History of the Peloponnesian War
Thucydides
Thucydides, Vol. 1-4. Smith, Charles Foster, translator. London and Cambridge, MA: Heinemann and Harvard University Press, 1919-1923.
When finally the armies were at close quarters and the Peloponnesians outflanked with their left the right wing of their opponents and were about to encircle it, the Acarnanians, coming upon them from their ambush, fell upon their rear and routed them, so that they did not stand to make resistance and in their panic caused the greater part of their army to take to flight also; for when they saw the division under Eurylochus, their best troops, being cut to pieces, they were far more panic-stricken. And it was the Messenians, who were in this part of the field under the command of Demosthenes, that bore the brunt of the battle.
On the other hand, the Ambraciots and those on the enemy's right wing defeated the troops opposed to themselves, and pursued them to Argos; and indeed these are the best fighters of all the peoples of that region.
When, however, they returned and saw that their main army had been defeated, and the victorious division of the Acarnanians began to press hard upon them, they made their escape with difficulty to Olpae; and many of them were killed, for they rushed on with broken ranks and in disorder all except the Mantineans, who kept their ranks together during the retreat better than any other part of the army. And it was late in the evening before the battle ended.
On the next day, since Eurylochus and Macarius had been slain, Menedaius had on his own responsibility assumed the command, but the defeat had been so serious that he was at his wit's end how, if he remained, he could stand a siege, blockaded as he was by both land and sea by the Athenian fleet, or, if he retreated, could get away safely. He therefore made overtures to Demosthenes and the Athenian generals regarding a truce for his retreat and also about the recovery of his dead.