Each year at this time — the anniversary of the bombing of Hiroshima — the world pauses. The pause is less to mourn the dead than to debate a moral question: whether the bombing was justified and, by extension, whether the United States unnecessarily slaughtered tens of thousands of people on Aug. 6, 1945. The debate rarely focuses on a careful analysis of war and morality and is more frequently framed by existing views of the United States. The debate is rarely about Hiroshima or about World War II . It is a debate about the moral character of the United States. This is not an illegitimate subject, and Hiroshima might be a useful point with which to begin the debate. But that isn't possible until after we consider the origins of Hiroshima, which can be found in the evolution of modern warfare.