Imagine you’re a poor boy growing up in Rome. Your parents can’t help you get a start in life. Most of the work in the city is done by enslaved people.
This means there are few opportunities for you. But General Gaius Marius has recently changed the rules for joining the army. Before, only citizens who owned property could serve. Now, anyone can join.
Say you join the army and serve loyally. Your general may reward you with land in one of the conquered lands. Then you’ll be able to retire, and you will have something to leave to your children. Many of the soldiers in the armies of the late Roman Republic are like you. They will follow their general wherever he asks. Even if he asks them to go into battle against other Romans.