Roman army of the mid-Republic

The Roman army of the mid-Republic, also called the manipular Roman army or the Polybian army, refers to the armed forces deployed by the mid-Roman Republic, from the end of the Samnite Wars (290 BC) to the end of the Social War (88 BC). The first phase of this army, in its manipular structure (290–c. 130 BC), is described in detail in the Histories of the ancient Greek historian Polybius, writing before 146 BC.

The central feature of the mid-Republican army was the manipular organisation of its battle line. Instead of a single, large mass (the phalanx) as in the Early Roman army, the Romans now drew up in three lines (triplex acies) consisting of small units (maniples) of 120 men, arrayed in chessboard fashion, giving much greater tactical strength and flexibility. This structure was probably introduced in c. 300 BC during the Samnite Wars. Also probably dating from this period was the regular accompaniment of each legion by a non-citizen formation of roughly equal size, the ala, recruited from Rome's Italian allies, or socii. The latter were about 150 autonomous states which were bound by a treaty of perpetual military alliance with Rome. Their sole obligation was to supply to the Roman army, on demand, a number of fully equipped troops up to a specified maximum each year. Evidence from Roman army camps near Numantia in Spain suggests that a much larger tactical unit, the cohort (480 men, equivalent to 4 maniples) already existed, alongside maniples, in the period 153–133 BC. By c. 100 BC, cohorts appear to have fully replaced maniples as the basic tactical unit.

The Second Punic War (218–201 BC) saw the addition of a third element to the existing dual Roman/Italian structure: non-Italian mercenaries with specialist skills lacking in the legions and alae: Numidian light cavalry, Cretan archers, and Balearic slingers. From this time, these units always accompanied Roman armies.

The Republican army of this period, like its earlier forebear, did not maintain standing or professional military forces, but levied them, by compulsory conscription, as required for each campaigning season and disbanded thereafter (although formations could be kept in being over winter during major wars). Service in the legions was limited to property-owning Roman citizens, normally those known as iuniores (age 16–46). The army's senior officers, including its commanders-in-chief, the Roman consuls, were all elected annually at the People's Assembly. Only members of the Roman equestrian order—the equites—were eligible to serve as senior officers. Iuniores of the highest social classes (equites and the First Class of commoners) provided the legion's cavalry, the other classes the legionary infantry. The proletarii (the lowest and most numerous social class, assessed at under 400 drachmae wealth in c. 216 BC) were until c. 200 BC ineligible for legionary service and were assigned to the fleets as oarsmen. Elders, vagrants, freedmen, slaves and convicts were excluded from the military levy, save in emergencies. During a prolonged such emergency, the Second Punic War, severe manpower shortages necessitated that the property requirement be ignored and large numbers of proletarii conscripted into the legions.^{[citation needed]} After the end of this war, it appears that proletarii were admitted to the legions as volunteers (as opposed to conscripts) and at the same time the property requirement was reduced to a nominal level by 150 BC, and finally scrapped in the consulship of Gaius Marius (107 BC).

The legionary cavalry also changed, probably around 300 BC onwards from the light, unarmoured horse of the early army to a heavy force with metal armour (bronze cuirasses and, later, mail coats). Contrary to a long-held view, the cavalry of the mid-Republic was a highly effective force that generally prevailed against strong enemy cavalry forces (both Gallic and Greek) ^{[citation needed]} until it was decisively beaten by the Carthaginian general Hannibal's horsemen during the second Punic War. This was due to the greater operational flexibility Hannibal’s Numidian light cavalry allowed.

For the vast majority of the period of its existence, the Polybian levy was at war. This led to great strains on Roman and Italian manpower, but forged a superb fighting machine. During the Second Punic War, fully two-thirds of Roman iuniores were under arms continuously. In the period after the defeat of Carthage in 201 BC, the army was campaigning exclusively outside Italy, resulting in its men being away from their home plots of land for many years at a stretch. They were assuaged by the large amounts of booty that they shared after victories in the rich eastern theatre. But in Italy, the ever-increasing concentration of public lands in the hands of big landowners, and the consequent displacement of the soldiers' families, led to great unrest and demands for land redistribution. This was successfully achieved, but resulted in the disaffection of Rome's Italian allies, who as non-citizens were excluded from the redistribution. This led to the mass revolt of the socii and the Social War (91–88 BC). The result was the grant of Roman citizenship to all Italians and the end of the Polybian army's dual structure: the alae were abolished and the socii recruited into the legions. The Roman army of the late Republic (88–30 BC) resulted, a transitional phase to the Imperial Roman army (30 BC – AD 284).