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What makes the subject disquieting, however, is not that the decision seems so baffling but that it seems so explainable. Most Americans are convinced that their nation is among history's most moral. Yet a combination of factors, most of them justified and all of them understandable, created a momentum for unleashing a truly monstrous force. Will future historians someday find themselves sorting through the rubble of another atomic attack, by a nation as moral or one that is less so, and come to the same frightening conclusion? It would be far more comforting to think that the dropping of the Bomb made global war simply unthinkable, something that could never happen again. --By Walter Isaacson