The Peloponnesian War pitted the Athenians against the Spartans. The Peloponnesians’ were an alliance of city-states controlled by Sparta. These two powerful city-states became locked in a struggle for dominance of the eastern Mediterranean area. The roots of the conflict and in particular this expedition is highly complex. As Thucydides says in his history of the war, the underlying cause was Spartan fear of Athens' expansive power. But, the triggering event was Athens' aggressive behavior towards Corinth, an ally of Sparta.
In the early fifth century BC, Greece consisted of many city-states allied in various factions or leagues. The alliances between these cities are difficult to understand. The alliances created a patchwork where everyone was close to an enemy. This is important to remember when seeing the distance between Athens and Sicily. Many of Athens' enemies were closer than Sicily and much more dangerous.
In the late sixth and early fifth centuries BC, the Delian League, under Athenian leadership was the supreme sea power. Desiring additional power, Athens converted the Delian League into its own naval empire (Zook). This led to the period known as the Golden Age of Athens and their most expansionist era.
At the beginning of the war, Athens controlled the sea, but Sparta had a far superior army. Athens signed a treaty with one of Corinth's colonies. The following year, Athens simply took over a second Corinthian colony. Corinth, an ally of Sparta, asked for help in halting the Athenian aggression. This, combined with an Athenian embargo on commerce from a different Spartan ally, led to negotiations to mediate the dispute. When negotiations failed, Sparta declared war on Athens.
From 431 to 425 BC, Sparta invaded the outlying areas of Attica, an area north of Athens. They hoped to break the Athenian's will and encourage its subjects and slaves to rebel. Pericles, the Athenian leader, avoided land battles and concentrated on control of the sea. After Pericles' death, the Athenians signed a fifty year truce called the Peace of Nicias in 421. This provided that each side restore captured places and prisoners and remain at peace with each other. This was soon supplemented by an actual Athenian-Spartan alliance, which concluded for fifty years. The intention was chiefly to give each power a chance to put its own alliance in order while secure from an attack by the other. The war had been marked by numerous acts of brutality on both sides; prisoners had been slaughtered or enslaved, and agreements broken in a way that were shameful. (Winks)
The peace officially lasted only five years, years that saw the gradual rise to eminence in Athenian politics of Pericles’ cousin, Alcibiades, a brilliant, ambitious, dissolute, and unstable youth, who initially succeeded Cleon as leader of the lower-class war party against the restrained and unglamorous Nicias. Athenian intrigues to support Argos against Sparta only ended in...