The standard battle formation employed by the Roman legion during the late Republican and earlier Imperial Era was the triplex acies, a formation of three lines. In the first of these lines four cohorts stood side by side, while in each of the others, only three cohorts stood side by side. Within each cohort, the three maniples stood side by side, while within the maniple itself, two centuries stood one behind the other. The intervals between lines varied according to the nature of the ground. The cohort was usually eight ranks in depth. [1]

If a legion had a fighting strength of 3,600 men it would probably have a front of 180 soldiers. Each cohort would have a 45 man front. Allowing each soldier about three feet on all sides, so that he could use his weapons easily, it has been estimated that if there were no interval between cohorts, the front of the legion would cover over a thousand feet. [2]

The duplex acies, with five cohorts in each of two lines, [3] and the quadruplex acies, with four lines, were probably employed only when required by the nature of the ground while the simplex acies or single line, was probably seldom used except in defense of a fortified camp. Such a formation might also be described as an orbis. Whenever a Roman force was suddenly attacked, it might go into this rough oval or square formation with the officer in the center. [4]

Another formation called the testudo (tortise), was used when the legionaries were under heavy missile attack, or were advancing to besiege a fortified town or camp. The formation was a tight one, the soldiers in the first rank and flanking files held their shields before themselves and to their right and left, forming a screen and the soldiers in the center held up their shields above their heads so as to form a roof. The walls and roof of shields were kept tight so that javelins and arrows could not penetrate them. [5] The effectiveness of the testudo was increased by the uniform rectangular shape of the legions shields.

In the second century of our era, the legion was, according to Arrian, drawn up in a phalanx eight ranks deep. Probably a version of the duplex acies evolved, which had a milarian cohort stationed on the right of the first line. [6]

The most common battle formation was probably the unbroken front (fronte longa). The legions, deployed in the triplex acies, were posted side by side so as to form a continuous front. An interval separated the center (media acies) from the wings (cornua) which were composed of auxiliary infantry and horse. The third line may be considered a reserve, available to defend the flanks or attack the enemy's flank. Caesar used this line that way, both against the Helvetians and against Pompey at Pharsalus (48 B.C .) [7]

Another, although less common formation, was the oblique attack (acies Obliqua) in which the army advanced in echelon formation with one wing loading. The army could also advance in crescent forftlation (acies sinuata) with the center held back, or it could adopt a wedge formation (cuneus). On the other hand, if the enemy advanced in a wedge formation, the Romans might themselves adopt the V shaped forfex (shears) formation, in order to prevent the enemy from breaking the line. [8]

The oblique order was a good one to use against a numerically superior enemy. It was also a good one to use when a part of the Roman army consisted of levies or newly raised formation. The inexperienced or inferior troops could be posted on the refused wing where they would nevertheless hold the attention of a large portion of the enemy army. Another way of compensating for an inferiority in numbers was to draw the army up with one or both flanks resting on natural obstacles such as, for example, a sea coast, a river bank or a morass. In this case, all the light troops could be posted on the wing which lacked natural protection. Caesar used a similar formation at Pharsalus. When the crescent formation or attack on both enemy wings was contemplated, Vegetius recommended stationing light troops before the center which was to be refused. [9]

By the Second Century of our era, the quality of the Roman auxiliary troops had risen greatly. Instead of native warriors raised for a particular campaign, they had now become trained light infantry. For this reason, The Romans altered their system of frontier defense. Along the Frontier (limes) was a series of redoubts (castella) and camps (castra), manned by the auxiliaries, who could now be depended on to offer disciplined resistance to attack. Some distance to the rear, the legionaries had their camps. If the enemy attacked a certain point, the auxiliaries would meet them. Meanwhile, several legions could assemble to march on the enemy. If the auxiliaries were unable to repulse the attack, they could at least tire the enemy. The advance of the legions would then settle the issue. [10]

The Romans of the late Republican and earlier Imperial Era usually went into action with a quick charge. Once the army was in position and the light troops had finished skirmishing, the tuba and the cornu sounded. [11]

The Romans rushed forward in a charge, javelins poised for hurling. When they were close enough, the forward ranks of the legions hurled their javelins, drew their short swords and continued their rush upon the enemy who were almost certain to be rather disordered by the javelin volley. [12]

Along the front of each cohort, a series of mortal combats raged. Each legionary, his right foot advanced, thrust for his enemy with the point of his sword. [13]

Meanwhile, the supporting ranks pressed forward, hurling their javelins over their comrades heads, filling the gaps left by the fallen, and replacing the wounded or exhausted. When the first line of cohorts had done its utmost, and had begun to tire, it fell back through the second line and reformed. As soon as the second line wearied and became disordered the first fine resumed its old position. The enemy was worn down by the constant charges of fresh cohorts. The third line acted as a reserve, and was brought into action only if the first two lines failed. Meanwhile, auxiliary archers or slingers, who had retired behind the legions might pelt the enemy with missiles, and cohorts of light infantry might engage the enemy on the flanks.

The Roman cavalry was drawn up on the wings, with the heavier armed troopers nearest the infantry. Usually they probably deployed three or more ranks deep. When the battle began, they often engaged the enemy cavalry but only rarely his infantry. Usually, they were held in reserve,for the pursuit. Sometimes, light infantry were intermingled with the cavalry. [14]

Arrian, a Roman governor of Cappadocia, ordered the first four ranks of his legionary phalanx to thrust with their pild while the rear four ranks were to hurl their lighter javelins (lancease) over the heads of their comrades. Thus they were to stop the attacking Alani cavalry, killing horses -and men! [15]

Footnotes