Soon after “Hiroshima” was published, the influential Saturday Review ran an editorial condemning “the crime of Hiroshima and Nagasaki.” America’s military establishment tried to quell the outrage with a piece in Harper’s by Henry Stimson, a retired Secretary of War. The article—ghostwritten by McGeorge Bundy, a future national-security adviser—claimed that dropping nuclear bombs on Japan had averted further war, saving more than a million American lives. Kai Bird, a co-author of “American Prometheus,” the definitive biography of J. Robert Oppenheimer, told me that this pushback was specious: “Bundy later admitted to me that there was no documentary evidence for this ‘million’ casualty figure. He just pulled it out of thin air.”