In 418 BC Sparta defeated its neighbor and oldest rival, Argos, a city theoretically allied to Athens. One again, hostilities began to brew.

In Athens, the charismatic aristocrat Alcibiades had been elected strategos (general) of the city. Alcibiades had been raised by Pericles and tutored by thinkers such as Socrates. A desperate attention-seeker, (he famously cultivated a lisp), he would stop at nothing to increase his influence over Athens, and now used Sparta's renewed aggression as an excuse to increase his own power.

An expedition to Argos followed in which those that had supported the Spartan invasion were deported from the city. Soon after, Athens besieged the island of Melos, which had always refused to become a member of the Delian League and whose inhabitants were primarily of Spartan decent.

Like the struggle between the USA and the USSR in modern times, the rivalry between Athens and Sparta had become a Cold War, with each fighting the other only indirectly. So when the Sicilian city of Segesta appealed for Athenian help against its opponent Selinus, the Athenian assembly saw an ideal opportunity to not only enrich itself, but take a decisive tactical step against Sparta which was allied to Sicily's chief city Syracuse.