The anniversaries of the atomic bombings of Hiroshima and Nagasaki present an opportunity to demolish a cornerstone myth of American history — that those twin acts of mass civilian slaughter were necessary to bring about Japan’s surrender, and spare a half-million US soldiers who’d have otherwise died in a military conquest of the empire’s home islands.Those who attack this mythology are often reflexively dismissed as unpatriotic, ill-informed or both. However, the most compelling witnesses against the conventional wisdom were patriots with a unique grasp on the state of affairs in August 1945 — America’s senior military leaders of World War...