I highly recommend Thomas Cahill’s crystalline little book Sailing the Wine-Dark Sea. Among the many gems you’ll find inside is this quotation from Thucydides; it’s part of the great historian’s reflections on the consequences of the Peloponnesian war.

“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.... 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 member; 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-defense. Anyone who held violent opinions could always be trusted, and anyone who objected to them became a suspect.... As a result...there was a general deterioration of character throughout the Greek world. The plain way of looking at things, which is so much the mark of a noble nature, was regarded as a ridiculous quality and soon ceased to exist. Society became divided into camps in which no man trusted his fellow.”