History of the Peloponnesian War

After the two armies had effected a junction, at daybreak they took post at a place called Metropolis and made camp. Not long afterwards the Athenians with their twenty ships arrived in the Ambracian Gulf, reinforcing the Argives; and Demosthenes also came with two hundred Messenian hoplites and sixty Athenian bowmen.

The ships lay at sea about the hill of Olpae, blockading it; but the Acarnanians and a few of the Amphilochians—for most of these were kept from moving by the Ambraciots—had already gathered at Argos and were preparing for battle with their opponents, having chosen Demosthenes to command the whole allied force in concert with their own generals. And he, leading them close to Olpae, encamped;

and a great ravine separated the two armies. For five days they kept quiet, but on the sixth both sides drew up in order of battle. Now the army of the Peloponnesians was larger than that of Demosthenes and outflanked it; he, therefore, fearing that he might be surrounded, stationed in a sunken road overgrown with buses an ambush of hoplites and light-troops, about four hundred all together, his purpose being that in the very moment of collision these troops should leap from their hiding-place and take the enemy in the rear at the point where his line overlapped.