The Greek historian Thucydides has long been a favorite of American secretaries of state. For George Marshall, the History of the Peloponnesian War illustrated many of the diplomatic pitfalls of the cold war. A generation later, framed on Colin Powell’s State Department desk was the more ambiguous and ultimately ironic paraphrase of Thucydides: “Of all manifestations of power, restraint impresses men most.”

Restraint doesn’t much preoccupy Yale classicist and political commentator Donald Kagan, who spells out his version of the lessons the ancient historian has to offer in Thucydides: The