The Geography of the Greek Peninsula offers protection from invaders, because the Balkan Mountain Range sits between Europe and Hellas. Nonetheless, there were at least two occasions in antiquity when the mountains were not high enough to protect the Greeks.
Invaders from the North spilled into Greece at the beginning (1900 B.C) and end (1100B.C) of the Second Millenium. Take a look at the following map. Blue is invasion one; Magenta invasion two.
The first of these invasions is marked by evidence of fire in many Greek settlements including Asea, Korakou, and Eutresis. Corinth was deserted afterward and Asine (Argolis) badly damaged.
The second invasion, more relevant to this post, was much more widespread. All of the Eastern Mediterranean was in decline and vulnerable, so the stage was set for traumatic changes to the early civilized world. Egypt, furthest from the source of the invaders, beat off attacks in 1230 and 1190 B.C. The coasts of Cyprus, Palestine, and Syria were attacked. The Assyrians were humbled and rendered impotent as a power and the Hittite Kingdom disappeared all together.
In Greece, Mycenae fell. Recovered tablets from Pylos record an effort to bolster coastal defenses against the invaders, to no avail. From Thessaly to Messenia, Delphi, and Attica, all were destroyed.
The Greeks, more shallow rooted than the cultures of the Fertile Crescent, fell hard and writing disappeared. The winners were the Dorians – barbarians who invaded a civilized land. The invasion was a catastrophe because it broke down a developed civilization, but the end of the Mycenaean Age at the hands of the Dorians was significant because the old ways were also destroyed. The Mycenaean view had been too tied to the outside – its predecessor Minoan culture. Now those external links were broken, freeing up the minds of the Greeks toward a new path. For three centuries the Greeks were separated from the east and moved forward in isolation. This new spirit was not Dorian. It was Greek forged by the invasion of the Dorians.