Until c. Around BC, during the period conventionally known as the rule of king Servius Tullius , it appears that a universal levy of eligible adult male citizens was instituted. This development apparently coincided with the introduction of heavy armour for most of the infantry. Although originally low in numbers, the Roman infantry was extremely effective and developed some influential battle strategies.
The early Roman army was based on a compulsory levy from adult male citizens which was held at the start of each campaigning season, in those years that war was declared. There were no standing or professional forces. During the regal period to c. When the kings were replaced by two annually elected praetores in c.
It is likely that the hoplite element was deployed in a Greek-style phalanx formation in large set-piece battles. However, these were relatively rare, with most fighting consisting of small-scale border-raids and skirmishing. In these, the Romans would fight in their basic tactical unit, the centuria of men. In addition, separate clan-based forces remained in existence until c. In BC, shortly after the establishment of the Roman Republic , Rome concluded a perpetual treaty of military alliance the Foedus Cassianum , with the combined other Latin city-states.
The treaty, probably motivated by the need for the Latins to deploy a united defence against incursions by neighbouring hill-tribes, provided for each party to provide an equal force for campaigns under unified command. It remained in force until BC. The central feature of the Roman army of the mid-Republic , or the Polybian army, was the manipular organization 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 consisting of small units maniples of men, arrayed in chessboard fashion, giving much greater tactical strength and flexibility.
This structure was probably introduced in c. 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 approximately autonomous states which were bound by a treaty of perpetual military alliance with Rome.
