The long anticipated war between Athens and its allies and Sparta and its allies finally broke out in 431 BC. Like the more recent antagonism between NATO and the Soviet Union, it was a war between two power blocs, with two very different ideologies.

The Delian League of democratic Athens had begun as a naval alliance to protect Greece from Persia, but by the outbreak of the war it more closely resembled an empire with subject states. Its opponent, Sparta, was the greatest land power of Greece, a strict military oligarchy that had brutally conquered neighboring states in its early days. It attempted to use the war to pose as the 'liberator of Greece' because of the highhanded way Athens had treated its allies.

The war started slowly at first with the Athenians retreating behind their long city walls and receiving shipments of food from their harbor. The Spartans meanwhile set up camp eight miles north of the city and set about destroying the city's crops. Numerous small-scale skirmishes took place, but Pericles insisted that Athens avoid land battles and use its navy to cripple Sparta by attacking the coasts of the Peloponnese, the Spartan homeland.

When plague broke out in Athens in 430 BC, things took a dramatic turn for the worse. With Pericles dead, the popular assembly became indecisive and years were to pass before a decisive encounter.