Structural history of the Roman military
During this period Rome itself was probably little more than a fortified hilltop settlement and its army a relatively small force, whose activities were limited "mainly [to] raiding and cattle rustling with the occasional skirmish-like battle".
Mommsen uses philological arguments and references from Livy and others to suggest that the greater mass of foot-soldiers probably consisted of pilumni (javelin-throwers), with a smaller number possibly serving as arquites (archers).
By the close of the century, the Romans had lost their struggle for independence, and the Etruscans had conquered Rome, establishing a military dictatorship, or kingdom, in the city.
The first class of the richest citizens served as heavy infantry with swords and long spears (resembling hoplites), and provided the first line of the battle formation.
[21] Men without property, who were thereby excluded from the qualifying social classes of the adsidui, were exempted from military service on the grounds that they were too poor to provide themselves with any arms whatsoever.
The maniples were typically deployed into three discrete lines (Latin: triplex acies) based on the three heavy infantry types of hastati, principes and triarii.
They carried an iron bossed wooden shield, 120 cm (4 ft) tall and rectangular in shape with a curved front to partially protect the sides.
There was an additional class of troops (Latin: accensi, also adscripticii and later supernumerarii) who followed the army without specific martial roles and were deployed to the rear of the triarii.
[citation needed] The light infantry of 1,200 velites[29] consisted of unarmoured skirmishing troops drawn from the youngest and lower social classes.
[citation needed] The Roman levy of 403 BC was the first to be requested to campaign for longer than a single season,[36] and from this point on such a practice became gradually more common, if still not typical.
[citation needed] A small navy had operated at a fairly low level after the Second Samnite War, but it was massively upgraded during this period, expanding from a few primarily river- and coastal-based patrol craft to a full maritime unit.
[40] By the time of Polybius, the triarii or their successors still represented a distinct heavy infantry type armed with a unique style of cuirass, but the hastati and principes had become indistinguishable.
[41] Where accepted allies could not provide the required force types, the Romans were not averse during this period to hiring mercenaries to fight alongside the legions.
[47] Nor did the legions meaningfully professionalise: as, in general, both soldiers and commanders served only for short periods intending, respectively, to secure plunder or political advancement from military victory.
[citation needed] After having declined in size following the subjugation of the Mediterranean, the Roman navy underwent short-term upgrading and revitalisation in the late Republic to meet several new demands.
[37] By the time of Julius Caesar in 54 BC, regular legionary units were supplemented by exploratores, a body of scouts, and speculatores, spies who infiltrated enemy camps.
[68] Due to the demands of the civil war, the extraordinary measure of recruiting legions from non-citizens was taken by Caesar in Transalpine Gaul (Latin: Gallia Transalpina), by Brutus in Macedonia, and by Pompey in Pharsalus.
[71] The experience of Caesar and, earlier, Marius and Sulla, had demonstrated the willingness of "emergency" (re-activated previously decommissioned) legions containing troops keen for plunder to follow their generals against the state.
Although entire legions were occasionally transferred into theatres of war, they remained largely rooted in one or more legionary bases in a province, detaching into smaller bodies of troops (Latin: vexillationes) on demand.
This low figure is probably a direct result of the changing needs of military staffing: a system of fixed border defences (Latin: limes) were established around the Empire's periphery under Hadrian, consolidating Trajan's territorial gains.
Antonio Santosuosso argues that the strict discipline and high motivation of the days of Marius had lapsed,[95] but Andrew Alfoldi states that the Illyrian troops were both valiant and warlike,[93] and Tacitus described German recruits as being natural mercenaries (Latin: vivi ad arma nati).
[98] The gradual inclusion of greater numbers of non-citizen troops into the military was taken a further step by the creation under Hadrian of a new type of force in addition to the legions and auxilia, known as numeri.
[103] Additionally, the Emperor Gallienus took the revolutionary step of forming an entirely cavalry field army, which was kept as a mobile reserve at the city of Milan in northern Italy.
[109][110][111] While the limitanei were supposed to deal with policing actions and low-intensity incursions, the duty of responding to more serious incidents fell upon the regional or provincial troops of the reduced field reserves of the comitatenses.
[124] There is some dispute about whether this new military structure was put into place under the Emperor Diocletian or Constantine since both reorganised the Roman Army in the late 3rd and early 4th centuries to some degree.
[127] Recruitment from amongst Roman citizens had become greatly curtailed as a consequence of a declining population,[128][129] "cripplingly numerous" categories of those exempted from military service and the spread of Christianity with its pacifist message.
In return for being allowed to settle as foederatii in northern Gallia on the near side of the Rhine, the Franks were expected to defend the Empire's borders in their territory and provide troops to serve in Roman units.
[citation needed] In 376, a large band of Goths asked Emperor Valens for permission to settle on the southern bank of the Danube River on terms similar to the Franks.
Santosuosso argues that foederati regiments consisted mostly of cavalry[145] that were raised both as a temporary levy for a specific campaign need and, in some cases, as a permanent addition to the army.
As barbarian incursions continued, some advancing as far as the heart of Italy, Rome's borders began to collapse, with frontier forces swiftly finding themselves cut off deep in the enemy's rear.
