Changes that had been happening slowly in the Roman military were formalized by the Consul Marius around 100 BCE. These included the state providing the soldiers with equipment, paying the soldiers better and more regularly and giving each soldier a plot of land after his service as a pension. This also made the Roman military a more professional force that was no longer recruited for a season but multiple years depending on the situation when one joined. It also broke down the old formations of Hastati, Pricipes and Triarii as all heavy infantry were equipped the same and organized into legions. This made the old social order less relevant to the battlefield and made the legion far more effective as it was better equipped, trained and motivated.
Around this time armies also became less and less loyal to the Senate and more loyal to their own general as he was the one who directly payed them and commanded them for a longer period of time than in the past. This was never a formal change from the old system but gradually happened and enabled civil war as many generals could rely on their soldiers to side with them against the Senate.
The template provided by Marius remained the basic structure of the Roman military and is thought of as the classic roman legion by many.
Around this time armies also became less and less loyal to the Senate and more loyal to their own general as he was the one who directly payed them and commanded them for a longer period of time than in the past. This was never a formal change from the old system but gradually happened and enabled civil war as many generals could rely on their soldiers to side with them against the Senate.
The template provided by Marius remained the basic structure of the Roman military and is thought of as the classic roman legion by many.