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.
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-existent. There simply weren't enough landowners available who weren't already fighting the Germanics or Jugurtha to field a new army.
Marius 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 to confront the Germanic tribes, he also gained the loyalty of these men. The recruiting of the masses changed the entire relationship between citizens, generals, the Senate and Roman institutional ideology. Before Marius, armies fought for the survival or the expansion of the state, including their own lands, but after Marius, they fought for their Legate who gave them equipment and salaries, and who they came to like. 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. Now the Legates with their armies fought for their own glory. This extreme loyalty to the Legate led to rebellion and civil war as epitomised by Julius Caesar, who with his legions gained supreme power. Eventually this led to the crowning of the 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 armour, 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 their 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 favour 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 favoured 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 kit and baggage were carried was completely adjusted. From this point on, the legionary would carry their entire standard package including weapons, armour, 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.
