Marian Reforms

The Marian reforms of 107 BC were a group of military reforms initiated by Gaius Marius, a statesman and general of the Roman Republic.

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) comprised 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.

...
Wikipedia