Answer: Rome was able to gain its empire in large part by extending some form of citizenship to many of the people it conquered. Military expansion drove economic development, bringing enslaved people and loot back to Rome, which in turn transformed the city of Rome and Roman culture.
Explanation: It is perhaps surprising to learn that the Roman Empire is only (by an estimate) at its greatest extent in the early second century. But Rome's mighty empire did grow from what was little more than a village and Rome expanded to the east, greedily acquiring land in Greece, Egypt, Syria and Macedonia.