Romulus is credited with the founding of the Roman legion (legio, legionis derives from the Latin legere meaning to choose or levy). The Latin word populus originally meant army.
The Phalanx Legion (753-300 BC) The organization and command structure of the king’s army, a combined force of 3,300 infantry and cavalry, is unknown. Each of the three (Etruscan-named) Roman tribes (from the Latin word tribus for thirds), the Luceres, Ramnes and Tities, contributed 1,000 infantry (10 centuries) and 100 cavalry (10 decuries).Levies were self-equipped and the wealthiest citizens formed the cavalry (equites). The word for the common soldier, miles, originates from the 1,000 man (milles) tribal contingent.
The Greek hoplite army emerged around 675 BC and by 600 BC the Etruscans had adopted its equipment (round shields and spears) and tactics (the tight ‘phalanx’ formation). The Romans followed suit .
The Servian Reforms (6th Century BC) Unlike the Etruscan model, the basis for recruitment into the Roman army revolved around citizenship and voting rights. King Servius Tullius (578-535 BC ), whose Etruscan name was Mastarna, is credited with the creation of the comitia centuriata. He organized Rome’s citizenry into five propertied classes.
Historians Titus Livius (59BC-12 AD) and Dionysius of Halicarnassus (died 8BC) tell us that each class participated according to its means, and individual soldiers financed their own equipment. Men over age 46 (seniores) comprised half of each class, and they defended Rome while the juniores (17-45) took to the field. Livy breaks down the classes in his History of Rome as follows (Dionysius' description differs in some respects):
The 1st class (assessed at over 100,000 asses) consisted of 80 centuries of heavy infantry (plus 2 centuries of engineers) and 18 centuries of cavalry. The infantry was equipped in the same manner as Greek hoplites with swords and spears and bronze shields (the round clipeus), breastplates, helmets, and greaves.
The 20 centuries of the 2nd class (assessed at 75,000 to 100,000 asses) were equipped as the 1st Class but with rectangular shields and no breastplates.
The 20 centuries of the 3rd class (assessed at 50,000 asses) were outfitted as in the 2nd but without greaves.
The 20 centuries of the 4th class (assessed at over 25,000 asses) consisted of men with only spears and shields.
The 5th class (assessed at 11,000 asses) was composed of 30 centuries of slingers plus 2 centuries of trumpeters and mechanics.
The capite censi (citizens, with minimal to no property) were excluded from service and organized into a single century). They supplied the necessary labour to build and maintain equipment .
The 40 1st Class centruries of juniories formed a solid phalanx of 4,000 to 4,200 infantry (principes, hastati, and triarii) plus 600 cavalry. Eventually the total number of centuries rose to 60 (possibly from the inclusion of the juniories' ten 2nd Class and ten 3rd Class centuries).
Following the abolition of the monarchy in 509 BC, armies were raised by magistrates (two praetors) for a March-October campaign season. A legion was typically disbanded in the autumn and reformed over the winter. During extended sieges (i.e., against the Veii), or periods when it was necessary to permanently garrison a conquered region, only soldiers eligible for retirement were discharged.
The Manipular Legion (4th-3rd Century BC) The sack of Rome, around 390 BC, by the Gauls exposed the tactical limitations of the phalanx. Marcus Furius Camillus (446-365 BC), known as the 2nd founder of Rome, is credited (according to Livy and Plutach) with introducing:
the oval scutum (shield)
and pilum (javelin),
compensating troops with pay,
and adopting the maniple (from manipuli or handfuls) as the basic tactical unit of the legion.
The citizens were reorganized on the basis of age versus property. Legionary centuries were paired (one prior and one posterior) in maniples and the annual levy grew from one to two legions by 362 BC.
Livy (writing in the 1st century BC) described the legion in 340 BC as 4,000-5,000 men organized into three lines (triplex acies) of heavy infantry arrayed in a checkerboard (quincunx) fashion. There were 15 maniples of hastati, 15 maniples of principes, and 15 ordos of mixed troops. Each ordo was broken down into three vexilla of 60 men each – triarii (veterans), rorarii (young men) and accensi (the least dependable). The gaps between maniples (covered by the line immediately to the rear) facilitated the interchange of lines and the harmless passage of enemy chariots or elephants. Cohorts (maniples of mixed units) existed for administrative purposes only.
Everyone with a net worth of 11,000 asses between the ages of 17-46 was compelled to serve and subject to conscription (dilectus).
Polybius tells us that by the 2nd Punic War (221-202 BC) the army had expanded to 4 legions under two consuls . The hastati, the youngest and least experienced men, formed the 1st line (1,200 men in 10 mniples). The 2nd line (1,200 men in 10 maniples) was comprised of principes (leaders), experienced soldiers and family men in their late twenties and early thirties. The triarii (third-liners) were veterans (600 men in 10 maniples) who formed an emergency reserve.
Soldiers of the first three lines were each equipped with a 4’ by 2' oval shield (scutum) weighing 22 pounds and a Spanish short thrusting sword (the gladius Hispaniensis encountered in combat against Carthaginian Iberian mercenaries). Armour (helmets [Montefortino, Attic or Corinthian, greaves and breast plates) varied as the cost was funded by the individual soldiers. The hastati and principes carried two javelins (pila), possibly of Etruscan or Samnite origin, whereas the triarii used the thrusting spear (hasta).
The rorarii and accensi may have become the 1,200 velites (cloak-wearers), inexperienced young men who preceded the other lines as a screen. Each veles was equipped with a light 4 foot javelin, a sword, and a hide-covered 3 foot wicker shield. They were not organized into formal units and could advance and retreat through the heavy infantry as required. The 300 legionary cavalrymen were equipped with body armour, a circular shield, a sword and a lance.
The Cohortal Legion (2nd Century BC) The resistance of draft-eligible property owners to state subsidization of the army and recruitment changes waned as the duration of wars became longer, the theatres of operation more distant, and the year-round need to garrison territorial gains essential. The wealthy viewed military service as a burden that limited the time they could devote to wealth accumulation.
Around 101 BC Gaius Marius (known as the 3rd founder of Rome) abolished the velites and replaced the maniple (which was small, had too many gaps and was vulnerable to collapse under aggressive enemy attack) with the cohort as the basic tactical unit. Non-propertied citizens (men of free birth irrespective of social rank) were allowed to enlist, and the legions swelled with volunteers (aged 17 and older). Recruits escaped poverty, learned a trade, enjoyed superior medical attention and received a bonus of farmland upon retirement.
Military kit was standardized. Greaves were dropped (except in the case of centurions) and men were equipped with a helmet, shield (scutum), armour (typically Celtic ring mail known as the lorica hamata), sword (gladius), dagger (pugio), hob-nailed sandals (caligae), entrenching tools and two javelins (pila). Each of “Marius’ mules”, as the soldiers were nicknamed, shouldered a 66 pound (35 kg) load of equipment (inclusive of three days of rations).
Marius reintroduced the use of the shield as a weapon to push and corral the enemy. The pilum, universally carried by all ranks, was designed to bend (the iron shank was not hardened) or break apart upon impact. Marius stressed fitness and martial training to endow his men with self-confidence. He conferred a silver eagle (associated with the god Jupiter) upon each legion as its new primary standard.
In battle, the legion’s typical triplex acies (quincunx formation) consisted of 4 cohorts in the first line, 3 in the second and 3 in the rear. At 60 to 80 men per century, legion strength was 4,800 to 6,000 men including 120 cavalry. Marius is likely responsible for reducing the number of 1st cohort centuries from six to five and they were subsequently doubled in size by the mid 1st century AD.
By the end of the Social War in 87 BC, nearly the entire free population of Italy south of the Po was eligible to enlist. The alae sociorum (units previously provided by allied cities) vanished, military salaries were state funded and elected magistrates no longer commanded the legions.
Julius Caesar initiated recruitment of “Latins” north of the Po and native Gauls. The conflict between Pompeii and Caesar, the war with Brutus, and the clash between Antony and Augustus completed the transformation of the army from a militia into a professional fighting machine.