Nobody should ever say that it was a good call, but it was the only one a US President was likely to make in 1945. The decision to drop an atomic bomb on Hiroshima on Aug. 6, 80 years ago, was the almost-inevitable outcome of Japanese intransigence and of the technical success of the Manhattan Project, which brought into being nuclear weapons. The anniversary is generating a wave of commemorations and renewing the arguments for, and against, the mission of Colonel Paul Tibbets to drop “Little Boy” from his B-29, named Enola Gay, over Japan on that summer morning. In...