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.
But the Acarnanians at first thought that all the fugitives were going away without covenant or truce and therefore set off in pursuit of the Peloponnesians; and when some of the generals tried to prevent this, saying that a truce had been made with them, someone hurled javelins at them, believing that they had been betrayed. Afterwards, however, they let the Mantineans and Peloponnesians go, but began to kill the Ambraciots.
And there was much dispute and uncertainty as to whether a man was an Ambraciot or a Peloponnesian. About two hundred of the Ambraciots were slain; the rest of the fugitives escaped into the neighbouring country of Agraea, and were received by Salynthius the king of the Agraeans, who was friendly to them.
Meanwhile the troops from the city of Ambracia arrived at Idomene. Now it consists of two lofty hills, and of these the higher had already been seized unobserved during the night by the troops which Demosthenes had sent forward from his main army; but the lower had previously, as it chanced, been ascended by the Ambraciots, who spent the night there.
After dinner Demosthenes and the rest of the army set out immediately after nightfall, he himself with half of them making for the pass, while the rest took the road through the Amphilochian mountains.