The whole war started because Athens supported those aforementioned Ionian Greeks when they were rebelling in Anatolia against the Persians. That made the Persian king Xerxes mad, so he led two major campaigns against the Athenians, and the Athenians enlisted the help of all the other Greek city-states. In the wake of that shared Greek victory, the Greeks began to see themselves as Greeks rather than as Spartans or Athenians or whatever. And then Athens emerged as the de facto capital of Greece and then got to experience a Golden Age, which is something that historians make up. But a lot of things did happen during the Golden Age, including the Parthenon, a temple that became a church and then a mosque and then an armory until finally settling into its current gig as a ruin. You also had statesmen like Pericles, whose famous funeral oration brags about the golden democracy of Athens with rhetoric that wouldn't sound out of place today. If we look to the laws, they afford equal justice to all in their private differences.