Can Pakistan learn from tragedy? As horrific details continue to emerge of Tuesday’s massacre in a Peshawar army-run school—at least 148 persons killed, 132 of them children—this is the central question facing the troubled country.

If Pakistan’s powerful generals use this moment to rethink their long-standing support for Islamist militancy, then the country has a hope of emerging as a better place for its 180 million people. But if the generals view the atrocity as merely an excuse for vengeance against the Pakistan Taliban,...