The earliest Roman battle‐order was probably the spear‐armed hoplite phalanx, a single, close‐order infantry formation. In the 4th cent. bc this was replaced by the more flexible manipular organization whereby the legion was drawn up in three lines of maniples behind a screen of light infantry and with cavalry on the wings. Each line was supported by, and could fall back upon, the line behind. All were spear‐armed initially, but by the 2nd cent. the first two (hastātī, principēs) had javelins (pīla).
From the late 3rd cent. three maniples were grouped into a cohort by taking one maniple from each line. First a tactical expedient, by the 1st cent. bc this became a permanent organization, coinciding with the equipping of all legionaries with pila. Roman close‐order infantrymen evolved from being primarily spearmen to swordsmen with javelins. The manipular organization proved flexible and resilient enough to defeat Celtic, Carthaginian, and Hellenistic infantry (see rome (history), 1.3–4). The cohorts were sizeable tactical units with uniform equipment, reflecting the move away from socio‐economic differentiation within the ranks. Shortages of light infantry and cavalry (esp. against Hannibal; see punic wars) were serious weaknesses only partially remedied during the republic.
Throughout the Principate legionaries remained armoured swordsmen, although fencing styles evolved from short‐ to long‐sword. The pilum alone gave way to javelins and spears. Imperial commanders now had at their disposal the regular support troops (auxilia). The legions continued as the main‐line troops, drawn up in the centre between wings and behind screens of auxiliaries. The tactical problems posed by mounted enemies in the Danubian and eastern theatres necessitated deepened legionary formations and increased effectiveness of missiles. Specialists were also transferred around the empire to best tactical advantage, e.g. oriental archers used against Germans, infantry bowmen and Moorish cavalry against Sarmatians, western cavalry against eastern horse‐archers. An increasing emphasis was placed on cavalry of all types.
Military treatises and accounts of campaigns demonstrate the care with which the imperial army's orders‐of‐march were formulated. Scouts, interpreters, and guides were institutionalized to ease passage and avoid ambushes of an advancing army. Itineraries were used on known routes. Emphasis was placed on fortified camp‐sites for protection overnight and as fall‐back positions.
The adoption and development of Hellenistic military technology gave Roman armies the option of ‘scientific’ siege operations. During the Principate military installations were lightly defended, and Roman armies hoped to operate offensively beyond the frontiers. However, Roman units tended to be strung out along frontier lines, and already in the 2nd cent. ad troops were becoming attached to their bases (see limes). To form campaigning armies increased recourse was had to drawing away detachments for service in other theatres. Many never returned to parent units.
Technical expertise for battle order, command/control, siegework, and camp‐designing resided within the legions, esp. at centurial level, where a continuous tradition of skills was developed and passed on (see centurions). A militarized aristocracy exercised higher command during the republic and the Principate (see careers, Roman), but from the later 2nd cent. a military élite developed within the army, providing professional commanders and emperors who preserved the empire. See armies, roman; artillery; booty; fortifications; mercenaries; siegecraft, roman.
Subjects: Classical Studies.