Professionalisation during the Republican period.  
Manipular legion (315–107 BC).  
The army of the early Republic continued to evolve, and although there was a tendency among Romans to attribute such changes to great reformers, it is more likely that changes were the product of slow evolution rather than singular and deliberate policy of reform. The manipular formation was probably copied from Rome's Samnite enemies to the south, perhaps as a result of Roman defeats in the Second Samnite War.  
During this period, a military formation of around 5,000 men was known as a legion (Latin: legio). However, in contrast to later legionary formations of exclusively heavy infantry, the legions of the early and middle Republic consisted of both light and heavy infantry. The term manipular legion, a legion based on units called maniples, is therefore used to contrast the later cohortal legion of the Empire that was based around a system of cohort units. The manipular legion was based partially upon social class and partially upon age and military experience. It therefore represents a theoretical compromise between the earlier class-based army and the class-free armies of later years. In practice, even slaves were at one time pressed into the army of the Republic out of necessity. Normally a single legion was raised each year, but in 366 BC two legions were raised in a single year for the first time.  
Maniples were units of 120 men each drawn from a single infantry class. The maniples were small enough to permit tactical movement of individual infantry units on the battlefield within the framework of the greater army. The maniples were typically deployed into three discrete lines (Latin: triplex acies) based on the three heavy infantry types of hastati, principes and triarii. The first type, the hastati, typically formed the first rank in battle formation. They typically wore a brass chest plate (though some could afford mail), a helmet called a galea, and occasionally, greaves (shin guards). They carried an iron bossed wooden shield, 120 cm (4 ft) tall and rectangular in shape with a curved front to partially protect the sides. Traditionally they were armed with a sword known as a gladius and two throwing spears known as pila: one the heavy pilum of popular imagination and one a slender javelin. However the exact introduction of the gladius and the replacement of the spear with the sword as the primary weapon of the Roman legions is uncertain, and it's possible that the early manipular legions still fought with the hastati and principes wielding the hasta or spear.  
The second type, the principes, typically formed the second rank of soldiers back from the front of a battle line. They were heavy infantry soldiers armed and armoured as per the hastati. The triarii, who typically formed the third rank when the army was arrayed for battle, were the last remnant of hoplite-style troops in the Roman army. They were armed and armoured as per the principes, with the exception that they carried a pike rather than two pila. A triarii maniple was divided into two formations each six men across by 10 men deep. A manipular legion typically contained between 1,200 hastati, 1,200 principes and 600 triarii. The three classes of unit may have retained some slight parallel to social divisions within Roman society, but at least officially the three lines were based upon age and experience rather than social class. Young, unproven men would serve as hastati, older men with some military experience as principes, and veteran troops of advanced age and experience as triarii.  
The heavy infantry of the maniples was supported by a number of light infantry (Latin: velites) and cavalry (Latin: equites) troops, typically 300 horsemen per manipular legion. The cavalry was drawn primarily from the richest class of equestrians, but additional cavalry and light infantry were drawn at times from the socii and Latini of the Italian mainland. The equites were still drawn from the wealthier classes in Roman society. There was an additional class of troops (Latin: accensi, also adscripticii and later supernumerarii) who followed the army without specific martial roles and were deployed to the rear of the triarii. Their role in accompanying the army was primarily to supply any vacancies that might occur in the maniples, but they also seem to have acted occasionally as orderlies to the officers.  
The light infantry of 1,200 velites consisted of unarmoured skirmishing troops drawn from the youngest and lower social classes. They were armed with a sword and shield (90 cm (3 ft) diameter), as well as several light javelins, each with a 90 cm (3 ft) wooden shaft the diameter of a finger, with a c. 25 cm (10 in) narrow metal point. Their numbers were swollen by the addition of allied light infantry and irregular rorarii.  
The Roman levy of 403 BC was the first to be requested to campaign for longer than a single season, and from this point on such a practice became gradually more common, if still not typical.  
A small navy had operated at a fairly low level after the Second Samnite War, but it was massively upgraded during this period, expanding from a few primarily river- and coastal-based patrol craft to a full maritime unit. After a period of frenetic construction, the navy mushroomed to a size of more than 400 ships on the Carthaginian pattern. Once completed, it could accommodate up to 100,000 sailors and embarked troops for battle. The navy thereafter declined in size. This was partially because a pacified Roman Mediterranean called for little naval policing, and partially because the Romans chose to rely during this period on ships provided by Greek cities, whose peoples had greater maritime experience.  
Proletarianisation of the infantry (217–107 BC).  
The extraordinary demands of the Punic Wars, in addition to a shortage of manpower, exposed the tactical weaknesses of the manipular legion, at least in the short term. In 217 BC, Rome was forced to effectively ignore its long-standing principle that its soldiers must be both citizens and property owners when slaves were pressed into naval service; around 213 BC, the property requirement was reduced from 11,000 to 4,000 asses. Since the Romans are unlikely to have preferred to employ slaves over poor citizens in their armies, it must be assumed that, at this point, the proletarii of the poorest citizens must also have been pressed into service despite their lack of legal qualification. By 123 BC, the financial requirement for military service was slashed again from 4,000 asses to just 1,500 asses. By this time, therefore, it is clear that many of the property-less former proletarii had been nominally admitted into the adsidui.  
During the 2nd century BC, Roman territory saw an overall decline in population, partially due to the huge losses incurred during various wars. This was accompanied by severe social stresses and the greater collapse of the middle classes into lower classes of the census and the proletarii. As a result, both the Roman society and its military became increasingly proletarianised. The Roman state was forced to arm its soldiers at the expense of the state, since many of the soldiers who made up its lower classes were now impoverished proletarii in all but name, and were too poor to afford their own equipment.  
The distinction between the heavy infantry types of hastati, principes and triarii began to blur, perhaps because the state was now assuming the responsibility of providing standard-issue equipment to all but the first class of troops, who alone were able to afford their own equipment. By the time of Polybius, the triarii or their successors still represented a distinct heavy infantry type armed with a unique style of cuirass, but the hastati and principes had become indistinguishable.  In addition, the shortage of available manpower led to a greater burden being placed upon its allies (socii) for the provision of allied troops. Where accepted allies could not provide the required force types, the Romans were not averse during this period to hiring mercenaries to fight alongside the legions. 

Wikipedia: Text is available under the Creative Commons Attribution-ShareAlike 4.0 License.

This episode includes AI-generated content.