The ancient Greeks were brilliant and fierce warriors. In one of the few times when they more or less pulled together they defeated the greatest empire ever known until then.
They seemed to just not be able to agree on anything. If some outside force had forced them to cooperate they could have conquered the world. I guess in a way that is what Phillip and Alexander did.
Alexander's conquest of the Persian Empire was a Macedonian achievement, with some Greek troops among his forces--while other Greeks were fighting on the Persian side as mercenaries.
During the Cold War Turkey was a reliable ally because of their enmity with Russia, while the Greeks were seemingly less worried about the Soviet danger (after all, the Russians were also Orthodox, and many Greeks had supported the Communist side in the Greek Civil War) than about Turkish threats to Greek interests (in Cyprus and elsewhere). But after the Cold War and the rise of more aggressive forms of Islamism in the Near East, I don't think we can rely on Turkey.