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In September 2014, when the Islamic State (ISIS) was at the height of its power, Director of U.S. National Intelligence James Clapper acknowledged that the United States had underestimated the terrorist group’s will to fight. “We underestimated the Viet Cong and the North Vietnamese and overestimated the will of the South Vietnamese,” he told The Washington Post. “In this case, we underestimated ISI[S] and overestimated the fighting capability of the Iraqi army … It boils down to predicting the will to fight, which is an imponderable.”

Scholars and policymakers have long sought to determine what drives people to keep fighting when the chips are down, and, if need be, to give their lives to a cause. Traditional explanations, based on rational choice theory or focused on mental abnormalities, have largely failed to explain what motivates the members of extremist insurgent movements. But Clapper was wrong to suggest that the

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