In the early and middle Republic legionaries were drafted from all Roman citizens meeting a certain property qualification. The height of the property qualification has been reduced at least once (during the Second Punic War, to increase available manpower) and possibly a second time (around 130 B.C.; De Ligt, p. 125-6). In the late Republic, after the Social War, the number of citizens increased because the Italian allies received citizen status. From then on the Italian allies would no longer provide auxiliary troops, as they were now eligible to serve in the legions. Census figures indicate this doubled the available manpower for the legions from about 463,000 to 910,000. Any auxiliaries would now be mainly recruited in the overseas territories (for example Numidian and Iberian horsemen, Ligurian and Thracian light forces, Balearic slingers and Cretan archers, though confusingly not everyone serving in such units would be from that region - it gives the right idea though).

From the above it should be clear that during the Republic recruitment for the legions took place exclusively on the Italian peninsula. This changed during the Empire. Legions would initially be raised in Italy, but after moving to a border region, they'd start accepting local recruits. Non-citizens could only serve as auxiliaries, but auxiliaries could earn citizenship by service. Other citizens who might enlist would be sons of Roman colonists, traders, or indeed soldiers. Later on military careers would become more or less heritable. In the first few centuries of the Empire generally there would be enough volunteers so that a draft was not needed. The citizen's obligation to serve if drafted was never lifted though, so in times of crisis a draft could take place.

---

Sources:

(Both from P. Erdkamp (ed.), A Companion to the Roman Army, Oxford 2007)

• Luuk de Ligt, Roman Manpower and Recruitment During the Middle Republic p.96-113
• Gabriele Wesch-Klein, Recruits and Veterans p.453-450