Thucydides, History of the Peloponnesian War
Benjamin Jowett, Ed.

107. About this time the Athenians began to build their Long Walls extending to the sea, one¹ to the harbour of Phalerum, and the other to the Piraeus. [2] The Phocians² made an expedition against the Dorians, who inhabit Boeum, Cytinium, and Erineum, and are the mother people of the Lacedaemonians; one of these towns they took. Thereupon the Lacedaemonians under the command of Nicomedes the son of Cleombrotus, who was general in the place of the king Pleistoanax the son of Pausanias (he being at that time a minor), came to the assistance of the Dorians with fifteen hundred hoplites of their own, and, of their allies, ten thousand, and compelled the Phocians to make terms and to restore the town. They then thought of returning; but there were difficulties. [3] Either they might go by sea across the Crisaean Gulf, in which case the Athenian fleet would be sure to sail round and intercept them, or they might march over Mount Geraneia; but this seemed dangerous when the Athenians were holding Megara and Pegae. The pass was not easy, and was always guarded by the Athenians, who were obviously intending to stop them by that route also. [4] So they determined to remain in Boeotia and consider how they could best get home. They had another motive:—Certain Athenians were privately making overtures to them, in the hope that they would put an end to the democracy and the building of the Long Walls. [6] But the Athenians were aware of their embarrassment, and they also suspected their design against the democracy. [5] So they went out to meet them with their whole force, together with a thousand Argives and contingents from the other allies; they numbered in all fourteen thousand men. [7] Among them were some Thessalian cavalry, who came to their aid in accordance with the treaty³, but these deserted to the Lacedaemonians during the engagement.