The Athenian historian Thucydides (c.460-c.395) wrote the History of the Peloponnesian War, which was fought between Athens and Sparta in the years 431-404. In books 6 and 7, he describes the Athenian expedition against Sicily in the years 415-413, which culminated in the Siege of Syracuse, the largest city on the island. The Athenian generals Nicias and Demosthenes, however, were outwitted by Hermocrates of Syracuse and Gylippus, a Spartan who commanded a part of the Syracusan army. Finally, the Athenians were forced to retire, and a battle took place near a river.
Thucydides 7.85-87 is offered here in the translation by Richard Crawley.
Thucydides on the Destruction of the Athenian Army
[7.85] At last, when many dead now lay piled one upon another in the stream, and part of the army had been destroyed at the river, and the few that escaped from thence cut off by the cavalry, Nicias surrendered himself to Gylippus, whom he trusted more than he did the Syracusans, and told him and the Spartans to do what they liked with him, but to stop the slaughter of the soldiers.
Gylippus, after this, immediately gave orders to make prisoners; upon which the rest were brought together alive, except a large number secreted by the soldiery, and a party was sent in pursuit of the 300 who had got through the guard during the night, and who were now taken with the rest. The number of the enemy collected as public property was not considerable; but that secreted was very large, and all Sicily was filled with them, no convention having been made in their case as for those taken with Demosthenes. Besides this, a large portion were killed outright, the carnage being very great, and not exceeded by any in this Sicilian war. In the numerous other encounters upon the march, not a few also had fallen. Nevertheless many escaped, some at the moment, others served as slaves, and then ran away subsequently. These found refuge at Catana.
[7.86] The Syracusans and their allies now mustered and took up the spoils and as many prisoners as they could, and went back to the city. The rest of their Athenian and allied captives were deposited in the quarries, this seeming the safest way of keeping them; but Nicias and Demosthenes were butchered, against the will of Gylippus, who thought that it would be the crown of his triumph if he could take the enemy's generals to Sparta. One of them, as it happened, Demosthenes, was one of her greatest enemies, on account of the affair of the island and of Pylos;note[Demosthenes had been able to capture a group of Spartan soldiers at the island of Sphacteria near Pylos, one of the greatest defeats in the history of Sparta.] while the other, Nicias, was for the same reasons one of her greatest friends, owing to his exertions to procure the release of the prisoners by persuading the Athenians to make peace. For these reasons the Spartans felt kindly towards him; and it was in this that Nicias himself mainly confided when he surrendered to Gylippus. But some of the Syracusans who had been in correspondence with him were afraid, it was said, of his being put to the torture and troubling their success by his revelations; others, especially the Corinthians, of his escaping, as he was wealthy, by means of bribes, and living to do them further mischief; and these persuaded the allies and put him to death. This or the like was the cause of the death of a man who, of all the Greeks in my time, least deserved such a fate, seeing that the whole course of his life had been regulated with strict attention to virtue.
[7.87] The prisoners in the quarries were at first hardly treated by the Syracusans. Crowded in a narrow hole, without any roof to cover them, the heat of the sun and the stifling closeness of the air tormented them during the day, and then the nights, which came on autumnal and chilly, made them ill by the violence of the change; besides, as they had to do everything in the same place for want of room, and the bodies of those who died of their wounds or from the variation in the temperature, or from similar causes, were left heaped together one upon another, intolerable stenches arose; while hunger and thirst never ceased to afflict them, each man during eight months having only half a pint of water and a pint of grain given him daily. In short, no single suffering to be apprehended by men thrust into such a place was spared them. For some seventy days they thus lived all together, after which all, except the Athenians and any native Sicilians or Italiots who had joined in the expedition, were sold. The total number of prisoners taken it would be difficult to state exactly, but it could not have been less than 7,000.
This was the greatest Greek achievement of any in this war, or, in my opinion, in Greek history; at once most glorious to the victors, and most calamitous to the conquered. They were beaten at all points and altogether; all that they suffered was great; they were destroyed, as the saying is, with a total destruction, their fleet, their army, everything was destroyed. Few out of many returned.