The Ancient Greeks and Romans both began their histories as city-states. While the irregular coastline and the mountainous terrain of the Greek peninsula isolated the various Greek city-states from one another, the city of Rome was located in the geographical middle of a generally north-south plain bordered on the east with mountains and on the west by the sea. Therefore, Rome was exposed to the migrations and invasions of people from the Po River in the north and Sicily in the south.
The two primary ethnic and cultural influences upon the Romans were determined to a degree by this geography. That is, the first influence was that of the Etruscans in the north, and the second major influence was that of the Greeks in the south. By the time the city-state of Rome had emerged as a distinct entity out of its Etruscan origins and was prepared to expand its own unique influence, Greek civilization had spread throughout the Mediterranean basin. However, the fierce exclusiveness of the Greek city-states from one another, stemming from their geographical isolation, had determined that Greek colonization of the Mediterranean would be an extension of isolated city-states. The Greek polis did not permit the building of a Greek empire, and the strict barriers to the extension of citizenship prevented any one city-state from becoming dominant. As we have seen from Greek history, the Athenians were on the way to creating an empire through their domination of the Delian League, but this trend was reversed in the Pelopponesian Wars.
The Romans, on the other hand, brought other communities on the Italian peninsula under their control, first by conquest, and then by extending Roman citizenship to elements of the conquered peoples. Over time, in the crucible of fierce, unremitting conflict during the Punic Wars, the people of the Italian peninsula came to identify themselves as Romans. There are, therefore, two key components in the success of the Romans in building an empire. One surely was their military prowess, and the other was their organizational/political/legal skill in extending their governance over the conquered peoples into the empire.