The study of Thucydides and his famous History of the Peloponnesian War has never been so intense, so widespread or influential, as in our time. Thucydides claimed that his work is “a possession forever” meant to be useful to “such men as might wish to see clearly what has happened and what will happen again, in all human probability, in the same or a similar way.” More than twenty-four-hundred years later, political leaders and students of politics treat it in just that way.

In the ancient world, Thucydides’ focus on politics routed the broader but shallower purview of his predecessors. Herodotus, with his meandering style full of discursive side trips into the customs and habits of various peoples and his serious consideration of the causal role of the gods in human affairs, did not become the model for what was thought to be the best historic ...