Roman army of the mid-Republic

The Roman army of the mid-Republic (also known as 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–ca. 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 ca. 300 BC during the Samnite Wars. Also probably dating from this period was the regular accompaniment of each legion by an 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 ca. 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 slingers from the Balearic islands. From this time, these units always accompanied Roman armies.

...
Wikipedia