How did the Romans manage to effectively combine their whole empire into one unit?

The Romans had two key ways of becoming stronger as one unit and extending their empire. One way was by dedicating life to warfare and the second was by providing main roads to central Rome.

Constant warfare reflected Rome's character. Originally struggling for survival, it was necessary to provide a strong defense from attacking empires. Following their four main virtues- simplicity, seriousness, dignity, and piety, they grew strong and safe as a military, and focused on harsh discipline and endurance throughout life. Because of this, they were able to use their military superiority to conquer many empires and not only expand their power, but gain other empires' riches, goods and lands.

All roads led to Rome and they used the roads to transport these riches and goods in and out of the city-state and it became the center of trade. The roads also helped to spread new ideas, technologies, religions and cultures. This kept good communication and contact with other societies. So having these roads of Rome played an important part in helping to develop Rome's empire into a single unit.