When Invasions Purify
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.