Thucydides summarised the situation before the war as: "The growth of the power of Athens, and the alarm which this inspired in Lacedaemon, made war inevitable". The nearly 50 years before the War had been marked by the development of Athens as a major power in the Mediterranean world. Its empire began as a small group of city-states, called the Delian League – from the island of Delos, on which they kept their treasury – that formed to ensure that the Greco-Persian Wars were over. After defeating the Second Persian invasion of Greece in the year 480 BC, Athens led the coalition of Greek city-states that continued the Greco-Persian Wars with attacks on Persian territories in the Aegean and Ionia. What ensued was a period which Thucydides called the Pentecontaetia, in which Athens increasingly became an empire, carrying out an aggressive war against Persia and increasingly dominating other city-states. Athens brought under its control all of Greece except for Sparta and its allies, ushering in a period now called the Athenian Empire. By mid-century, the Persians had been driven out of the Aegean and had ceded control of vast territories to Athens. Athens had greatly increased its own power; a number of its formerly independent allies were reduced, over the course of the century, to the status of tribute-paying subject states of the Delian League. This tribute was used to fund a powerful fleet and, after the middle of the century, massive public works in Athens, causing resentment.
Thucydides summarised the situation before the war as "The growth of the power of Athens and the alarm which this inspired in Lacedaemon made war inevitable" The nearly 50 years before the War had been marked by the development of Athens as a major power in the Mediterranean world Its empire began as a small group of city-states called the Delian League – from the island of Delos on which they kept their treasury – that formed to ensure that the Greco-Persian Wars were over After defeating the Second Persian invasion of Greece in the year 480 BC Athens led the coalition of Greek city-states that continued the Greco-Persian Wars with attacks on Persian territories in the Aegean and Ionia What ensued was a period which Thucydides called the Pentecontaetia in which Athens increasingly became an empire carrying out an aggressive war against Persia and increasingly dominating other city-states Athens brought under its control all of Greece except for Sparta and its allies ushering in a period now called the Athenian Empire By mid-century the Persians had been driven out of the Aegean and had ceded control of vast territories to Athens Athens had greatly increased its own power; a number of its formerly independent allies were reduced over the course of the century to the status of tribute-paying subject states of the Delian League This tribute was used to fund a powerful fleet and after the middle of the century massive public works in Athens causing resentment