"The history of the Peloponnesian War finds an appropriately severe and penetrating recorder in the exiled Athenian Thucydides, unquestionably one of the greatest historians of all time. George Marshall said that no-one could understand the Second World War in our own time unless they had read him. Thucydides too wished to record great events; but he had no illusions about war: "In peace and prosperity both states and individuals are activated by higher motives because they do not fall under the dominion of imperious necessities. But war, which takes away the comfortable provision of daily life, is a violent teacher and assimilates men's characters to their conditions..." War, Thucydides believed like Homer, was a prerequisite of human society – "the ultimate constraint by which all settled societies protect themselves", as John Keegan puts it. He also believed that human nature will under necessity use ultimate force."