>>1
Originally the main barbarian threat to the Greeks was... the Greeks. It was the Dorian invasion which caused the collapse of the mycenaeans. The Persians were a threat but the poleis were capable of removing them. It was the Macedonians which subjugated the poleis. Even in the Hellenic period it was nominally Greek empires fighting one another for the most part. Before the arrival of the Slavs, Turks, and Arabs generally speaking the Greeks withstood barbarians.