History of the Peloponnesian War
Thucydides. The Peloponnesian War. London, J. M. Dent; New York, E. P. Dutton. 1910.
Uniting here at daybreak, they sat down at the place called Metropolis, and encamped. Not long afterwards the Athenians in the twenty ships came into the Ambracian Gulf to support the Argives, with Demosthenes and two hundred Messenian heavy infantry, and sixty Athenian archers.
While the fleet off Olpae blockaded the hill from the sea, the Acarnanians and a few of the Amphilochians, most of whom were kept back by force by the Ambraciots, had already arrived at Argos, and were preparing to give battle to the enemy, having chosen Demosthenes to command the whole of the allied army in concert with their own generals.
Demosthenes led them near to Olpae and encamped, a great ravine separating the two armies. During five days they remained inactive; on the sixth both sides formed in order of battle. The army of the Peloponnesians was the largest and outflanked their opponents; and Demosthenes fearing that his right might be surrounded, placed in ambush in a hollow way overgrown with bushes some four hundred heavy infantry and light troops, who were to rise up at the moment of the onset behind the projecting left wing of the enemy, and to take them in the rear.
When both sides were ready they joined battle; Demosthenes being on the right wing with the Messenians and a few Athenians, while the rest of the line was made up of the different divisions of the Acarnanians, and of the Amphilochian darters. The Peloponnesians and Ambraciots were drawn up pell-mell together, with the exception of the Mantineans, who were massed on the left, without however reaching to the extremity of the wing, where Eurylochus and his men confronted the Messenians and Demosthenes.