Take a look at the previous years to see how the roman army recruited. The interest of the soldiery was to protect the land which they already had. As rome expanded the traditional campagning season expired and extend in to campagning years, therefore the landowners could not return afterwards to tend to what they had fought to protect. Marius opened the ranks to the lower classes to fill the gaps which was previously occupied by the landowners but he needed to give these lowerclasses incentive, therefore rewards with land, pensions ect. This practice is continued today in the form of pensions within plenty of modern armies. Marius' mistake was not to identify who was responcible for these pensions. To gain sole loyalty of the legions to the state the state MUST provide the reward however this provision was left to the generals who then inturn gained the full loyalty of his legions giving him the tools to use the legions has he pleased. Marius was not to blame for the doom of the republic, his reforms were visionary but he missed the point of identifying who was to be the master who fed the dog.