Thucydides. History of the Peloponnesian War.

So savage was the progress of this revolution [at Corcyra], and it seemed all the more so because it was one of the first which had broken out. Later, of course, practically the whole of the Hellenic world was convulsed, with rival parties in every state -- democratic leaders trying to bring in the Athenians, and oligarchs trying to bring in the Spartans.... In the various cities these revolutions were the cause of many calamities -- as happens and always will happen while human nature is what it is, though there may be different degrees of savagery, and, as different circumstances arise, the general rules will admit of some variety. In times of peace and prosperity cities and individuals alike follow higher standards, because they are not forced into a situation where they have to do what they do not want to do. But war is a stern teacher; in depriving them of the power of easily satisfying their daily wants, it brings most people's minds down to the level of their actual circumstances.

So revolutions broke out in city after city, and in places where the revolutions occurred late the knowledge of what had happened previously in other places caused still new extravagances of revolutionary zeal, expressed by an elaboration in the methods of seizing power and by unheard-of atrocities in revenge. To fit in with the change of events, words, too, had to change their usual meanings. What used to be described as a thoughtless act of aggression was now regarded as the courage one would expect to find in a party membership; to think of the future and wait was merely another way of saying one was a coward; any idea of moderation was just an attempt to disguise one's unmanly character; ability to understand a question from all sides meant that one was totally unfitted for action. Fanatical enthusiasm was the mark of a real man, and to plot against an enemy behind his back was perfectly legitimate self-defence. Anyone who held violent opinions could always be trusted, and anyone who objected to them became a suspect. To plot successfully was a sign of intelligence, but it was still cleverer to see that a plot was hatching.... If an opponent made a reasonable speech, the party in power, so far from giving it a generous reception, took every precaution to see that it had no practical effect. (3.82) (Tansl. Rex Warner. Penguin Books, 1954)

(Savior of the West against Persian aggression and the paragon of democratic ideals, 5th-century Athens accumulated a war chest and an empire that none in the Mediterranean could rival. And then, as the result of an interminable war, arrogance and atrocity, and a disastrous expedition to the distant island of Sicily, it all went -- as the Brits say -- to smash. No wonder that Thucydides' judicious, cold-blooded account of the rise and fall of the Athenian empire has been the reading of generals for the past four-hundred years. Or that Thomas Hobbes translated it from out of the Greek. For a footnote to the ways in which words lose their shape and meaning on the anvil of terror, see also George Orwell's "Politics and the English Language.")