When was the decision made to use atom bombs on Hiroshima and Nagasaki? Was it one decision — or several that made their use inexorable and inevitable? What were the forums in which those decisions were made? When was Japan targeted? And Germany? Seventy-five years after those cities were obliterated, these remain insistent questions. The first step, the establishment of the Uranium Committee in October 1939, after President Roosevelt read Einstein’s letter urging a bomb project, was not really a decision to use the bomb. It was a scientific exploration that more or less had a deterrence aim to beat the Nazis to the bomb. Vannevar Bush (no relative of the two presidents to come), who headed the National Defense Research Committee in the White House, slow walked the project until well into 1941. Only $10 million was spent in the first two years. He reported directly to FDR and was the central decider in weapons development during World War II. Bush was an electrical engineer, inventor, Vice-President of the Massachusetts Institute of Technology and, from 1939 onward, President of the Carnegie Institution. He came to the White House job in 1940 determined to bring the full force of U.S. science, including that in academia, to bear on the development of weapons in this war, not the next. He knew that the generals considered academic scientists to be eggheads who, at best, would develop weapons for the next war — or in the words of Harvey Bundy, confidante of Secretary of War, Henry Stimson, the military “would naturally have the feeling that these damn scientists weren’t very practical men; they were visionaries.” Bush was determined to prove them wrong. At first, the atom bomb project seemed too speculative to his purpose; who knew if it would work at all? But in July 1941 the British MAUD (Military Application of Uranium Detonation) Committee concluded that a modest amount of uranium-235 would be sufficient to produce a massive atomic explosion. Now the bomb was not...