Tactics
Just as the armour and weapons of the late army were fundamentally similar to those of earlier eras, so the army's tactics were based on traditional principles. The key elements of systematic scouting, marching formation, battle array, fortified camping, and siegecraft were all followed intact in the late period. This section examines aspects of late tactics that differed significantly from Principate tactics.
One striking difference was that late army doctrine (and practice) aimed at avoiding open battle with the enemy if possible, unlike the early Principate doctrine of seeking to bring the enemy to battle as often and as quickly as possible. The main motivation was likely not a reduced ability to win such encounters. The late army continued to win the great majority of its battles with barbarians. Rather, the primary concern seemed to be the need to minimise casualties. Pitched battles generally resulted in heavy losses of high-grade comitatenses troops, which could not be easily replaced. This in turn supports the hypothesis that the late army had greater difficulty than the Principate in finding sufficient recruits, and especially high-quality recruits. The late army preferred to attack the enemy by stealth or stratagem: ambushes, surprise attacks, harassment and manoeuvres to corner the enemy in zones where they could not access supplies and from which they could not escape (e.g. by blocking mountain passes or river crossings).
Where battle could not be avoided, the late army broadly followed traditional practice as regards array. Heavy infantry would be drawn up in a main line, normally straight and several ranks deep. Mounted archers were stationed, together with light-armed slingers, in front of the main infantry line. Cavalry would be posted on the wings (light cavalry on the outside). Foot archers would form the rear rank(s) of the main infantry line. There would be a reserve infantry and cavalry line of variable size to the rear of the main line, in order to deal with breaches in the main line and to exploit opportunities. At a distance of a mile or so to the rear of the army, its fortified camp of the previous night would contain its assistants and baggage, guarded by a small garrison. The camp could act as a refuge if the army was put to flight. Roman armies in the field never camped overnight without constructing defences. A ditch would be dug around the perimeter of the camp, and the spoil used to erect a rampart, which would then be topped with a palisade of sharpened wooden stakes arranged cross-hatched to form an impenetrable screen. Such defences, systematically patrolled, effectively precluded surprise attacks and enabled the troops to get a good night's sleep.
Where the late army appears to have evolved to some extent is in battle tactics. The Principate army had relied on a barrage of heavy javelins (pila) followed by a shock infantry charge, which was often sufficient to shatter, or at least disorganise, the barbarian line. After that, legionaries were trained to engage in aggressive mano-a-mano combat, striking the enemy in the face with the boss of their heavy shields (scuta) and stabbing them viciously with short swords (gladii). In close combat, the Romans had the crucial advantage of superior armour, and such tactics very often resulted in the rout of the less well-equipped and trained barbarian foe. The mounted archers and slingers in front of the main infantry line would loose their missiles on the enemy before the infantry lines engaged and would then hastily retreat to the rear of their own infantry line, whence, in conjunction with the foot archers already there, they would loose a continuous rain of missiles on the enemy foot by shooting over the heads of their own infantry. The cavalry's task on each wing was to scatter the enemy cavalry facing them and then, if possible, to encircle the main body of enemy infantry and attack them from the flanks and rear.
In the late army, while the role of archers and cavalry remained similar, the infantry's tactics were less aggressive, relying less on the charge and often waiting for the enemy to charge. During the battle, the Roman line would exert steady pressure in close formation. The thrusting-spear (2–2.5 m long) had replaced the gladius (just 0.5 m (1 ft 8 in) long) as the primary mêlée weapon. The extended reach of the thrusting-spear, combined with the adoption of oval or round shields, permitted a battle array where shields were interlocked to form a "shield wall". (Spears would protrude through the 'V' shaped gaps formed between overlapping shields). The late army also relied more heavily on missiles, replacing the single volley of pila with a more prolonged discharge of javelins and darts.
This kind of combat was consistent with the aim of minimising casualties and its efficacy is illustrated by the Battle of Strasbourg. The battle was primarily a struggle of attrition where steady pressure on the barbarians resulted in their eventual rout. Despite a long and hard-fought struggle, Roman casualties were negligible.
Read more about this topic: Late Roman Army, Strategy and Tactics