The extensive campaigning abroad by Roman generals, and the rewarding of soldiers with plunder on these campaigns, led to a general trend of soldiers becoming increasingly loyal to their generals rather than to the state. Rome was also plagued by several slave uprisings during this period, in part because vast tracts of land had been given over to slave farming in which the slaves greatly outnumbered their Roman masters. In the 1st century BC at least twelve civil wars and rebellions occurred. This pattern continued until 27 BC, when Octavian (later Augustus) successfully challenged the Senate's authority, and was made princeps (first citizen).
Between 135 BC and 71 BC there were three "Servile Wars" involving slave uprisings against the Roman state. The third and final uprising was the most serious, involving ultimately between 120,000 and 150,000 slaves under the command of the gladiator Spartacus. In 91 BC the Social War broke out between Rome and its former allies in Italy when the allies complained that they shared the risk of Rome's military campaigns, but not its rewards. Although they lost militarily, the allies achieved their objectives with legal proclamations which granted citizenship to more than 500,000 Italians.
The internal unrest reached its most serious state, however, in the two civil wars that were caused by the clash between generals Gaius Marius and Lucius Cornelius Sulla starting from 88 BC. In the Battle of the Colline Gate at the very door of the city of Rome, a Roman army under Sulla bested an army of the Marius supporters and entered the city. Sulla's actions marked a watershed in the willingness of Roman troops to wage war against one another that was to pave the way for the wars which ultimately overthrew the Republic, and caused the founding of the Roman Empire.
The prior era saw great military successes and great economic failures. The patriotism of the plebeians had kept them from seeking any new reforms. Now, the military situation had stabilised, and fewer soldiers were needed. This, in conjunction with the new slaves that were being imported from abroad, inflamed the unemployment situation further. The flood of unemployed citizens to Rome had made the assemblies quite populist.
During this period, an army formation of around 5,000 men (of both heavy and light infantry) was known as a legion. The manipular army was based upon social class, age and military experience. Maniples were units of 120 men each drawn from a single infantry class.
The maniples were typically deployed into three discrete lines based on the three heavy infantry types:
1. Each first line maniple were leather-armoured infantry soldiers who wore a bronze breastplate and a bronze helmet adorned with 3 feathers approximately 30 cm (12 in) in height and carried an iron-clad wooden shield. They were armed with a sword and two throwing spears.
2. The second infantry line was armed and armoured in the same manner as was the first infantry line. The second infantry line, however, wore a lighter coat of mail rather than a solid brass breastplate.
3. The third infantry line was the last remnant of the hoplite-style (the Greek-style formation used occasionally during the early Republic) troops in the Roman army. They were armed and armoured in the same manner as were the soldiers in the second line, with the exception that they carried a lighter spear.
The three infantry classes 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 in the first line, older men with some military experience would serve in the second line, and veteran troops of advanced age and experience would serve in the third line.
The heavy infantry of the maniples were supported by a number of light infantry and cavalry troops, typically 300 horsemen per manipular legion. The cavalry was drawn primarily from the richest class of equestrians. There was an additional class of troops who followed the army without specific martial roles and were deployed to the rear of the third line. Their role in accompanying the army was primarily to supply any vacancies that might occur in the maniples. The light infantry consisted of 1,200 unarmoured skirmishing troops drawn from the youngest and lower social classes. They were armed with a sword and a small shield, as well as several light javelins.
Rome's military confederation with the other peoples of the Italian peninsula meant that half of Rome's army was provided by the Socii, such as the Etruscans, Umbrians, Apulians, Campanians, Samnites, Lucani, Bruttii, and the various southern Greek cities. Polybius states that Rome could draw on 770,000 men at the beginning of the Second Punic War, of which 700,000 were infantry and 70,000 met the requirements for cavalry. Rome's Italian allies would be organized in alae, or wings, roughly equal in manpower to the Roman legions, though with 900 cavalry instead of 300.
A small navy had operated at a fairly low level after about 300 BC, but it was massively upgraded about forty years later, during the First Punic War. After a period of frenetic construction, the navy mushroomed to a size of more than 400 ships on the Carthaginian ("Punic") pattern. Once completed, it could accommodate up to 100,000 sailors and embarked troops for battle. The navy thereafter declined in size.
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, near the beginning of the Second Punic War, Rome was forced to effectively ignore its long-standing principle that its soldiers must be both citizens and property owners. 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. As a result, the Roman state was forced to arm its soldiers at the expense of the state, which it did not have to do in the past.
The distinction between the heavy infantry types began to blur, perhaps because the state was now assuming the responsibility of providing standard-issue equipment. In addition, the shortage of available manpower led to a greater burden being placed upon Rome's allies for the provision of allied troops. Eventually, the Romans were forced to begin hiring mercenaries to fight alongside the legions.