The basic building blocks of Roman armies in the Middle Republic are the citizen legion and the socii alae or “wing”. A “standard” Roman army generally consisted of two legions and two matching alae. but larger and smaller armies were possible by stacking more legions or enlarging the alae. We’re not nearly so well informed as to the structure of the alae of socii (the socii being Rome’s “allied” – really, subject – peoples in Italy), except that they seem to have been tactically and organizationally interchangeable with legions. Combined with the fact that they don’t seem archaeologically distinctive (that is, we don’t find different non-Roman weapons with them), the strong impression is that at least by the mid-third century – if not earlier – the differences were broadly ironed out and these formations worked much the same way.¹ So, for the sake of simplicity, I am going to discuss the legion here, but I want you to understand (because it will matter later) that for every legion, there is a matching ala of socii which works the same way, has effectively the same equipment, fights in the same style and has roughly the same number of troops.

With that said, we reach the first and arguably most important thing to know about the legion: the Roman legion (and socii ala) of the Middle Republic is an integrated combined arms unit. That is to say, unlike a Hellenistic army, where different “arms” (light infantry, heavy infantry, cavalry, etc.) are split into different, largely homogeneous units, these are “organic” to the legion, that is to say they are part of its internal structure (we might say they are “brigaded together” into the legion as well). Consequently, whereas the Hellenistic army aims to have different arms on the battlefield in different places doing different things to produce victory, the Roman legion instead understands these different arms to be functioning in a fairly tightly integrated fashion with a single theory of victory all operating on the same “space” in the enemy’s line.

And you may well ask, before we get to organization, “What is that theory of victory?” As we saw, the Hellenistic army aims to fix the enemy with its heavy infantry center, hold the flanks with lighter, more mobile infantry (to protect that formation) and win the battle with a decisive cavalry-led hammer-blow on a flank. By contrast, the Romans seem to have decided that the quickest way to an enemy’s vulnerable rear was through their front. The legion is thus not built for flanking, its cavalry component – while ample in numbers – is distinctly secondary. Instead, the legion is built to sandpaper away the enemy’s main battle line in the center through attrition, in order to produce a rupture and thus victory.

To do that, you need to create a lot of attrition and this is what the manipular legion is built to do.

The legion of the Middle Republic is built out of five components: three lines of heavy infantry (hastati, principes and triariivelites), and a cavalry contingent (the equites). Specifically, a normal legion has 1200 each of velites, hastati and principes, 600 triarii and 300 equites, making a total combined unit of 4,500. Organizationally, the light infantry velites were packaged in with the heavy infantry (Polyb. 6.24.2-5) for things like marching and duties in camp, but in battle they typically function separately as a screening force thrown forward of the legion.

So to take the legion as an enemy would experience them, the first force were the velites. These seem to have been deployed in open order in front of the legion to screen its advance. These fellows had lighter javelins, the hasta velitaris (Livy notes they carried seven, Livy 39.21.13), no body armor and a “simple headcovering” (λιτός περικεφάλαιος, Polyb. 6.22.3), possibly hide or textile; they also carried a smaller round shield, the parma, and the gladius Hispaniensis for close-in defense (Livy 38.21.13). These are, all things considered, fairly typical ancient javelin troops, aiming to use the mobility their light equipment offers them to stay out of close-combat.

Behind the velites was the first line of the heavy infantry, the hastati. These fellows were organized into units called maniples (lit: “a handful”) of 120, which in turn are divided into centuries of 60 each. The maniples are their own semi-independent maneuvering units (note how much smaller they are than the equivalent taxeis in the phalanx, this is a more flexible fighting system), each with its own small standard (Polyb. 6.24.6) to enable it to maintain coherence as it maneuvers. That said, they normally form up in a quincunx (5/12ths, after a Roman coin with the symbol of five punches, like on dice) formation with the rear ranks, as you can see above.

The hastati (and the principes, who are equipped the same way) have the large Roman shield, the scutum, two heavy javelins (pila), the gladius Hispaniensis sword, a helmet (almost always a Montefortino-type in bronze in this period) and body armor. Poorer soldiers, we’re told, wore a pectoral, wealthier soldiers (probably post-225, though we cannot be certain) wore mail. That is, by the standards of antiquity, quite a lot of armor, actually – probably more armor per-man than any other infantry formation on their contemporary battlefield. That relatively higher degree of protection – big shield, stout helmet (Montefortino’s in this period range from 1.5-2.5kg, making them unusually robust), and lots of body armor – makes sense because these fellows are going to aim to grind the enemy down.

Note that a lot of popular treatments of this assume that the hastati were worse equipped than the principes; there’s no reason to assume this is actually true. The principes are older than the hastati, but the way to understand this formation is that the velites are young or poor, whereas for the upper-classes of the infantry (probably pedites I-IV) after maybe the first year or so, they serve in the heavy infantry (hastati, principes, triarii) based on age, not on wealth (and then the equites are the truly rich, regardless of what age they are; the relevant passage here is Polyb. 6.21.7-9, which is, admittedly, not entirely clear on what is an age distinction and what is a wealth distinction).

We’ve discussed the combat width these guys fight with already – somewhat wider spacing than most, so that each man covers the other’s flanks but they all have room to maneuver. It seems like the standard depth in the Middle Republic was either base-3 (so 3 deep on close order, 6 deep for “fighting” open order) or base-4 (so 4 and 8). Even in open-order with the maniples stretched wide (possibly by having rear centuries move forward), there would have been open intervals (10-20m) between maniples, which reinforces the role of a maniple as a potentially independent maneuvering unit – it has the space to move.²

Behind the hastati are the principes, with the same equipment and organization, slightly off-set to cover the intervals between the hastati, with a gap between the two lines (we do not know how large a gap). These men are slightly older, though not “old”. The whole field army generally consists of iuniores (men under 46) and given how the Romans seem to like to conscript, the vast majority of men will be in their late teens and 20s. So we might imagine the velites to be poorer men, or men in their late teens (17 being the age when one become liable for conscription) or so, while the hastati are early twenties, the principes mid-twenties and the handful of triarii being men in their late twenties or perhaps early 30s. The positioning of the principes isn’t to spare older men the rigors of combat, but rather to put more experienced veterans in a position where they can steady the less experienced hastati.³

Finally, behind them are the triarii, who trade the pila for a thrusting spear, the hasta, the Roman version of the Mediterranean omni-spear. These men are, as noted, the oldest and so likely the calmest under pressure and thus form a reserve in the rear. The three-line system here is what the Romans call a triplex acies (“three battle lines”). This wasn’t the only way these armies engaged and they could sometimes be formed up into a single solid line, but the triplex acies seems to have been the standard. We don’t know exactly how deep such a formation would run, but we have fairly good evidence that a legion might occupy a space around 400m wide (with some variation), meaning a whole Roman army’s core heavy infantry component (the two legions and two alae) might be some 1.6km (about a mile) across.

The equites, while organic to the legion organizationally, will be tactically grouped in battle to form cavalry screens on the edges of the army, not as a grand flanking cavalry “hammer”, but as flank-protection for the advancing infantry body (as a result, they tend to fight more cautiously). The equites in this period are heavy cavalry, with armored riders (after c. 225, that would be mail), using a shield and a hasta, along with a gladius as a backup weapon and thus serving as “shock” cavalry. Roman cavalry, if we look at their deployments, is generally ample in numbers, but the Romans seem to have been well aware it wasn’t very good, and sought allied cavalry (especially non-Italian allied cavalry) whenever they could get it. But the cavalry, Roman or not, was almost never the decisive part of the army.

Polybius tells us that the socii supplies more cavalry than the Romans and implies that there was a standard rule of three socii cavalrymen to every Roman equites, while socii infantry matched Roman infantry numbers (Polyb. 6.26.7). Looking at actual deployments though, we see that the socii tend to outnumber the Romans modestly, on about a 2:3 ratio, with socii cavalry only modestly outnumbering Roman cavalry.⁴ Consequently a normal Roman consular field army (of which the Romans generally had at least two every year) was 8,400 Roman infantry, around 12,600 socii infantry, 600 Roman cavalry and perhaps a thousand or so socii cavalry, for a combined force of 21,000 infantry (c. 5,000 light 16,000 heavy, so that’s a lot of heavy infantry) and 1,600 cavalry. That somewhat undersells the cavalry force the Romans might bring, as Roman armies also often move with auxilia externa (allied forces not part of the socii), which are very frequently cavalry-heavy (especially, after 203, that really good Numidian cavalry).⁵ By and large, it’s not that the Romans bring a lot less cavalry (as a percentage of army size), but that Italian cavalry tends to perform poorly and the as a result the Romans do not built their battle plans around their weakest combat arm.

Perhaps ironically, the Romans used their cavalry like Alexander and Hellenistic armies used their light infantry: holding forces designed to keep the flanks of the battlefield busy while the decisive action happened somewhere else.

Bret Devereaux, “Collections: Phalanx’s Twilight, Legion’s Triumph, Part IIa: How a Legion Fights”, A Collection of Unmitigated Pedantry, 2024-02-09.

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1. On this, see Burns, M. T. “The Homogenisation of Military Equipment under the Roman Republic”. In Romanization? Digressus Supplement I. London: Institute of Archaeology, University College London, 2003.

2. On this, M.J. Taylor, “Roman Infantry Tactics in the Mid-Republic: A Reassessment”, Historia 63.3 (2014): 301-322.

3. To expound at some length on my own thoughts on how I think the wealth/age issue was probably managed, Dionysius (4.19.2) claims that the Romans recruited by centuries in the comitia centuriata such that the wealthy, divided into fewer voting blocks, served more often, and we know from Polybius that the maximum period of service for the infantry was sixteen years and from some math done by N. Rosenstein in Rome at War (2004) that the average service must have been around seven years. My suspicion, which I cannot prove is that the very poorest Roman assidui (men liable for conscription) might have only been serving fewer years on average and so it wasn’t a problem having them do all of their service as velites (the only role they can afford), whereas wealthier Romans (my guess is pedites IV and up) are the ones who age into the heavy infantry, with pedites I, whose members probably serve more than the seven-year average (perhaps around 10?) might make up close to 40% of the actual heavy infantry body (which is their balance in the comitia centuriata). The velites thus serves two important functions: a place to “blood” wealthier young Roman men to prepare them to stand firm in the heavy infantry line, as well as a place for poorer Romans to contribute militarily in a way they could afford. But I think that, once in the heavy infantry, the division between hastati, principes and triarii was – as Polybius says (6.21.7-9 and 6.23.1) – an age division, not a wealth division. Instead, the next wealth line is for the equites.

4. The data on this is compiled by Taylor, Soldiers & Silver (2020), 26-28.

5. Taylor, op. cit., 54-7 compiles examples.