In 550 BC, King Servius Tullius instituted a universal levy of eligible adult male citizens. This development coincided with the introduction of heavy armour for most infantrymen. Before this moment, Rome relied on clan-based war-bands that only coalesced into a united force during serious external threats. The standard levy probably numbered 9,000 men consisting of 6,000 heavily armed infantry and 2,400 light-armed infantry. A small group of 600 light cavalry also participated in these early campaigns. When kings were replaced by two annually elected praetores around 500 BC, the levy remained the same size but divided equally between them. Each praetor commanded one legion of 4,500 men. These early forces fought in Greek-style phalanx formations during large set-piece battles. Most fighting consisted of small-scale border raids and skirmishing instead. Romans fought in their basic tactical unit called the centuria containing 100 men. Separate clan-based forces existed until at least 358 BC under Praetors authority. In 493 BC Rome concluded a perpetual treaty of military alliance known as the Foedus Cassianum with other Latin city-states. This treaty provided for each party to provide an equal force for campaigns under unified command.
The Marian Transformation
Gaius Marius assumed command in 107 BC during a period when maniples were gradually phased out as the main tactical unit. Large cohorts used in allied armies replaced them in this process. The Republican army did not maintain standing or professional military forces before this era. Conscripts served only for each campaigning season and disbanded thereafter. Service in legions was limited to property-owning Roman citizens aged 16 to 46 years old. Elders, vagrants, freedmen, slaves and convicts were excluded from the military levy except in emergencies. During the Second Punic War extreme manpower needs forced the army to ignore minimum property requirements. Many volunteers drawn from the poorest social class served much longer periods than before. These volunteers were willing to serve beyond the six consecutive years limit imposed by law. The standard levy doubled during Samnite Wars to four legions totaling approximately 18,000 Roman troops. Four allied alae of similar size accompanied these forces. The army campaigned exclusively outside Italy after Carthage fell in 201 BC. Men remained away from their home plots of land for many years at a stretch. They received large amounts of booty shared after victories in rich eastern theatre. Disaffection among Rome's Italian allies led to mass revolt known as Social War between 91 and 88 BC. Grant of Roman citizenship to all Italians ended Polybian army dual structure.