With the encouragement of Alcibiades, the Athenians take the island of Melos (which has remained neutral during the Peloponnesian War). Its inhabitants are treated with great cruelty by the Athenians, with all the men capable of bearing arms being killed, while the women and children are made slaves.
In Sicily, the Ionian city of Segesta asks for Athenian help from the Dorian city of Selinus (which is supported by the powerful Sicilian city of Syracuse). The people of Syracuse are ethnically Dorian (as are the Spartans), while the Athenians, and their allies in Sicily, are Ionian. The Athenians feel obliged to assist their ally and therefore prepare an armada to attack Sicily.