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Until the last decade of the 2nd century BC, the eligibility requirements to become a Roman soldier in the service of the Republic were very strict:
In particular, that produced a division among Roman census classes (distinct from the usual plebeian/patrician divide) in which four standardized unit types (based on how much money the soldier could spend on his own arms and armor) composed each legion:
When war threatened, the consuls of the day would be charged with the duty of recruiting an army from the eligible citizenry of the Republic. As a rule, one of the consuls would lead this mainly volunteer army into battle. As it can be imagined, not all elected consuls were adept at leading an army. For example, in 113 BC, the consul Gnaeus Papirius Carbo was defeated at the Battle of Noreia by invading tribes of the Cimbri and the Teutons. That was followed by a protracted war in Africa against King Jugurtha of Numidia. Consul Quintus Caecilius Metellus Numidicus was sent to defeat Jugurtha. Metellus never lost any armies and won some battles but after two years, he had not achieved total victory. Gaius Marius, one of his legates, requested Metellus to release him from his duties so he could return to Rome and run for consul at the end of 108 BC.
When Marius became junior consul in 107 BC and was appointed the task of concluding the war with Jugurtha, he had no army. The army Metellus had commanded in Africa was assigned to the senior consul, Lucius Cassius Longinus, to expel the Cimbri, who were once again encroaching on the Roman province of Transalpine Gaul (Gallia Transalpina). Marius had no troops with which to conduct the war in Africa, as the eligible citizenry from whom he could recruit an army was severely depleted by previous military disasters and the expansion of the latifundia at the expense of small landowners. To overcome that problem, he introduced a number of reforms.
The foremost of the Marian reforms was the inclusion of the Roman landless masses, the capite censi, men who had no property to be assessed in the census. Instead, they were "counted by the head". The men were now among the ranks of those who could be recruited even though they owned no significant property. Because the poor citizens could not afford to purchase their own weapons and armor, Marius arranged for the state to supply them with arms. He thus offered the disenfranchised masses permanent employment for pay as professional soldiers and the opportunity to gain spoils on campaign. With little hope of gaining status in other ways, the masses flocked to join Marius in his new army. The professional soldiers were recruited for an enlistment term of 16 years, later to rise to 20 years' full service and 5 years as evocati under the reforms of Augustus.
The second important reform implemented by Marius was the formation of a standing army. Marius was able to standardize training and equipment throughout the Roman legions. Drilling and training took place all year round, even in times of peace, not just when war threatened.
Marius organized the legions as follows. The total number of men in a full strength legion was about 4,800 soldiers. The internal organization of a legion consisted of 10 cohorts of 6 centuries each. The century consisted of 80 men. Each century was divided again into 10 contubernia led by a decanus. The contubernium contained 8 legionaries. The century fought as a unit, marched as a unit and camped as a unit. The century carried with it all the arms and accoutrements required to feed and maintain it as a fighting unit. Each man was responsible for carrying his own supplies, weapons and several days' worth of rations. The theory that a century contained 20 non-combatants is modern speculation and is not supported by the ancient evidence. That change drastically reduced the size of the baggage train required as support and made the army much more mobile. Between 2 and 6 legions grouped together constituted an army. The legions were kept in peak physical condition and discipline by constant training, one of the best in the ancient world.
The third reform that Marius was able to introduce was legislation that offered retirement benefits in the form of land grants. Members of the head count who had completed their term of service would be given a pension by their general and a plot of land in the conquered region on which to retire. Officers and commanders were given monetary rewards that were 10 to 25 times greater than that of a common foot soldier.
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The first and most obvious result was the improvement in the military capability of the army. A general, when war threatened the Republic, no longer hastily had to recruit a citizen army, train it to fight and obey military commands and discipline and then march it off to do battle, raw and un-blooded. That was instrumental in the growth and success of the Roman military machine and resulted in the continued success of the Romans on the battlefield.
Another benefit of the reforms was the settlement of retired legionaries in conquered land. That helped to integrate the region into a Roman province and "Romanize" its citizens, reducing unrest and revolt against Roman rule.
However, loyalty of the legions shifted away from the Roman state and towards the generals who led the army, as soldiers now had a direct financial incentive to support their generals' ambitions. It became alarmingly common for a general to prolong his imperium by using the army to influence the senate and consolidate his power. Some even went as far as to declare war on their political enemies leading to civil war.
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The cohort legions of the late republic and early empire are often called Marian legions. Following the Battle of Vercellae in 101 BC Marius granted all Italian soldiers Roman citizenship. He justified this action to the Senate by saying in the din of battle he could not distinguish Roman from ally. This effectively eliminated the notion of allied legions; henceforth all Italian legions would be regarded as Roman Legions. Thus the three different types of heavy infantry (the Hastati, the Principes and the Triarii, which composed the pre-Marian Roman armies) were replaced by a single, standard type of legionary based on the Principes.
The role of allied legions would eventually be taken up by contingents of allied/auxiliary troops, called Auxilia. Each legion had a same size or near same size Auxilia (auxiliary), which contained specialist units, engineers and pioneers, artillerymen and siege craftsmen, service and support units plus units made up of non-citizens (who were granted Roman citizenship upon discharge) and undesirables. These were usually formed into complete units such as light cavalry, light infantry or velites, and laborers. There was also a reconnaissance squad of 10 or more light, mounted infantry called speculatores who could also serve as messengers or even as an early form of military intelligence service.
During these reforms, the legions were also organized into permanent cohorts for the first time. Prior to this cohorts had been temporary administrative units or tactical task forces of several maniples, even more transitory than that of the legions of the early republic themselves. Now the cohorts were six to ten permanent units, composed of five to eight centuries each led by a centurion assisted by an Optio, a soldier who could read and write. These came to form the basic tactical unit of the legions. The senior centurion of the legion was called the Primus pilus, a career soldier and adviser to the legate; he was generally 50 years of age or older. There were also additional officers assigned to each legion, an Aquilifer, Imaginifer (Imperial Rome only), a Tesserarius, and a Cornicen. The aquilifer was in charge of the legion standard, so there was only one per legion. The imaginifer carried an image of the emperor (whichever one was in power at the time). The tesserarius was in charge of the guard outposts for each century. The cornicen was crucial in the heat of battle, as he blew the formation, attack, withdrawal, and many other notes. This was the only way legionaries and their officers could hear or issue orders in the din of battle.
Every legion had a baggage train of 500-550 mules, or about 1 mule for every 10 legionaries. To keep these baggage trains from becoming too large, Marius had each man carry as much of his own equipment as he could, including his own armor, weapons and 15 days' rations or about 50-60 pounds (22.5-27 kg) of load total. To make this easier, he issued each legionary a forked stick to carry his load on his shoulders. The soldiers were nicknamed Marius' Mules (muli mariani in Latin) due to the amount of gear they had to carry themselves.
A typical legion of this period had around 5,000-6,000 legionaries as well as a large number of camp followers, servants and slaves. Legions could contain as many as 6,000 fighting men divided among several cohorts. Numbers would also vary depending on casualties suffered during a campaign; Julius Caesar's legions during his campaign in Gaul often only had around 3,500 men and on one occasion during his civil war against Pompey he had to join two of his battle-reduced legions together to achieve the strength of one conventional legion.
Hundreds of years later, under the Emperor Diocletian and his successors, new legions raised for the field armies, as opposed to those stationed along the frontiers, were recruited to only about 1,000 men and were, therefore, the size of military auxiliary cohorts. This was a response to the logistical needs of the late Empire: the smaller units were more easily dispatched as needed to trouble spots than were the older, larger units, and they were no longer made up exclusively of fully armored heavy infantry. Instead, they often consisted of light infantry or archers. Except with regard to Roman citizenship (and even then not always), they were in fact no longer sharply distinguished, if distinguished at all, from auxiliary units raised from barbarians within and without the Empire. These later legions (comitatenses) should not be confused with the legions of heavy infantry of the earlier empire.