Herodotus and thucydides

The struggle against the Persian invasion had occasioned a rare interval of inter-state cooperation in ancient Greek history. The two most powerful city-states, Athens and Sparta, had put aside their mutual suspicions stemming from their clash at the time of Cleisthenes' reforms in order to share the leadership of the united Greek military forces. Their attempt to continue this cooperation after the repulse of the Persians, however, ended in failure. Out of this failure arose the so-called Athenian Empire, a modern label invented to point out the political and economic dominance Athens came to exercise over other Greek states in an alliance originally set up as a voluntary association of its members against Persia.

9.1. �The Establishment of an Athenian Empire

The victorious Greeks decided in 478 B.C. to continue a naval alliance in order to attack the Persian outposts that still existed in far northern Greece and western Anatolia, especi Thucydides!