socii were absorbed directly into the legions. The dual structure that had defined the Roman army for over two centuries collapsed into a single institution.

  Since the Second Punic War, the property requirements that had once barred the poorest Romans from legionary service had been quietly eroding. Extreme manpower pressure during that conflict forced commanders to overlook the minimum wealth threshold, and the practice continued for decades. The proletarii, Romans assessed at under 400 drachmae of wealth, had been ineligible for the legions. They were assigned instead to the fleet as oarsmen. Their gradual incorporation into the infantry was the quieter side of the same transformation. The army that emerged from these accumulated changes was larger and more inclusive than any that had come before. After a century of continuous overseas campaigning, it was ready to be remade into something permanent.

• 5. Augustus and the Professional Army

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  The standard legionary term under the reformed army was 25 years of full-time voluntary service. This was the hallmark of the army Augustus built after emerging from the civil wars as Rome's first emperor. Previously, legionaries had served for an average of ten years as a mix of conscripts and volunteers. Conscription was now reserved for emergencies. By the end of Augustus's reign in 14 AD, the imperial army numbered approximately 250,000 men. These were split equally between citizen legionaries and the auxilia, a new corps recruited from the peregrini, the non-citizen inhabitants of the empire. The peregrini made up around 90 percent of the empire's population in the first century. Augustus created the auxilia to supply cavalry, light infantry, archers, and other specialists that the citizen legions could not provide. Auxiliaries served a minimum of 25 years. On completion of their term, they were awarded Roman citizenship, a grant that carried significant legal, fiscal, and social advantages.

  The auxilia were organized into units of approximately 500 men. Three types existed: the cohors for all-infantry, the ala for all-cavalry, and the cohors equitata for mixed forces. Around 80 AD, a minority of auxiliary regiments were doubled in size. By 211 AD, the army had grown to roughly 450,000 men, organized into 33 legions and around 400 auxiliary regiments. At that point, auxiliaries outnumbered legionaries substantially. In 212, the emperor Caracalla granted Roman citizenship to all the empire's inhabitants. The legal distinction that had separated legionaries from auxiliaries became moot, and over the following century the two corps gradually converged.

• 6. Life Beyond the Battlefield

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  On completing their service, legionaries received a discharge bonus equivalent to 13 years of salary. Their regular pay already exceeded what common labourers of the time earned at home, and periodic cash bonuses accompanied occasions such as the accession of a new emperor. Centurions, the mid-level officers who commanded tactical sub-units of approximately 80 men, were paid several multiples of the basic rate. The most senior centurion in each legion, the primus pilus, was elevated to equestrian rank on completing his single-year term of office. Senior officers, including legion commanders and staff, were paid at least 50 times the basic legionary rate.

  Hadrian's Wall, in northern Britain, stands as one visible legacy of the legions' construction work. Across the provinces they served, soldiers built roads, bridges, ports, and public buildings. They founded entire new cities, classified as Roman colonies. Large-scale forest clearance and marsh drainage expanded available arable land. The legions also performed the function of a provincial governor's police force. Soldiers worshipped their own native gods, Roman deities, and the local gods of whatever province they served in. The polytheistic Roman system afforded wide freedom of worship. A few religions were banned as politically subversive, notably Druidism and Christianity. Among Eastern mystery cults that spread through the military in the later Principate, Mithraism proved by far the most popular. It was a syncretist religion, apparently originating largely in Asia Minor, whose rituals were secret and revealed only to initiates. In Hadrian's reign, from 117 to 138 AD, roughly 17 of the 42 imperial provinces housed standing military units. The chaotic 3rd century, with its barbarian invasions and waves of usurpation, would trigger a significant transformation in where the army was stationed and how it was structured.

• 7. The Demographic Advantage

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  Aristotle had already noted, in his own era, that Sparta's military system was built on a self-defeating logic. The Spartiates, the full citizens eligible for the highest military roles, declined steadily in number from the fifth century BC onward. Because Spartiate status was effectively irreversible once lost, military casualties translated directly into demographic collapse. Athens faced a parallel constraint: major military reverses frequently triggered political instability and factional conflict, weakening the city's capacity to recover.

  Under Philip II, Macedon resolved part of this problem by professionalizing military service and extending it to subject communities and non-citizens. This allowed Macedon to field larger armies than any individual Greek city-state. Even so, Macedonian manpower remained closely tied to royal authority and a finite agrarian base. When Alexander died, the capacity for multi-front recovery diminished sharply.

  Rome differed structurally from all three models. From the Middle Republic onward, the incorporation of Italian allies widened the recruitment base. The extension of citizenship after the Social War expanded it further. The subsequent absorption of provincial populations into the auxilia widened it further still. As the Greek historian Polybius observed, Roman defeats did not lead to political collapse but triggered renewed mobilization. Each major loss prompted the state to draw more deeply on its population rather than to fold. Systems that treated citizenship as an exclusive privilege tended toward demographic fragility. Those capable of expanding it could convert population growth and military defeat alike into renewed strength. By around 270 AD, plague and multiple major barbarian invasions had probably driven total numbers well below the 3rd-century high. The army that would emerge from the subsequent rebuilding would bear little resemblance to the force described by Polybius.

• 8. Byzantine Twilight

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  Diocletian, who took power in 284, rebuilt the army to roughly its early second-century strength of around 390,000 men. The legions were broken into cohort-sized units comparable to the old auxiliary regiments, and legionary armour was abandoned in favour of auxiliary equipment. A new force, the comitatus praesentales, consisted of large armies based near the emperor rather than on the frontiers. It became central to the late imperial system. Its primary role was not to repel invaders but to deter internal usurpations. The late army also relied more heavily on conscription than its Principate predecessor. Its soldiers were more poorly paid than those of the second century. Barbarians from outside the empire probably supplied a substantially larger share of recruits than before. Scholars disagree sharply about the late army's total size. Earlier estimates placed it at half again as large as the Principate army, and some as much as twice as large. More recent archaeological work suggests it was probably no larger than its predecessor.

  After the empire's formal division into eastern and western halves in 395, the western army disintegrated within decades as the Western Empire collapsed. The eastern army continued intact and essentially unchanged until the 7th century. A reorganization around themes then transformed it into what historians call the Byzantine army. Units tracing their lineage back to the Late Roman army fought on through centuries of Byzantine history. At the Battle of Manzikert in 1071, the professional core of that eastern army, the tagmata, was destroyed. The loss of Asia Minor that followed deprived the empire of its main recruiting ground. In 1097, after years of rebuilding under Alexios I Komnenos, the Byzantine army had grown to approximately 70,000 men. By the death of Manuel I Komnenos in 1180, the field army stood at 40,000 men. That recovery proved temporary. By around 1350, following a destructive civil war and the outbreak of the Black Death, the empire could no longer raise an army. It came to rely on troops provided by Serbs, Bulgarians, Venetians, Latins, Genoans, and Ottoman Turks to fight its own civil wars. The Ottomans, among those foreign forces, had campaigned for the empire just decades before they besieged it. Constantinople fell on the 29th of May 1453. The last remnants of the Byzantine state were conquered by 1461. The line that had begun with the first levy of Servius Tullius, more than two thousand years before, finally reached its end.