Polybius remarks both on the superior flexibility of Roman soldiers (18.31.9-11) and the intensity and effectiveness of Roman rewards and punishments (6.35-38). Josephus, a Greek-speaking Jewish man from the province of Judaea who first rebelled against the Romans and then switched sides offers the most famous endorsement of Roman drills, “Nor would one be mistaken to say that their drills are bloodless battles, and their battles bloody drills” (BJ 3.5.1).

It is hard to tell if the Roman triple-line (triplex acies) fighting system created the demand for synchronized discipline or if the Romans, having already developed a tradition of drill and synchronized discipline, adopted a fighting style that leveraged that advantage. Probably a bit of both, but in any event our evidence for the Roman army before the very late third century is very poor. By the time we truly see the Roman army clearly (c. 225 BC) the system seems to already [have been] in place for some time.

A Roman consular army was a complex machine. It was composed of an infantry line of two legions (in the center) and two socii “wings” (alae) to each side, along with cavalry detachments covering the flanks. Each of those infantry blocks (two legions, two alae) in turn was broken down into thirty separate maneuvering units (called maniples, generally consisting of 120 men; half as many for the triarii), which were in turn subdivided into centuries, but centuries didn’t really maneuver independently. In front of this was a light infantry screening force (the velites). So notionally there were in the heavy infantry of a standard two-legion consular army something like 120 different “chess pieces” that notionally the general could move around on their own and thus notionally the legion was capable of fairly complex tactical maneuvers.

You may have noted that word “notionally” because now we get into the limits of drill and synchronized discipline, because this isn’t a system for limitless tactical flexibility of the sort one gets in video games. Instead, recall that the idea here is to create coordinated movement and fighting (the synchronized discipline) through rigorous, repeated practice (drill). Of course one needs to practice specific things. Some of those things are going to be obvious: a drill for marching forward, or for turning the unit or for advancing on the charge.

In the Roman case, a “standard” battle involved the successive engagement and potentially retreat of each heavy infantry line: first the hastati (the first line) formed a solid line (filling the gaps) and attacked and then, if unsuccessful, retreated and the next line (the principes) would try and so on. Those maneuvers would need to be practiced: forming up, then having each maniple close the gap (we don’t quite know how they did this, but see below), the attack itself (which also involved usually throwing pila – heavy javelins), then retreat behind the next line if things went poorly. It’s also pretty clear from a battle like Cynoscephelae (197) or Bibracte (58) that individual maniples or cohorts (the Romans start using the larger 480-man-cohort as the basic maneuver unit during the second century BC) could be “driven” over the battlefield to a degree so there were probably drills for wheeling and turning.

Now even in this “standard” battle there is a lot of movement: maniples need to open and close gaps, advance and retreat and so on. This is what I mean by saying this army is a complex machine: it has a lot of moving parts that need to move together. The men in a maniple need to move together to make that mutually-supporting line and the maniples need to move together with each other to cover flanks and allow retreats. In terms of how the individual men moved, I’ve tended to think in terms of a “flow” model akin to this video of South Korean riot police training, rather than the clunkier Spartacus (1960) model.

But once an army has practiced all of these drills, it creates the opportunity for great improvisation and more complex tactics as well. Commanders, both the general but also his subordinates, can tell a unit to perform a particular maneuver that they have drilled, assuming the communication infrastructure exists in terms of instruments, standard shouted commands and battle standards (and note [that] Roman methods of battlefield communication were relatively well developed). That, for instance, allowed Aemilius Paullus to give orders to his first legion at Pydna for each of those maneuver units to either push forward or give ground independently, presenting the Macedonian phalanx with a tactical problem (an unevenly resisting line) it did not have a good solution for (Plut. Aem. 20.8-10). Having good junior officers […] was required but it wasn’t enough – those officers needed units which were already sufficiently drilled so that their orders (to press hard or retreat and reform in this case) could actually be carried out by soldiers for whom the response to those calls had become natural through that very drill.

At the same time I don’t want to give the wrong impression: even for the Romans battles where there was this sort of on-the-field improvising led by the general were uncommon (though not extremely rare). For the majority of battles, the legionary “machine” simply pushed forward in its standard way, even when – as at Cannae (216) – pushing forward normally proved to be disastrous. Just because an army can fight flexibly doesn’t mean it will or even that it should.

Bret Devereaux, “Collections: Total Generalship: Commanding Pre-Modern Armies, Part IIIa”, A Collection of Unmitigated Pedantry, 2022-06-17.