Marian legion (107–27 BC).  
Modern historiography has regularly cast Marius as abolishing the propertied militia and replacing it with landless soldiers motivated largely by pay. This belief emerges from the ancient literary sources, but rests on a relatively weak basis. 
Despite enrolling some three to five thousand volunteers during the Jugurthine War, Gaius Marius assumed command of consular legions recruited via hitherto normal procedure in the following Cimbric War. Conscription continued after Marius's time, especially during the Social War, and the wealth and social background of the men who joined before and after the opening of recruitment changed little. There is little evidence that later Roman armies during the 1st century BC were made up of volunteers; almost all ancient references to army recruitment, outside private armies, involve conscription.  
For much of the 20th century, historians held that the property qualification separating the five classes and the capite censi was reduced over the course of the second century to a nugatory level due to a shortage of manpower. The basis for that belief, however, was merely three undated Roman figures for the amount of property required to serve which would serve as evidence for reductions only if forced into a descending order. Many scholars have also now abandoned the notion that Italy suffered in the second century BC any deficit of manpower which would have driven such putative reductions.  
Modern historians have also sometimes credited to Marius the abolition of Roman cavalry and light infantry and their replacement with auxilia. There is no direct evidence for this contention, which is driven largely by literary sources' silence on those branches after the 2nd century; continued inscriptional evidence attests both citizen cavalry and light infantry into the end of the republic. The decline of Roman light infantry has been connected not to reform but cost. Because the logistical cost of supporting light infantry and heavy infantry was relatively similar, the Romans chose to deploy heavy infantry in extended and distant campaigns due to their greater combat effectiveness, especially when local levies could substitute for light infantry brought from Rome and Italy.  
The changes to the Roman army during the 1st century BC are now more attributed to the Social War and the civil wars from 49 to 31 BC. The large-scale downsizing of Roman cavalry detachments likely emerged from the extension of citizenship to all of Italy. Because Italy's enfranchisement meant that Rome was now directly liable for the cavalry's upkeep rather than their local communities, Rome instead levied auxilia from allies who, by treaty, were responsible for their contingents' upkeep.  
Pay remained extremely low – only five asses per day – and irregular. Moreover, although the surviving sources frequently characterise soldiers as "poor", these sources largely reflect the perspectives of the elite, by whom the vast majority of the population were considered "poor" and for whom poverty needed not entail actual landlessness. Many of the soldiers of the 1st century BC possessed modest lands. Nor did the legions meaningfully professionalise: as, in general, both soldiers and commanders served only for short periods intending, respectively, to secure plunder or political advancement from military victory.  
After the Social War, the state also started to keep men under arms for longer periods to maintain available experienced manpower, and coupled this with longer terms for commanders, particularly Caesar and Pompey. Client armies emerged but not in the 100s BC but rather in the decades before Caesar's civil war, which broke out in 49 BC.  
The legions of the late Republic were, structurally, almost entirely heavy infantry. The legion's main sub-unit was called a cohort and consisted of approximately 480 infantrymen. The cohort was therefore a much larger unit than the earlier maniple sub-unit and was divided into six centuriae of 80 men each. Each centuria was separated further into 10 "tent groups" (Latin: contubernia) of 8 men each. Legions additionally consisted of a small body, typically 120 men, of Roman legionary cavalry (Latin: equites legionis). The equites were used as scouts and dispatch riders rather than battlefield cavalry. Legions also contained a dedicated artillery crew of perhaps 60 men, who would operate devices such as ballistae.  
Each legion was normally partnered with an approximately equal number of allied (non-Roman) auxiliae troops. The addition of allied troops to the Roman army was a formalisation of the earlier arrangement of using light troops from the Socii and Latini, who had received Roman citizenship after the Social War. Auxiliary troops could be formed from either auxiliary light cavalry known as alae, auxiliary light infantry known as cohors auxiliae, or a flexible mixture of the two known as cohors equitata. Cavalry types included mounted archers (Latin: sagittarii) and heavy shock cavalry (Latin: cataphracti or clibanarii). Infantry could be armed with bows, slings, throwing spears, long swords, or thrusting spears. Auxiliary units were originally led by their own chiefs, and, in this period, their internal organisation was left to their commanders.  
However, "the most obvious deficiency" of the Roman army remained its shortage of cavalry, especially heavy cavalry; even auxiliary troops were predominantly infantry. Luttwak argues that auxiliary forces largely consisted of Cretan archers, Balearic slingers and Numidian infantry, all of whom fought on foot. As Rome's borders expanded and its adversaries changed from largely infantry-based to largely cavalry-based troops, the infantry-based Roman army began to find itself at a tactical disadvantage, particularly in the East.  
After having declined in size following the subjugation of the Mediterranean, the Roman navy underwent short-term upgrading and revitalisation in the late Republic to meet several new demands. Under Caesar, an invasion fleet was assembled in the English Channel to allow the invasion of Britain; under Pompey, a large fleet was raised in the Mediterranean Sea to clear the sea of Cilician pirates. During the civil war that followed, as many as a thousand ships were either constructed or pressed into service from Greek cities.  
Non-citizen recruitment (49–27 BC).  
By the time of Julius Caesar in 54 BC, regular legionary units were supplemented by exploratores, a body of scouts, and speculatores, spies who infiltrated enemy camps. Due to the demands of the civil war, the extraordinary measure of recruiting legions from non-citizens was taken by Caesar in Transalpine Gaul (Latin: Gallia Transalpina), by Brutus in Macedonia, and by Pompey in Pharsalus. This irregular and extraordinary recruitment was not, however, typical of recruitment during this period, and Roman law still officially required that legions were recruited from Roman citizens only.  


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