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Talking about the "Roman legion" is kind of talking about the "American transport network" - what do you mean? Rivers? Railroads? The interstate highway system? Or airliners? The term is static, but the reality was in constant flux. Still, it's possible to discuss such topics if you pick a well-defined window of time when certain things were constant; for the Roman legion, the time from its inception (5th C. BCE) to the Marian reforms of the 1st C. BCE works fairly well.

The "original" legion, like the phalanx it replaced, was made up of conscripted citizens. In the early days, legions I-IV were raised (the Latin legio means "levy") in times of trouble, and disbanded with the fighting ended. This changed as Rome became embroiled in larger and larger conflicts, however - during the second Punic War (211 BCE), 23 legions were raised. We don't know for certain the exact size of these early legions, but they most likely consisted of 4,000-5,000 foot soldiers, with 200-300 cavalry.

The legion was divided into 3 parts, each consisting of 10 maniples; each maniple was further divided into two centuria (centuries). The first group was made up of hastati, young first-time soldiers; in a typical 4200-man legion there were 1200 hastati, meaning 120 per maniple. The hastati formed the first line. Behind them were the principes, men at the prime age and in good condition, in equal numbers. The third line was made up of triarii - older, experienced men, and veterans. There were only 600 triarii in a legion, no matter its size. The balance of the soldiers in a legion, equal in number to the hastati and principes (1200 in a standard unit) were velites, lightly-armed skirmishers made up of the youngest and poorest recruits. The velites were broken into 60 small groups, each of which was assigned to a century. The legion's cavalry wing (literally - Latin ala) was divided into ten squadrons (turmae), each divided into 3 decuriae - so called because each consisted of 10 men in a standard 300-man ala.

Each sub-unit had its own officer and specialist corps. Each maniple had two centurions; the senior commanded the maniple as a whole and the century that deployed on the right. There was also a junior officer (optio) and a standard bearer (vexilarius) for each century. Every cavalry squadron had three decurions and three optiones, as well as its own standard-bearer.

The soldiers of the three lines all wore armor of some sort. The poorest had only a "heart guard" (called a pectorale), a 20cm square breastplate; mail shirts were preferred, although they were heavy. Each soldier had a single greave on his left leg - the left was considered the soldier's weaker side, and was most the side usually exposed in battle - and a helmet. Every legionary carried a shield: a parma, or buckler, in the case of the velites. The heavy infantry had a scutum instead, a heavy shield made of wood faced with canvas and leather, with a metal boss and rim. Cavalrymen wore mail shirts and carried a horseman's parma.

Weapons varied among the different infantry types, as well. Velites carried the hasta velitaris, a throwing spear suitable for their role as skirmishers. The hastati and principes carried a short sword called a gladius, as well as the pilum, a heavy throwing spear, designed to bend on impact to make it impossible for an enemy to throw it back at the legion. The triarii and the cavalry carried the hasta, a thrusting spear, which supports the suggestion that the triarii may have continued to deploy in a phalanx-like formation for some time.

The legion deployed for battle in three lines, presenting a front 975 yards across and between 110 and 130 yards deep. The maniples of hastati deployed in front, with a maniple-wide space between them; the principes were similarly drawn up behind, with their line staggered so that their maniples covered the gaps between those of the hastati. The maniples of the front two lines initially deployed with one century standing behind the first, to make the formation more maneuverable. The triarii deployed in the rear, assuming the legion's commanding officer felt they would be necessary, and the cavalry was deployed on the wings of the battle line. The velites were deployed as a screen and intermixed with the cavalry to add to its strength.

The tactics of this early legion were simple, as befits a non-standing army. Typically, after some skirmishing and missile fire by the cavalry and velites, the rear century of each maniple would move up and partially close the gaps in the first two lines, and the legion would advance. As each of the front ranks reached striking distance of the enemy, they would hurl their pila, before continuing to advance. The hastati would close with the enemy and engage them with their swords; the principes would move in to close gaps in the front line, or if the hastati were forced to retreat through the gaps in their line, they would provide covering fire before continuing the sword-fighting. The triarii would sometimes move up around the flanks of the main line, or they might remain behind to constitute a third line if the first two failed.

The simplicity of these early tactics sometimes backfired - the Carthaginian general Hannibal was particularly adept at exploiting the Romans' predilection for the straightforward advance. At the battle of Cannae in 216 BCE he was able to defeat a numerically superior Roman force by pulling back the center of his lines while his wings and auxiliaries moved in to attack the Romans' flanks and rear. The surrounded Roman force, despite a fierce and stubborn resistance, was annihilated almost to a man.

Nevertheless, the legion was a superlative fighting force, surpassing the feared Macedonian phalanx quite early on. It was flexible, durable, and maneuverable, capable of internal mutual support, and well-equipped, and Roman strategy made sure it was deployed in optimal conditions whenever possible. It was by far the finest military unit of its time, and the Romans would only improve on it in the years to come.

I did most of the research for the paper this is condensed from using Michael Grant's History of Rome, Daniel Green's The Glory that was Rome, Harry Judson's Caesar's Army, Henry Parker's The Roman Legions, Edward Salman's Roman Colonization Under the Republic, and Livy's Ab Urbe Condita. Check 'em out.

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