How Was the Early Roman Legion Structured?

Myke Cole

5 mins

07 Dec 2018

Header image credit: Artist’s depiction of a Roman legion engaging an enemy. Artwork © Giuseppe Rava.

Like its phalanx-centred predecessor, the Polybian legion (c.290-107 BC) was divided into wealth classes. However, more divisions were added to provide a variety of legionaries that could fight in differing roles, and who were uniquely suited to the positions they would occupy in the new Roman ‘manipular’ (from the Latin manipulus for ‘handful’) system of the triplex acies (three lines).

The skirmishers

The first class were the youngest and poorest members of Roman society who still met the minimum property requirements to serve in the legion. These were originally known as the leves, but by Polybius’ day were known as the velites (fast movers).

They served in light skirmishing roles, armed usually with a handful of javelins, a small round shield and a bronze helmet.

Polybius describes the velites as covering their helmets with animal skins in an effort to attract the attention of their officers, ‘to recognize them and judge if they fight bravely or not’. The velites deployed as a screen out in front of their more heavily armed comrades, softening up the enemy lines with missile fire, and keeping the enemy skirmishers from doing the same to the Romans.

An artist’s depiction of a phalanx-eye view of an approaching Roman legion. The velites deployed as a screen ahead of the advancing Romans. Artwork © Giuseppe Rava.

The first line

Next came the hastati (Latin for spearmen, further evidence that the Romans once fought as spear-wielding hoplítēs, as these hastati were swordsmen), older and wealthier than the velites. They formed the first line of the triplex acies and were armed with the sword as their primary weapon.

They also carried the two pila, and the scutum. They usually wore a bronze helmet and pectorale (a square of bronze that protected the heart), and a single greave on their leading leg.

The second line

The second line of the triplex acies was made up of the principes (leaders), men in the prime of their lives, armed as the hastati, save that the wealthier ones may have had a heavier mail shirt instead of the pectorale.

A reconstruction of the tactics of an individual princeps legionary. Artwork by Gerry Embleton © Osprey Publishing.

The final line

The last line was the triarii (the guys in the third line, literally), older men, grizzled veterans of multiple campaigns, the elite of the infantry. They were armed much as the principes and the hastati save that their equipment would likely have been finer, and they did not carry the pila, opting instead for the old hoplite thrusting spear.

As the last line of the legion, the triarii gave their name to a Roman euphemism (according to the turn of the 1st century AD Roman historian Titus Livius, better known as Livy), ‘ad triarios redisse’, in English ‘it comes down to the triarii’, meaning that a person was truly down to their last resort.

A detail of the carved relief on the Altar of Domitius Ahenobarbus, depicting Polybian Roman soldiers: 122-115 BC.

How were these units organised?

Each class of legionary was organized into the previously described maniples, rectangular blocks of men who were deployed in three to six ranks. These maniples were further divided into two centuries (from the Latin centum for ‘100’), each of which ideally consisted of 100 men, but in reality the number was closer to 60.

Each century was led by a centurion, an officer often compared to a modern sergeant, but actually much closer to a captain in terms of responsibility and span-of-control.

The Latin titles of these two centurions – centurio prior and centurio posterior – have led some historians to speculate that the centuries made up the maniple with one behind the other, though this is debated, with some scholars speculating they were positioned side by side.

The maniples of the triarii were smaller and less numerous, with closer to 60 legionaries. Ideally, the legion at full strength would consist of roughly 10–20 maniples of each class of infantry, for a rough total of 5,000 men.

A diagram of a Roman legion of the period showing the maniples with their attached officers and staff. (Bounford © Osprey Publishing).

The socii

These 5,000 were paired with their associated socii, ‘allied’ (read: subjugated) troops from neighbouring Italian regions that had treaties of friendship with Rome. The Romans kept a formula togatorum (toga-wearers list), a list of military obligations owed from allied regions.

These allied troops fought in a variety of roles, most notably serving as critical cavalry alongside the Roman equites, the Roman cavalry corps made up of those Romans rich enough to afford the tremendous expense of maintaining a horse.

The socii deployed as alae (wings), a word that gives us a hint of where they stood in the battle line, likely occupying the wings while the Roman troops occupied the centre.

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The elite of these allied troops were termed extraordinarii (the extraordinary), and were assigned to especially difficult tasks, such as serving as the vanguard or rearguard when the army was on the march, or handling reconnaissance duties.

The socii usually numbered the same as the Roman legionaries (and often slightly more) and triple the number of cavalry – to give a full strength of around 10,000 troops to a legion.

Conquering the Mediterranean

Rome threw out her last king, Tarquinius Superbus, in 509 BC, and established herself as a republic. The republic elected two senior officials for the year, the famed consuls, who also served as Rome’s campaigning generals. In the Polybian army each consul commanded two legions for a nominal strength of 20,000 troops.

It was this army of javelin-armed knife-fighters who would face the Hellenistic phalanx commanded by the successors of Alexander the Great, first suffering defeats, but innovating and adapting as only Romans could to eventually achieve great success – against Pyrrhus of Epirus in the Pyrrhic wars, and later against Philip of Macedon and his son Perseus in the Macedonian Wars.

The Battle of Pydna, 168 BC, where the Romans defeated King Perseus’ Macedonian army. In this image a Roman legion engages Antigonid Chalkaspides (Bronze Shields). (Angus McBride © Osprey Publishing).

The legion would suffer its greatest defeats against the famous Carthaginian general Hannibal Barca, including one of the greatest military disasters in history, Cannae, but would come back to win the Second Punic War and see Carthage utterly humiliated.

Hannibal fled to the court of the Seleucid king Antiochus III, and was there when the Roman legion took on the Seleucid phalanx at Magnesia and crushed it.

The Polybian legion enjoyed this success until an increased pace of campaigns on multiple fronts over longer periods made the Roman levy system untenable, leading to the reforms of Gaius Marius which completely transformed the legion into a new army that would usher Rome from an expanding republic into a dominating empire.

It was this new legion that Caesar would take over the Alps to conquer Gaul, but Rome’s imperial heights would always have their roots firmly in the republican system that Polybius described.

Myke Cole is the author of Legion versus Phalanx: The Epic Struggle for Infantry Supremacy in the Ancient World, published by Osprey Publishing.