Answer:
by conquering neighboring territories
Step-by-step explanation:
The Romans were initially a relatively small tribe based around Rome, and they were not even the biggest tribe in Italy. They started to reform their military though, and one by one started to attack and conquer the neighboring countries, giving the initial rise of the Roman Empire. In order to be able to control the conquered people, the Romans were quick to assimilate everyone that they have conquered, thus making them Romans, which instead of being a burden were contributing to the empire. As the empire grew stronger in wealth, military power, numbers, and ambitions, they started to attack the other tribes, like the numerous Celtic tribes, and Germanic tribes. Later the expansion continued with taking over the Iberian Peninsula, the Phoenician colonies, part of the Barber territory, the northern half of the Balkans, and after three wars with Macedonia they managed to conquer the southern part of the Balkans too. Everywhere they went, the Romans used the same method as they started, assimilation through imposing their culture and language, and of course rule.