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One historic weakness of U.S. foreign policy has been the excessive confidence of policy makers who believe they can foresee the future. As a result, they lock themselves into policies and fail to adjust in time, even when their underlying assumptions prove unfounded. This has been the case repeatedly over the past thirty years in Afghanistan. President Obama’s call for a reduction of the future force structure of Afghan National Security Forces (ANSF) poses the risk of repeating this tragic cycle.

During the 1980s, U.S. policy makers assumed that the Soviets would ultimately prevail and the Afghan resistance could only impose costs on Soviet occupation forces. It might have been a reasonable belief, but it proved wrong. And it was a costly mistake. On the...