The legions of the Republic fought in a disciplined, tight formation that was essentially a hinged phalanx with greater flexibility. The Polybian legion had formed the traditional three ranks of the manipular-based triplex acies with three separate formations of soldiers arrayed in depth, staggered behind each other. This formation was primarily intended to allow for great endurance, not tactical flexibility. The cohort-based principate legion retained the triplex acies, but each cohort was a distinct unit with its own command structure. This allowed for greater unit flexibility and initiative, which in turn allowed the principate legion to more easily shift forces from areas less engaged to those hard pressed. This same attribute allowed for greater use of reserve forces, further increasing the legion's combat capabilities.
The principate legion's order of march was relatively unchanged from its predecessor's. The vanguard of the formation was typically cavalry or light infantry, allowing for scouting and flexibility in case of ambush or attack. Behind the vanguard came the main body, generally lead by the finest of the legionary forces. The baggage moved either behind or in front of the legion, dependant upon the most likely source of danger, and the trail element was normally the lesser legionary units, perhaps accompanied by the auxilia (Bohec 128). This formation was fairly standard throughout the Roman army, indicating it was deeply imbedded within the legionary training.
The order of battle for the legions is both well established and somewhat mysterious. We have a great deal of information pertaining to how the legion formed for battle, and yet some of the information is contradictory or appears to make little sense. Certainly the legion formed in three primary elements, as it traditionally had ("ala" means "wing" and referred to the allied forces that formed on the left and right flanks of the Polybian legion). This tripartite formation allowed for tactical flexibility, which the legion used to maneuver against its opponents weak points (Bohec 141). The interval between individual soldiers in the formation, however, is unclear. Polybius informs us that each soldier required six feet on all sides in battle, but Vegetius indicates three feet to each side and two feet of depth (Goldsworthy, Roman Army 179). The cohort's formation is similarly contradictory. Vegetius and Josephus stated that the cohort marched in a column three men across, deploying into lines either three or six men deep. In contrast, Arrian wrote that the column was four men across on the march, and deployed into lines four or eight men deep. This would indicate that a standard formation for a Roman unit on the march was either three or four across, deploying into the triplex acies with similar depth under normal circumstances and doubling that depth when facing cavalry or situations requiring greater depth (Goldsworthy, Roman Army 179). It is clear that the optio was positioned in the rear of the formation, controlling the alignment and positioning of the century, but the location of the centurions is less clear. There is no question the centurion led from the front, but it seems unlikely he was positioned directly in front of the unit. It is more probable he stood to the right of the formation, possibly in the gaps between the elements. From this position he could both lead and maintain control of the front ranks.
The battle began when the two sides closed within artillery range. The Romans utilized artillery fire to disrupt enemy lines as they approached, followed by archers and other ranged weapons as the distance decreased. The final approach was perhaps the most startling illustration of the difference between the Polybian legion and that of the principate. The Polybian legion's approached accompanied by loud war cries and the banging of shields. This din was intended to both defeat the enemy's morale and to bolster the legion's own confidence. In sharp contrast, the imperial legion closed in silence-a sight that must have been unnerving to its foes for its unearthly quality. When the enemy closed within 15 meters, the Romans unleashed a volley of pila, followed up by a tremendous battle cry and charge (Goldsworthy, Roman Army 197). This double shock, combined with the damage inflicted by artillery and missile fire, often proved devastating, shattering the enemy charge and resulting in a rout. If the enemy lines did not break, then close combat ensued. The triplex acies of the imperial army had similar staying power to that of the Polybian legion, and the arms and armor were also similar. The great scutum of the legionary was more than merely a defensive weapon, the great boss in the middle was also used to strike at an opponent, shoving them off balance and opening the foe to a sharp upward thrust from the gladius (Goldsworthy, Roman Army 218). Eventually, even a valiant foe was normally overcome by the legion, and it was during the rout that an army suffered the majority of its casualties.
Roman culture lent itself well to such aggressive tactics. The stresses of hand-to-hand combat demand excellent leadership and individual courage to motivate the rankers to commit to an all out assault. Roman culture virtually demanded the pursuit of individual glory, ensuring the presence of legionaries willing to act as champions and thereby providing the example to their fellow soldiers in combat. Such bravery was well rewarded in Roman society, and it was equally well rewarded in the legions. Units were recognized for exceptional performance, as exemplified by Caesar's lavish praise of the LEGIO X in his Commentaries. Even greater praise was rewarded to the permanent legions of the principate, as illustrated by the additional designation earned by LEGIO IVX MARTIA VICTRIX after its success in suppressing the Boadicean revolution in Britain (Goldsworthy, Roman Army 254). Individual courage was also recognized in a variety of ways, ranging from payment in gold to awards, such as the corona muralis, which was awarded to the first man over the wall during a siege (Goldsworthy, Roman Army 277). This system of rewards, combined with the cultural imperative of individual virtus, is well illustrated by the story of Pullo and Vorenus in De Bello Gallico. Caesar describes these two centurions, archrivals in the pursuit of position and glory, as engaging in a remarkable contest of individual courage, as first Pullo, then Vorenus, alternately charge into the enemy and are in turn rescued by their rival. They eventually return safely to the Roman lines after inflicting a number of casualties on the enemy (Caesar V, 44). Such examples were lauded by the Romans, and surely contributed to a legion's ability to demoralize the opposition and encourage their own troops. And on the morale of the armies, victory turned.