Marius changed the roman army by promising land to the poor men if they became soldiers.
Near the beginning of the 1st century BC BCE, Marius was given almost unlimited powers to reorganize the legions. He introduced a cohort of approximately 550 men to 600 men and had each legion have its battle standards. This gave the men a rallying point. He had the Roman javelins break after thrown, thus disabling them to be used against his own soldiers. He abolished the use of personal weapons to allow for poorer men to join the ranks. The legions would be the main source of weapons supply. He also had the army carry equipment in their own backpacks. This left the supply wagons with less materials to carry and placed important equipment at hand for each soldier.
Another aspect of the Marius changes was to increase the loyalty of the legions to their generals rather than to the Senate. Later of course Augustus would have the military swear an oath to him rather than the Roman State.