Diodorus account mostly focuses on mainland Greece, covering the end of the Pentecontaetia (1-7, 22, 27-28), the first half of the Peloponnesian War (30, 31–34, 38–51, 55–63, 66-73), and conflicts during the Peace of Nicias (74-84). Most of the side narratives concern events in southern Italy, relating to the foundation of Thurii (9-21, 23, 35) and the secession of the Plebs at Rome (24-25). An account of the war between Leontini and Syracuse, culminating in the embassy of Gorgias to Athens (54-56), sets up the account of the Sicilian Expedition in book XIII.