As others have already comprehensively accounted for the role of geography in shaping Greek civilization, let me discuss the political effects of that influence. As has already been established, Greece is very mountainous, has poor farmland, and has proximity to the sea. As a result of these factors, Greek civilization is actually more far reaching than the peninsula of Greece, as there were Greek cities and colonies across the Mediterranean world in places such Asia Minor, throughout the Aegean, as well as in Italy and north Africa, to give some examples.
This had profound political implications on the development of Greek history. For one thing, we can expect a significant element of cultural exchange and interaction between Greeks and other ancient cultures, as is attested to by the historical tradition. We can see, even beyond the case of Greece, the vast cultural debt which the Romans owe their predecessors. Furthermore, the Greeks themselves might have owed a similar debt to the Ancient Egyptians, considering that Pythagoras spent time learning from the Egyptians, and so too did other luminaries such as Plato.
In addition, there is the example of the famous Persian Wars (which had far-reaching effects on Greek History). The Persian Wars actually had their origins in the Greek colonies of Ionia, which had fallen into Persian rule. The Ionian city-states rebelled, and Athens sent assistance to aid in the rebellion. The initial battle at Marathon was actually a reprisal for Athenian complicity in that earlier rebellion. This, in a way, illustrates the connection between geographic context and political reality. Far from being merely a collection of colonies in Asia Minor, the Ionian city-states exerted a profound influence on Greek history.