The civil war at Corcyra 427-425 BC (= Thucydides, III, 69-85):

"So bloody was the march of the revolution, and the impression that it made was the greater as it was one of the first to occur. Later on, one may say, the whole Hellenic world was convulsed; struggles being everywhere made by the popular chiefs to bring in the Athenians, and by the oligarchs to introduce the Spartans. In peace there would have been neither the pretext nor the wish to make such an invitation; but in war, with an alliance always at the command of either faction for the hurt of their adversaries and their own corresponding advantage, opportunities for bringing in the foreigner were never wanting to the revolutionary parties. The sufferings which revolution entailed upon the cities were many and terrible, such as have occurred and always will occur, as long as the nature of mankind remains the same... Revolution thus ran its course from city to city, and the places which it arrived at last, from having heard what had been done before, carried to a still greater excess the refinement of their inventions, as manifested in the cunning of their enterprises and the atrocity of their reprisals. Words had to change their ordinary meaning and to take that which was now given them. Reckless audacity came to be considered the courage of a loyal ally; prudent hesitation, specious cowardice; moderation was held to be a cloak for unmanliness; ability to see all sides of a question, inaptness to act on any. ... The leaders in the cities, each provided with the fairest professions, on the one side with the cry of political equality of the people, on the other of a moderate aristocracy, sought prizes for themselves in those public interests which they pretended to cherish, and, recoiling from no means in their struggles for ascendancy engaged in the direst excesses... Meanwhile the moderate part of the citizens perished between the two, either for not joining in the quarrel, or because envy would not suffer them to escape. In the confusion into which life was now thrown in the cities, human nature, always rebelling against the law and now its master, gladly showed itself ungoverned in passion, above respect for justice, and the enemy of all superiority... Indeed, it is true that in these acts of revenge on others men take it upon themselves to begin the process of repealing those general laws of humanity that are there to give hope of salvation to all who are in distress, instead of leaving those laws in existence, remembering that there may come a time when they, too, will be in danger and will need their protection. "