Marius' reforms of the Army

To Marius are attributed some of the major reforms of the Roman Army. Yet his were the final touches to a process begun much earlier. Rome, and Rome's army in particular, by its very nature tended to resist any radical changes of direction. Far more it moved gradually.
Minor reforms of Gaius Gracchus had been such to make the state responsible for the supply of equipment and clothing to the legionaries and to forbid the enlistment of youths under seventeen.
Also the practice of filling in the ranks of depleted troops by raising extra troops and calling for voluteers from the so-called capite censi (meaning: head count), the Roman poor who owned no property, was common practice.
Marius however took the final step and opened the army to anyone who was poor, but fit and willing to fight. Rather than supplementing his ranks with the poor capite censi, he made an army out of them. These volunteers would sign up as soldiers for much longer periods then than the six years which conscripts had been obliged to serve. To these people drawn largely from the poor from the cities, being a soldier was a profession, a career, rather than a duty performed to Rome. Marius so created the first professional army Rome had ever had.
Marius, too, was careful to enlist experienced soldiers as well, by offering special inducements to veterans.
It was with this new army that Marius saved Italy from massive barbarian invasions by defeating the Germans at Aix-en-Provence and, together with Catulus, against the Cimbri at Vercellae.
Marius is also given credit for changing the construction of the pilum by replacing one of the iron nails with a wooden pin, so that the connection would break under impact and be impossible to return (the pilum had already been fashioned to bend on impact, as mentioned above, but it was notoriously difficult to temper the long metal head so that it was bent on impact, yet was strong enough to actually do damage.)
Also to Marius is attributed the allotment of land to legionaries at their demobilization - giving every legionary a prize to look forward to at the end of his service. A pension, so to speak.
Marius also is given credit for changing the construction of the legion, abolishing the three lines and the velites and instead founding the entire legion of soldiers of equal armour and weaponry.
Already under the great Roman general Scipio Africanus (who defeated Hasdrubal and Hannibal) the cohort had occasionally been the preferred tactical division.
I can not be clearly proven if it truly was Marius who made this change to the legion, or if it once more was not rather a gradual development within the army.
Though the most likely reasoning for the introduction of the cohort system for the legion was the change in recruitment policy under Marius. The previous system was based on the wealth and the experience of the individuals. Now, with the legionaries reduced to complete uniformity in recruitment, the same equal treatment was afforded when forming battle lines.
Under Marius the Roman legion reached a stage in its organization which in strength, resilience and flexibility had no equal.
In the period from Marius to Rome's first emperor Augustus, there is little change in the organization of the army itself.
Though one or two points of Marius' reforms changed the nature of the army in ways, which Marius himself would not have foreseen, nor intended.
Provincial governors could recruit to make up for losses without any reference to the consuls, who so far had enjoyed sole authority in recruitment. Changes such as these allowed for Julius Caesar to raise new troops in Cisalpine Gaul for his campaigns.
Also, and perhaps most importantly, the loyalty of the soldiery was transferred from Rome itself to their commanders. The non-Roman people of Italy had little loyalty to Rome itself and yet they made up greater and greater numbers of the army. Had the previous system of recruitment which drew only from the land-owning classes ensured that the legionaries had responsibilities and loyalties back home, then the urban poor had nothing to lose back home. The soldiers' loyalties lay with the one man who could provide them with the loot, a victorious commander.
Hence arose a specter to Roman authority which would haunt it for the rest of its history.