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Just as Herodotus is the father of history, Thucydides is the father of realism. To understand the geopolitical conflict zones of the 21st century, you must begin with the ancient Greeks. Among the many important lessons Thucydides teaches in his History of the Peloponnesian War is that what starts a war is different from what causes it.

Thucydides chronicles how the Peloponnesian War began in the latter part of the late fifth century B.C. with disputes over the island of Corcyra in northwestern Greece and Potidaea in northeastern Greece. These places were not very strategically crucial in and of themselves. To think that wars must start over important places is to misread Thucydides. Corcyra and Potidaea, among other locales, were only where the Peloponnesian War started; not...