Passage 3.9.9
οἱ δὲ ἐς τὸ φανερὸν τοῦ πολέμου παρασχόντες τὴν ἀρχὴν ἐγένοντο οἱ ἐξ Ἀμφίσσης Λοκροί. τοῖς γὰρ δὴ Λοκροῖς γῆ πρὸς τοὺς Φωκέας ἐτύγχανεν οὖσα ἀμφισβητήσιμος γῆ · ἐκ ταύτης ὑπὸ Θηβαίων ἐπαρθέντες τῶν περὶ Ἰσμηνίαν τόν τε σῖτον ἀκμάζοντα ἔτεμον καὶ ἤλασαν λείαν ἄγοντες· ἐνέβαλον δὲ πανδημεὶ καὶ οἱ Φωκεῖς ἐς τὴν Λοκρίδα καὶ ἐδῄωσαν τὴν χώραν.
The Locrians of Amphissa were openly responsible for initiating the war. There was disputed territory between the Locrians and the Phokians: at the instigation of the Thebans, especially Ismenias and his supporters, the Locrians harvested grain when it was still ripe, and carried off plunder. In response, the Phokians launched a full-scale invasion into Locris, laying waste to the countryside.