The Romans became powerful after Rome changed from a monarchy to a republic. As a monarchy, the city-state was unable to prevent foreign rulers from assuming power.
After Rome became a republic, its people made strategic treaties with some nations and broke existing treaties with others. This helped Rome form alliances that helped its people conquer other city-states. During this period, Romans started using new weapons and battle tactics, giving them the opportunity to become a powerhouse in Italy.
Several significant battles took place between the fourth and fifth centuries, cementing Rome's status as a powerful city-state. Although the Romans were not successful in every battle, they learned valuable lessons from their losses.