Final answer:
Gaius Marius abolished the property requirement for joining the Roman army in 107 BCE, which allowed the poor citizens, or proletariats, of Rome to enlist and serve under a general's command in exchange for loyalty.
Step-by-step explanation:
The rule that Gaius Marius abolished in 107 BCE to recruit more soldiers into the Roman army was the property requirement for membership in the army. Before Marius's reforms, soldiers had to demonstrate a certain level of wealth to join the army, effectively excluding the lower classes. By eliminating this rule, Marius opened the army to the proletariats, or the poor citizens of Rome, who were previously unable to serve due to their lack of property. As a result, these citizens were now able to join the army in exchange for an oath of loyalty to their general rather than to the Republic, which led to the development of professional client armies that were loyal more to their commander than to the state itself. This was a significant shift from the earlier Republican military system, where soldiers were primarily free, economically independent citizens who volunteered for campaigns and then returned to their civilian lives.