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.
The Romans did not intend to create an empire, but they responded to threats from their neighbors, first on the Italian peninsula, then from Carthage in the western Mediterranean, then from Macedonia in the east, and so on. As each adversary was defeated, the Romans found themselves drawn-in to keep the peace ( that is, to maintain their control) among the conquered peoples. This process led to the creation of armies made up of large numbers of Romans who were separated permanently from the land, became professional soldiers, and had to be supported by the state. The army and its generals became so powerful that they eventually posed a threat to the political institutions of the Roman Republic. In other words, the conquest of Rome's enemies, destroyed the Republic and led to the creation of an imperial government. From the very beginning of the history of Rome, the very force which created the empire, that is, the army and its generals, would also be the cause for its downfall. The Roman Empire would last for many centuries, however, and the foundations of its endurance rested upon the extension of the Roman sense of identity to conquered peoples; that is, to "barbarians". Also important were Roman law, and political skills exercised in the Senate and by some of the more outstanding emperors.
The Greek city-states had to be united by force, first through the invasion of the Macedonians, and then by the Romans. But the Greeks would, in a sense, have the last laugh. Their culture was more sophisticated, their learning and philosophy more advanced, and Roman culture would be overwhelmed as the Romans absorbed the cultural influences of the Hellenistic east. In the final outcome, as the Roman Empire declined in the west, Roman emperors transferred their capitol from Italy to Asia Minor. Long after the Roman empire was gone, the Byzantine Empire, an amalgam of Roman and Greek culture, centered in the city of Constantinople, would endure. The Greeks had conquered their conquerors.