Roman legion

• 1. Introduction

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  The Roman legion was the largest military unit the Roman army ever fielded, and for roughly a thousand years it stood as the most consistently effective fighting force in the ancient world. At its peak in the Imperial period, a single legion could number more than five thousand legionaries, backed by an equal number of auxiliary troops. What kept such a force together across centuries of warfare, political upheaval, and constant change? The answer lies not in any single weapon or tactic, but in a system built on structure, discipline, and a willingness to borrow from every enemy Rome ever faced. From the earliest citizen-soldiers of the Roman Kingdom to the reformed, smaller units of the late Empire, the legion's story is one of relentless adaptation. The aquila, the eagle standard, became the symbol of that system's soul, and losing it was considered the greatest dishonor a legion could endure.

• 2. From Kingdom to Republic

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  In the period before the formal legio existed, Roman forces were organized into centuriae of roughly one hundred men, grouped together as circumstances demanded and answering to whoever had raised them. During the rule of Servius Tullius, the census was introduced, dividing all able-bodied, property-owning male Roman men into five classes for military service based on wealth, then organizing them into centuries as sub-units of the greater army. Joining the army was both a duty and a mark of Roman citizenship; the wealthiest landowners were expected to serve the most years.

  After the overthrow of the Roman monarchy, the legio was subdivided into two separate legions, each assigned to one of the two consuls. In 494 BC, when three foreign threats emerged simultaneously, the dictator Manius Valerius Maximus raised ten legions, a number Livy records as greater than had ever been mustered at any one time before. Some warfare during this period still fell outside the legionary structure entirely; the campaign in 479 BC by the clan army of the gens Fabia against the Etruscan city of Veii ended with the clan's complete annihilation, a reminder that private military ventures operated at their own risk.

  Legions became more formally organized through the 4th century BC as Roman warfare shifted toward more frequent and deliberately planned operations. After a crushing defeat at the Battle of the Allia in 387 BC, the military structure was thoroughly reformed. Under the resulting Camillan system, legionaries were initially structured by social class, with the poorest men forming the first line of battle.

• 3. The Manipular Legion

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  The cavalry was originally the most prestigious unit within the mid-Republican legion. Wealthy young Romans displayed skill and prowess on horseback, laying the groundwork for eventual political careers. In a total force of around three thousand men, the legion had only about three hundred horsemen, divided into ten units called turmae of thirty men each, commanded by decurions.

  The heavy infantry was the principal unit, and it was subdivided by experience into three distinct lines. The hastati, the least experienced soldiers, were placed at the front deliberately; Rome could not afford to lose veterans early, and if the newest men broke and tried to run, experienced soldiers stood behind them to stiffen their resolve. The principes, more seasoned and better equipped, formed the second wave. The triarii, the veterans, rested on one knee when not engaged and were deployed only in extreme situations. Armed with long hastae rather than the pilum and gladius used by the two front lines, the triarii fought in a phalanx formation. Their appearance, an armored formation advancing on an enemy who believed they had already won, frequently halted pursuing foes in their tracks. The phrase Ad triarios redisse, meaning to fall back upon the triarii, became a Roman idiom for using one's last resort.

  Light infantry called velites, mainly poorer citizens who could not afford heavier equipment, acted as skirmishers. After throwing their javelins, they retreated through gaps between the maniples, screened by the heavy infantry lines behind them. Each maniple consisted of two centuries and was commanded by the senior of the two centurions. A century of hastati or principes held 60 men; a century of triarii held 30. Together with around 1,200 velites and 300 cavalry, the mid-Republican manipular legion reached a nominal strength of about 4,500 men.

• 4. Marius, Caesar, and Political Power

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  By the 3rd century BC, the class-based Camillan system was seen to be inefficient. Under the Polybian system that replaced it, rank was determined no longer by wealth but by age and experience. All legionaries had their hastae replaced by gladii, and each man also carried two pila, used as an opening volley before close combat. A typical late Republican legion carried 5,120 legionaries as well as a large number of camp followers, servants, and slaves. When including auxiliaries, a legion could contain as many as eleven thousand fighting men.

  Throughout the Late Republic, the legions accumulated political weight that proved impossible to contain. By the 1st century BC, the threat posed by a demagogue commanding devoted legions was openly recognized. Roman governors were forbidden to leave their provinces with their armies. When Julius Caesar broke this rule, crossing the Rubicon out of his province of Gaul into Italy proper, he triggered a constitutional crisis. The civil wars that followed ended the Republic and brought about the foundation of the Empire under Augustus in 27 BC.

  During those preceding civil wars, generals had formed their own legions and numbered them freely. At the civil war's end, Augustus was left commanding roughly fifty legions, with several duplicate numberings, including multiple units all calling themselves Legio X. For political and economic reasons he reduced that number to 28, a figure that shrank further to 25 after three legions were destroyed completely at the Battle of the Teutoburg Forest.

• 5. The Imperial Machine

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  Augustus did not merely cut the legions; he rebuilt the entire system on permanent foundations. Legions became standing, professionally paid units allocated to static bases with permanent legionary fortresses. He greatly increased the number of auxiliaries to the point where they equaled the legionaries in number, and he created the Praetorian Guard along with a permanent Roman navy manned by freed slaves called liberti.

  Augustus also standardized pay. A rank-and-file legionary received 225 denarii a year until the emperor Domitian raised it to 300. There was no further increase until Septimius Severus, who reigned from 193 to 211, pushed it to 500 denarii a year. Veterans who completed 25 or more years of service received a praemia, a discharge bonus of 3,000 denarii from the time of Augustus, along with the option of a plot of farmland; that land, given strategically, helped consolidate control of frontier regions and restive provinces. Under Caracalla the cash praemia rose to 5,000 denarii.

  The philosopher Favorinus is quoted as remarking that it is ill arguing with the master of thirty legions, a figure that captures the stabilized Imperial army under Augustus' successors. With each legion's 5,120 legionaries typically supported by an equal number of auxiliaries according to the historian Tacitus, the total force available to a legion commander during the Pax Romana probably ranged up to eleven thousand men, with more prestigious legions and those on hostile borders often commanding more auxiliaries. By the time of Severus, auxiliaries may have composed 55 to 60 percent of the total army, around 250,000 of the estimated 447,000 troops.

• 6. Discipline and the Eagle

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  From 104 BC onward, each legion carried an aquila, an eagle standard, as its defining symbol. The aquilifer who bore it was chosen from steady veterans with a thorough understanding of legionary tactics, and the position carried pay twice the base wage. Losing the aquila in battle was the gravest dishonor a legion could suffer, and often led to the unit's disbanding.

  Julius Caesar, in his Gallic War, records an incident from his first invasion of Britain in 55 BC that shows how powerful that bond was. When his troops hesitated to leave their ships because of the massed Britons waiting onshore, the aquilifer of the tenth legion threw himself overboard, carried the eagle forward, and advanced alone against the enemy. His comrades, fearing the disgrace of abandoning the standard, leapt from the ship after him, followed by troops from every other vessel in the fleet.

  Discipline within the legions was correspondingly severe. Minor punishments ranged from a centurion striking a soldier with his staff to reduction in rations, flogging, or dishonourable discharge. More serious failures could bring fustuarium, being beaten to death by cudgels in front of assembled troops. Decimation, reserved for mutiny or mass desertion, required that one man in every ten, chosen by lot, be beaten to death by the other nine. The primus pilus, the senior centurion of the entire legion, commanded the first century of the first cohort and was paid sixty times the base wage. The story of one centurion named Petronius Fortunatus, who made his rank in four years and then spent the next forty-two years serving across twelve different legions without ever reaching the primi ordines, illustrates both the depth of the centurionate and the long, demanding career it represented.

• 7. Adaptation and Longevity

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  Montesquieu wrote that the main reason for the Romans becoming masters of the world was that, having fought successively against all peoples, they always gave up their own practices as soon as they found better ones. The legion's history bears this out across every century of its existence. The gladius was adopted from Iberian opponents. Warship design was adapted from the Carthaginian quinquereme. Heavy mounted cavalry and mounted archers were copied from the Numidians and Parthians.

  The late Roman writer Vegetius, in his De re militari, records that each century in the later legion carried a ballista, and each cohort had an onager, giving the legion a siege train of 59 ballistae and 10 onagers, each manned by ten artillerymen called libritors and drawn by oxen or mules. These engines could reduce city walls, defend Roman fortified camps, and even serve as field artillery during river crossings.

  By the fourth century AD the legion had shrunk to units of around 1,000 to 1,500 men, a transformation that began when large formations were broken into smaller detachments to cover more territory. The final form of the late legion originated with the elite legiones palatinae created by Diocletian. The Notitia Dignitatum, the administrative register of the late Empire, lists 25 legiones palatinae, 70 legiones comitatenses, 47 legiones pseudocomitatenses, and 111 auxilia palatina in the field armies alone, plus a further 47 legions in the frontier forces. Legio V Macedonica, founded by Augustus in 43 BC, is listed in that same document still active as a comitatense unit, and records place it in Egypt during the Arab conquest of 637 AD, nearly seven centuries after its founding.