Focus on Military Organization: Rome’s “Polybian” Legion

When Rome first appeared as a city-state in the Tiber Valley sometime in the middle of the eighth century BCE, its first army differed little from those of other small communities in Latium. It is believed Rome’s first military organization was based on the tribal system, reflecting the three original Roman tribes (the Ramnes, the Tities, and the Luceres). Each tribe provided 1,000 infantry towards the army, made up of ten centuries consisting of 100 men. The tribal contingent was under the command of a tribunus or tribal officer.  Together, these 3,000 men made up a legio or levy. This infantry force was supplemented by a small body of 300 equites or “knights,” aristocratic cavalry drawn equally from the three tribes. Thus, from the very beginning of Roman military tradition cavalry was not held in high regard and would remain a subordinate arm to infantry during the Archaic and Republican periods (c.750-31 BCE). As we shall see, inferior cavalry units will hinder Roman efforts against the brilliant Carthaginian general Hannibal Barca (247-183 BCE) during the Second Punic War (218-201 BCE).

Right: Bas-relief of three second century BCE Republican-era legionaries in battle kit. Notice the oval shields (scutums) of the period and the wearing of Montefortino-type helmets and chain-mail body protection (lorica hamata). From the Altar of Domitius Ahenobarbus on the Field of Mars in Rome. Louvre Museum, Paris. 

Initially, the organization of the early Roman army was heavily influenced by their powerful neighbors to the north, the Etruscans.  Etruscan civilization emerged in Etruria around 900 BCE as a confederation of city-states. By 650 BCE, they had expanded in central Italy and became the dominant cultural and economic force in the region, trading widely with Greeks and Carthaginians on the Italian peninsula. Under direct occupation by the Etruscans between c.620-509 BCE, Rome benefited greatly from this cultural exchange, with Roman villages transformed into a thriving city-state. And although these Etruscan city-states were united in a league of usually twelve cities, they seldom operated together unless faced with an outside threat. Like the Greek poleis to the east, the Etruscan cities spent most of their energy fighting each other.   

Sometime in the sixth century BCE, the Etruscans adopted the Greek method of fighting and organized their militia-armies into phalanxes.  After conquering the Roman city-state in the late sixth century BCE, the newly created Etrusco-Roman army was composed of two parts: the Etruscans and their subjects, the Romans and Latins. The Etruscans fought in the center as heavy infantry hoplites, while the Romans and Latins fought in their native style with spears, axes and javelins on either wing. The army was divided into five classes depending on nationality. The largest contingent, or first class, was composed of Etruscan heavy infantry armed in Greek fashion with heavy thrusting spear and long sword and protected by breastplate, helmet, greaves and a heavy round shields. The second class were spearmen conscripted from subject peoples and armed in Italian fashion with spear, sword, helmet, greaves, and the oval shield or scutum, probably of Samnite origin. The third class was lightly armored heavy infantry spearmen with scutum, while the fourth and fifth classes were light infantry javelin throwers and slingers.

The second of the Etruscan overlords in Rome, Servius Tullius (578-535), is credited in the middle of the sixth century BCE with attempting to integrate the population by reorganizing the army according to wealth and not nationality.  The Servian reforms reflected an old Indo-European custom where citizenship depended on property and the ability to maintain a panoply and serve in the militia.  The reforms segregated Etrusco-Roman society into seven groups.  The wealthiest group formed the cavalry or equites, made up of Etruscan nobles and members of the Roman Patrician class.  The equites did not act in the capacity of heavy or light cavalry but served as mounted infantry and scouts.      

Left: The bowl of a Montefortino- type helmet, which was used by Roman heavy infantry from the Camillian legions to the Polybian legions. Here, the cheek guards are missing, although their hinges are visible. The knob on the top was used to hold black plumes. Museum Carnuntum, Altenburg, Austria.       

The second wealthiest group acted as heavy infantry, fighting in the phalangeal formation and armed as before in the Greek manner.  The third through sixth groups were armed in native Italian fashion identical to the pre-Servian period. The seventh class, or capite censi, were too poor to qualify for military service.  Tactically, the Servian army fought as before, with heavy infantry in the center phalanx, protected by lightly armored heavy infantry on the wings and light infantry skirmishers in the front until the phalanx engaged.  There is no mention of archers in the Servian reforms.  Like the Greeks, the Romans seemed to disdain the bow and arrow as a weapon of war, preferring it for hunting.          

The early Republican army was a citizen army.  In fact, the original meaning for the word legion (derived from legere, Latin for “to gather together”) was a draft or levy of heavy infantry drawn from the property owning citizen-farmers living around Rome.  The army continued to adhere organizationally to the Servian reforms and consisted of three legions, each of a thousand men, supplemented by light infantry provided by the poorer citizens and cavalry by the wealthy Patrician class.  Divided into ten centuries of a hundred men, each legion was commanded by a military tribune appointed from the Patrician class, while each century was commanded by a centurion promoted or elected from the ranks of the legionaries. 

During the first century of Republican rule, the Roman army continued to utilize the phalangeal-based tactical system.  But the battle square proved less effective against opponents unaccustomed to the stylized hoplite warfare favored by the Mediterranean classical civilizations.  When, in 390 BCE, 30,000 Gauls crossed the Apennines in search of plunder, the defending Roman legions were pushed against the Allia River.  The Roman phalanxes, outnumbered two-to-one and overwhelmed by the ferocity and physical size of the Celtic marauders, were defeated, unable to cope with the barbarians' open formation and oblique attacks. The sack of the "Eternal City" in 390 BCE left a lasting impression on the psyche of Roman civilization.  The surviving Romans who witnessed the violation of their city from a nearby hill vowed never again to fight unprepared.  The military reforms of the early fourth century BCE are associated with the leader Marcus Furius Camillus, a man credited with saving the city from the Gauls and remembered as a second founder of Rome.  Although history cannot precisely answer if Camillus himself was responsible for the Camillian military reforms, the changes that bear his name dramatically altered the character of the Roman legion in the fourth century BCE.

The Roman army's experience against Gauls in the north and campaigns against the Samnites (343-290 BCE) in the rough, hilly terrain of central Italy forced a change in tactical organization, with Roman commanders altering the panoply and tactical formation of the legions to meet the different fighting styles of their opponents, whether barbarian or civilized, giving individual legionaries more responsibility and greater tactical freedom.  This is sometimes referred to as the “Polybian” legion, so named because the historian Polybius gives us the most detailed description of its workings.  During this time the Roman army expanded from three to four legions, and the number of legionaries per legion grew to 4,200 infantry, supported by 300 Roman cavalry.  As before, the wealthiest men made up the cavalry (equites), divided into ten groups or turmae, commanded by decurions.  These men were armed and armored in Hellenistic fashion with spear and sword and protected by helmet, cuirass and small circular shield.

Right: Line drawing of a Celtiberian short sword, the template for what would become the famous Roman gladii hispaniensis beginning in the Middle Republican era (264-133 BCE. Notice the similar leaf blade shape of later Roman short swords, and the scabbard style and suspension system.

As part of the reorganization of the “Polybian” legion, the Roman century had been reduced from one hundred to sixty men apiece.  Each of these centuries was commanded by two centurions, members of a professional veteran officer class who drilled the legionaries and commanded them in battle.  The senior centurion led his troops from the front, while his hand-picked assistant or optio stood at the rear of the formation to maintain order.  Centurions also administered discipline to the troops when necessary.  Roman discipline was usually corporate in nature, though extraordinary circumstances could require a capital punishment known as decimation.  It was inflicted on legionaries who had given ground without cause in combat or exposed their neighbors to flank attack.  The process of decimation took place after the engagement, when a tenth of the offending unit was chosen by lot, then clubbed death by their own comrades.

During this time the Roman army abandoned the phalanx altogether in favor of a new linear formation organized around four classes of soldiers defined not only by wealth, but also by age and experience.  The Greek-styled battle square was replaced by three lines of heavy infantry, the first two-thirds armed in an innovative manner with two weighted javelins or pila, sword and protected by helmet, breastplate, greaves and the traditional oval scutum favored by the lower classes.  The ranks of the forward of these two lines or hastati were filled with young adult males in their twenties, while the center formation, or principes, was comprised of veterans in their thirties.  The third and last line or triarii were armored as above but armed in the older fashion with thrusting spear and scutum.  The triarii consisted of the oldest veterans and acted as a reserve.  The first line (hastati) and second line (principes) consisted of 1,200 legionaries each, while the third line (triarii) was made up of only 600 soldiers. This deployment is often referred to as the triplex acies (triple battle order). The poorest and youngest men served as velites or light infantry skirmishers, and are not formally included in the triplex acies, although their roles on the battlefield were important.  Armed with light javelins and sword and unprotected except for helmet and hide-covered wicker shield, the velites acted as a screen for their heavier armed and less mobile comrades.  Each legionary was still responsible for supplying his own panoply, but in order to maintain uniformity within each century, the weapons were frequently purchased from the state.

Before battle, the hastati, principes and triarii formed up in homogeneous rectangular units or maniples of 120 men (two centuries probably deployed side-by-side), protected by the velite light infantry.  Each maniple was commanded by the centurion from the right-hand century and organized around a signum or standard kept by the signifer, who led the way on the march and in combat.  Each maniple deployed as a small independent unit, typically with a twenty-man front and six-man depth and may have been separated from its lateral neighbor by the width of its own frontage, though this is still a matter of some debate.  Livy tells us that the maniples were “a small distance apart.”  Moreover, the hastati, principes and triarii maniples were staggered, with the principes covering the gaps of the hastati in front, and the triarii covering the gaps of the principes.  This checkerboard formation or quincunx provided maximum tactical flexibility for the maniple, allowing it to deliver or meet an attack from any direction.  As Polybius describes

The order of battle used by the Roman army is very difficult to break through, since it allows every man to fight both individually and collectively; the effect is to offer a formation which can present a front in any direction, since the maniples which are nearest to a point where danger threatens wheel in order to meet it. The arms that they carry both give protection and also instill great confidence into the men, because of the size of the shields and the strength of the swords which can withstand repeated blows.  All these factors make the Romans formidable antagonists in battle and very hard to overcome.

Left: The Polybian legion’s order of battle (triplex aciesor “triple battle order”) before engagement. In combat the triplex acies consisted of the hastati in the first rank, the principes in the second rank, and the veteran triarii in the third and final. When in danger of imminent defeat, the first and second lines, the hastati and principes, ordinarily fell back on the triarii to reform the line to allow for either a counter-attack or an orderly withdrawal.

In battle, the maniple legion presented a double threat to its adversaries.  After the screening velites withdrew through the ranks of the heavy infantry, the hastati moved forward and threw their light pila at thirty-five yards, quickly followed by their heavy pila.  Drawing their short thrusting Spanish swords or gladii hispaniensis, the front ranks of the hastati charged their enemy, whose ranks were presumably broken up by the javelin discharge.  As the Roman heavy infantry thrust into the enemy, the succeeding hastati threw their pila and engaged with swords.  The battle became a series of furious combats with both sides periodically drawing apart to recover.  When the two formations joined, the legionaries exploited the tears and stepped inside the spears of the first rank into the densely packed mass and wielded their swords with much greater speed and control than the closely packed spearmen could defend against.  During one of these pauses, the hastati retreated back through the open ranks of the battle-tested and fresh principes and triarii.  Meanwhile, the principes then closed ranks and moved forward, discharging their pila and engaging with swords in the manner of their younger comrades.  If there was a breach in the Roman line, the veteran triarii acted as true heavy infantry and moved forward to fill the tear with their spears. 

The new Roman system had many strengths.  By merging heavy and light infantry into the pila-carrying legionary, the Roman army gave its soldiers the ability to break up the enemy formation with missile fire just moments before weighing into them with sword and shield, in effect merging heavy and light infantry into one weapon system.  Once engaged, the maniple's relatively open formation emphasized individual prowess and gave each legionary the responsibility of defending approximately thirty-six square feet between himself and his fellow legionaries, a fact which placed special emphasis on swordplay in training exercises.  But even if the maniple failed, it could be replaced by a fresh one in the rear.  This ability to rotate fatigued legionaries with fresh soldiers gave the Romans a powerful advantage over their enemies.

Right: The numerous land campaigns during the Second Punic War is where we see the Polybian legion perfected in battle. Despite numerous losses to the Carthaginian general brilliant Hannibal Barca, the Romans persevered and defeated the Carthaginians at Zama in 202 BCE, after which the Polybian legion would expand Roman territory all around the Mediterranean basin.

One of the keys to the success of the Roman war machine was the adoption of the deadly gladius during the Punic Wars. Twenty inches long and three inches wide, the gladius was primarily a stabbing weapon, and legionaries were trained not to use it for cut-and-slash attacks, the preferred method of their enemies.  Instead, Roman soldiers would employ a shield parry, followed by a sharp under-thrust with the gladius into the enemy’s torso, creating the killing trademark of the Roman infantry.  This tactic was especially effective against poorly armored barbarians.      

The “Polybian” legion served the Republic well in its expansion against the Samnites, Etruscans and Gauls in northern and central Italy during the fourth century BCE.  But Rome faced new challenges in the third century BCE from the Greeks in southern Italy, the Carthaginians in Spain and North Africa and the Hellenistic Successor States in the Levant.  Rome's martial contacts with these other regional powers would test the effectiveness of the maniple legion against combined-arms tactical systems inspired by the success of the Macedonian art of war, while the military challenges of facing large Celtic and Germanic barbarian armies along its northern frontiers would also necessitate a restructuring of the Roman legion by the end of the second century BCE, resulting in the famous Marian reforms associated with the generalship and consulships of Gaius Marius (157-86 BCE).

Suggested Readings:

Primary Source

Livy. The War with Hannibal. Translated by Aubrey de Selincourt. Penguin Classics. Penguin Books, 1987. 

_____. Rome and the Mediterranean. Translated by Henry Bettenson. Penguin Classics. Penguin Books, 1976. 

 Secondary Sources

 Carey, Brian Todd, et al. Hannibal's Last Battle: Zama and the Fall of Carthage. Pen and Sword Military, 2007.

 Gabriel, Richard A., and Karen S. Metz.  From Sumer to Rome: The Military Capabilities of Ancient Armies.  Greenwood Press, 1991.

 Goldsworthy, Adrian. The Complete Roman Army. Thames and Hudson, 2011.   

 Santosuosso, Antonio. Soldiers, Citizens and the Symbols of War From Classical Greece to Republican Rome, 500-167 BC. Routledge, 2019.

Hannibal's Last Battle: Zama & The Fall of Carthage - Kindle Edition. Pen & Sword Military, 2007, By Brian Todd Carey.

From the publisher, "A 'crisply written, well researched, superb piece of scholarship about one of the most dramatic and decisive battles in the ancient world' (Journal of Military History). At Zama (in what is now Tunisia) in 202 BC, the armies of two great empires clashed: the Romans under Scipio Africanus and Carthaginians, led by Hannibal. Scipio’s forces would win a decisive, bloody victory that forever shifted the balance of power in the ancient world. Thereafter, Rome became the dominant civilization of the Mediterranean. Here, Brian Todd Carey recounts that battle and the grueling war that led up to it. He offers fascinating insight into the Carthaginian and Roman methods of waging war, their military organizations, equipment, and the tactics the armies employed. He also delivers an in-depth critical assessment of the contrasting qualities and leadership styles of Hannibal and Scipio, the two most celebrated commanders of their age. With vivid prose and detailed maps of the terrains of the time, Hannibal’s Last Battle is an essential text for fans of military history and students of the classical period."

About the Author: Brian Todd Carey is an Assistant Professor of History and Military History at the American Public University System, where he teaches ancient, classical, medieval and early modern military history. He is the author of dozens of history articles in numerous magazines and journals, including Aviation History, Command Magazine, History Magazine, Marine Corps Gazette, Medieval History Magazine, Military Heritage, Military History, Strategy and Tactics, World History Bulletin, World at War, World War II, and WWII Quarterly: The Journal of the Second World War and seventeen articles on ancient, classical and medieval Eurasian warfare for the twenty-one volume ABC-CLIO-World History Encyclopedia. In 2007 he was the recipient of the American Public University System's Excellence in Teaching and Learning Award for the School of Arts and Humanities. He is the author of Warfare in the Ancient World, Warfare in the Medieval World, Hannibal's last Battle: Zama and the Fall of Carthage, and Road to Manzikert: Byzantine and Islamic Warfare, 527-1071.  ☞  Pick up a copy on Amazon