Roman Battle Tactics 109BC–AD313

The study of Roman battle tactics has been likened to crossing a minefield. Doubt has been cast over previous attempts to reconstruct the 'battle mechanics' of the cohortal legion — the principal Roman unit of our period — because its size and organization, command structure and methods of deployment are imperfectly understood (Speidel 1992, 6; Wheeler 1998, 649). This book will focus on the tactics of the legion, because that is the formation for which we possess the most evidence, especially the legions of the Late Republic. The tactics of the auxiliary infantry cohorts and cavalry alae of the Empire will be considered where appropriate.

The time span of this book has been chosen to reflect the period in which the cohortal legion dominated the Roman battlefield. In 109 BC the last vestiges of the manipular legion can be discerned in the battle fought between Metellus and Jugurtha by the River Muthul; and AD 313 saw what was perhaps last great encounter of legion against legion (or at least of legionary 'vexillations' — detachments of one or two cohorts) near Adrianople. Soon after this date the legion was greatly reduced in size and status by the army reforms of the emperor Constantine. There are few detailed literary accounts of the legions of the late 3rd and early 4th centuries AD in action; but the evidence of inscriptions indicates the continuity of traditional centurial and cohortal organization, and we can assume that many of the tactics and manoeuvres carried out by the legions of Julius Caesar (our principal source for such matters) were still practiced.

During the 4th century AD the formation that had conquered the Roman Empire, and had successfully defended it for centuries, was whittled down to a unit of c.400 men. The reasons for this decline are outlined in my Imperial Roman Legionary, AD 161-284 (Osprey Warrior series 72). Yet even in this reduced form the legion lived on until the 7th century AD. When the Muslims invaded Syria and Egypt in the 630s and 640s, the Roman armies that met them at Yarmuk, Heliopolis and Babylon (Cairo) were composed, in part, of legions or units descended from them.

The size of legions varied considerably. The preferred number of soldiers in the legion of the Late Republic (the period c.133-31 BC) was 5,000 to 6,000, the latter being an optimum figure and probably seldom realized (Serv. Aen. 7.247). During the course of an extended war the effective fighting strength of a legion would fall dramatically. In 54 BC Julius Caesar marched with two legions to relieve the camp of Quintus Cicero, which was besieged by the Nervii, with two legions that totalled 7,000 men (Caes. BG 5.49). At the battle of Pharsalus in 48 BC Caesar's legions were even more reduced: having fought through the Ilerda and Dyrrhachium campaigns, the average strength of a legion was 2,750 men. The legions of the opposing Pompeian army were each about 4,000 strong (Caes. BC 3.88-89). Following Pharsalus, Caesar pursued the fugitive Pompey to Egypt with two legions, but their combined strength amounted to only 3,200 men (Caes. BC 3.106). One of the legions was the Sixth, recruited by Caesar in 52 BC; it suffered yet more casualties in Egypt, and arrived at the battlefield of Zela in Pontus ࿏ BC) with less than a thousand men (Anon. BAlex. 69). If legio VI had been raised with a strength of c.5,000, it had lost more than 80 per cent of its effectives in six years of campaigning.

The legions that Mark Antony took to Parthia in 36 BC, including a replenished Sixth (which by then bore the title Ferrata — 'Ironclad'), had an average strength of 3,750 each (Plut. Ant 37.3). In that year, however, the legions of Lepidus were only 'half full', so with c.2,500 to 3,000 men per legion, and potentially even fewer if they were only half the strength of the fuller legions of the period, i.e. those approaching 4,000 effectives (Vell. Pat. 2.80.1). The two legions with which Lucullus won his great victory at Tigranocerta ࿥ BC) were each a little over 4,000 strong — powerful units by the standards of the day (Plut. Luc. 27.2).

The Imperial legion had a paper strength of about 5,000 soldiers, but again, actual numbers were often far below this. During the early stages of the Illryian Revolt (AD 6-9) the Twentieth Legion was at only half strength when it won a striking victory against 20,000 of the enemy (Vell. Pat. 2.112.2).

What we can say with certainty of the legions of both periods is that they were divided into ten cohorts, and that each cohort was made up of six centuries divided between three maniples (cf Aul. Gell. NA 16.4.6).In the Early Imperial legion the century numbered 80 men, divided into ten contubernia (Hyg. De. Mut. Castr. 1). The eight soldiers of a contubernium formed a mess and tent group, and it has been suggested that they would form a file in the battle line, but there is no ancient evidence to confirm this.

The legion that preceded the cohortal formation was composed of 30 maniples and divided into three battle lines each of ten maniples. The first and second battle lines — the hastate &# 40'spearmen', although by now they fought with heavy javelins called pila), and principes &# 40'best men') — were organized into maniples each of 120 or 160 men. The triarii &# 40'third line men') — veterans equipped with thrusting spears — were always organized in maniples of 60 men. Each maniple was officered by two centurions (centurio — 'commander of 100'), one senior and one junior. The senior centurion was in overall command, but in battle the control of the left side of the maniple was delegated to the junior, who would assume complete command if the senior was incapacitated or killed. As well as two centurions, the maniple had two optiones to keep order at the rear of the maniple, and two standard–bearers (signiferi) (Polyb. 6.21-25).

The maniple would also have had at least one trumpeter. The standards provided a visual focus for advance or retreat; the trumpet provided audible signals and relayed commands from the general's trumpeters to the standard–bearers. Despite the duplication of officers and 'NCOs' (this term is convenient, though not really appropriate for the Roman army), the maniple was not divided tactically into two centuries: it was a single fighting unit. Polybius emphasizes that the pairing of officers was so that the maniple would never be without a leader.

Readers should also note that the manipular legion did not have a commander in the modern sense. Its six tribunes were men of extensive military experience and were far superior in rank to the centurions, but they did not have specific command functions (the same was true of legionary tribunes during the Empire). The only officers with clear command and tactical responsibility were the centurions (Isaac 1995, 23-24). Our instinctive need to interpret ancient organizations and ranks in familiar modern terms is often an obstacle to understanding; our distinctions between field and company officers, warrant officers and non–commissioned officers simply have no direct Roman equivalents. The essential fact seems to be that the legion was an organizational entity rather than a strictly 'pyramidal' fighting unit. It deployed in three mutually supporting lines, and its size might suggest a parallel with a modern brigade, but this resemblance is more deceptive than helpful. That the manipular legion functioned perfectly well without a commander indicates thorough standardized training, throughout its constituent sub–units, in relatively simple drill and formations.