Why do great powers fight great wars? The conventional answer is a story of rising challengers and declining hegemons. An ascendant power, which chafes at the rules of the existing order, gains ground on an established power—the country that made those rules. Tensions multiply; tests of strength ensue. The outcome is a spiral of fear and hostility leading, almost inevitably, to conflict. “The growth of the power of Athens, and the alarm which this inspired in Sparta, made war inevitable,” the ancient historian Thucydides wrote—a truism now invoked, ad nauseum, in explaining the U.S.-China rivalry.