State-building: the Greek polis

"Though the Greek city-states were unified to some extent in the face of an external threat, as that threat waned, conflicts between the city-states made a resurgence. Following the [Persian] wars, Athens emerged as the supreme naval power in Greece. It formed the Delian League, ostensibly to create a cohesive Greek network among city-states to ward off further Persian attacks. Under the leadership of Pericles, Athens grew so powerful that the Athenian Empire could effectively dictate the laws, customs, and trade of all her neighbors in Attica and the islands of the Aegean.

The might of the Athenian Empire encouraged an arrogance in the policy makers of the day which grew intolerable to the other city-states. When Athens sent troops to help Sparta put down a Helot rebellion, the Spartans refused the gesture and sent the Athenian force back home in dishonor, thus provoking the war which had long been brewing. Later, when Athens sent their fleet to help defend its ally Corcyra (Corfu) against a Corinthian invasion during the Battle of Sybota in 433 BCE, their action was interpreted by Sparta as aggression instead of assistance, as Corinth was an ally of Sparta."