What was the greatness of the Roman Empire based on?
If greatness can be defined as power, the Roman Empire can certainly be termed great. With its beginning as an average city-state on the Italian peninsula, Rome grew to be the most powerful empire in the ancient world, far eclipsing the Persian and Greek empires that preceded it, among others.
As the city of Rome began to grow in strength and population, it organized itself politically as a Republic, the form of government it perpetuated for approximately 500 years. Even as a Republic, the city of Rome expanded from its location along the Tiber River in the central-western part of the Italian peninsula to conquer and colonize the tribes and towns throughout Italy. Ultimately, Rome's imperial expansion led it to conquer around 20% of the world's known population, with the full extent of its empire reaching a climax around 117 A.D., during the reign of the emperor Trajan.
While conquered people naturally chafed under foreign rule, the first two centuries of imperial rule, after the demise of the Republic, is often called the Pax Romana by historians, a period marked by fairly widespread peace and prosperity. Rome's magnanimous approach to most of the people they conquered was to customarily integrate subject people into the Empire, assuring them the protection of Roman military might and participation in Rome's far-flung economy, laws and customs.
Roman laws were quite sophisticated and forward-looking for their day, and freed many conquered people from centuries of superstition and their attendant crude legal systems. In fact, many laws employed in the modern Western canon can be traced to Roman legal history.