With his election as Consul in 107 BC, and his subsequent appointment as commander of the Roman legions in Numidia, Marius faced a difficult challenge. Invasions of Germanic Cimbri and Teuton tribes into southern Gaul had forced large Roman armies to counter them. Thoroughly defeated in every engagement, Rome faced a manpower crisis similar to those faced during Hannibal's offensive in the Second Punic War. Prior to Marius, Rome recruited its main legionary force from the landowning citizen classes, men who could equip themselves and who supposedly had the most to lose in the case of Roman defeat. In previous wars, temporary relief from this traditional rule would be applied, but never on a long term basis. Recruits from the Roman capite cens (head count) and freed slaves (voluntarii), were used primarily in support and militia style functions.
Especially since the end of the Punic Wars and conquests in the east, the small landowning classes had dwindled to dangerous numbers. Wealthy senatorial aristocrats and equestrian elite land owners bought up small farms from struggling families and worked them with vast numbers of imported slaves. The jobless and landless mobs in Rome swelled out of control and led directly to the rise of the Gracchi, who championed political reform for the common citizens. By the time Marius came to power, the typical Roman recruiting base was literally non-existant. There simply weren't enough landowners available who weren't already fighting the Germanics or Jugurtha to field a new army.
Marius' idea would turn out to be the single greatest reform the Roman legions would undergo. Probably without realizing the massive implications his reform would have on a social or political basis, he had little choice but to 'break' the law in order to fulfill his political and military ambitions. He offered the disenfranchised masses permanent employment for pay as a professional army, and the opportunity to gain spoils on campaign along with retirement benefits, such as land. With little hope of gaining status in other ways, the masses flocked to join Marius in his new army.
Besides gaining an army, Marius gained something else: the extreme personal loyalty of the Roman head count. The recruiting of the masses would change the entire relationship between citizens, generals, the Senate and Roman institutional ideology. Prior to Marius, the armies may have been loyal to a general, but were fighting in theory for the survival or expansion of the state, including their own lands. After Marius, they fought for their Legate, provided they liked him of course, and for the plunder and glory he could provide. With nowhere to return to in Rome or beyond, these new soldiers became career full-time professional soldiers, serving terms from 20 to 25 years. A whole new class of citizen was developed from this simple change in military philosophy. While providing an immeasurable impact on the common people, this change would also have a profound effect on the entity of Rome itself. The extreme loyalty to generals rather than state would lead to open rebellion, civil war, military political power and eventually the crowning of emperors.
Besides the social impact of Marius' decision, he made several major changes to legion structure and tactical formations. Most importantly, he mostly replaced the maniple structure which consisted of four distinct legionary units (though it did continue as a style of formation at least until the mid 1st century AD). Each used different weapons, served different purposes tactically and were arranged in varying sizes and formations, essentially based on the class of citizen they were recruited from. Each soldier in the pre-Marian system provided his own gear and armor, resulting in wide ranges in quality and completeness. Marius supplied his new army's gear partially through the resources of the state, and through his own vast wealth. In the future, most new recruits would be uniformly equipped through the state treasury, or thier recruiting general.
To replace the maniple as a formation, the cohort was adopted (though the formation had been used in moderation at least since the Punic Wars). Each soldier was equipped the same and assigned to one of six identical centuries of 80 men, making up the cohort unit. There were then 10 cohors of 480 men making up a legion, which standardized the entire system. The legion was made into a single large cohesive unit with interchangeable parts, capable of tactical flexibility not available with the complex structure of the Republican manipular system. The long single lines used prior to Marius were also eliminated in favor of a tiered 3 cohort deep battle line. This allowed rapid and easy support or rotating of fresh troops into combat.
Additionally, officers began to be recruited from within the ranks on a regular basis. While political appointments and promotions based on social or client status would still occur, this now allowed the common soldier a way of advancing based on merit. This improved the strength of the legion as a whole and instilled confidence in the soldiers, knowing their officers were capable leaders, not favored clients of Senators in Rome. Marius, while adopting uniform gear for all, such as the gladius and scutum, also made significant changes to the common legionary spear (the pilum). It was made for the point to break off upon impact, making it ineffective to be thrown back by the enemy.
To eliminate another problem, the way the soldier's kits and baggage were carried was completely adjusted. From this point on, the legionary would carry their entire standard package including weapons, armor, food, tents, supplies and tools. The "Marius' Mules" allowed bulky, slow and cumbersome baggage trains to be shortened, making the infantry faster and more efficient. Finally, the legionary standards of the Eagle, wolf, minotaur, horse and boar were reduced to a single standard. The Eagle, representing Jupiter Optimus Maximus, replaced them all as the single symbol or loyalty, duty and pride among the soldiers.