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 speculative, though it was not yet a reality. Such was the certainty that, three years later, the uranium bomb was used directly on Hiroshima without ever being tested. (The July 1945 Trinity test was for the more tricky plutonium implosion design.) Now the argument that the U.S. should beat the Nazis to the nuclear punch could be joined to Bush’s ambition to put science in the service of U.S. weapons to be used in World War II and for global power after it (now more politely called “national security”).

Bush briefed FDR on October 9, 1941 recommending an all-out effort to make the bomb. Given his position and his ambition that the weapons whose development he was overseeing should be used in World War II, the first decision to use the bomb was made, in spirit and at least arguably, on that date, almost two months before the U.S. formally entered the war upon the bombing of Pearl Harbor. Of course, it had to be built in time. But course was set.

By May 1943, what came to be known as the Manhattan Project was firmly established. Leading U.S. physicists, including immigrants who had fled Europe, met in 1942 at the University of California, Berkeley for what should be the most famous summer study ever. They too concluded a uranium bomb would work. A nuclear chain reaction, the explosive heart of the bomb, was demonstrated at the University of Chicago on December 2, 1942. Hanford, Washington, had been selected as the site to make plutonium on an industrial scale. Los Alamos was selected as the place where scientists would design the bomb high on an isolated New Mexican mesa. It was a region that Robert Oppenheimer knew well.

On May 5, 1943, the Military Policy Committee, consisting of five men, chaired by Bush, met to review progress. It was, in effect, the Executive Committee of the Manhattan Project. James Conant, President of Harvard, was Vice-Chair. General Groves, who oversaw and coordinated the massive project on the ground, was a member, as were two other military personnel, General Styer, an expert in logistics, and Admiral Purnell, Deputy Chief of Naval Operations for Materiel. The Committee did not have a single general or admiral in charge of actually prosecuting the war that was raging. In fact, almost to a man, the the generals and admirals in the field did not know about the Manhattan Project.

It was on that fateful day, two years before the end of the war in Europe, that that the Military Policy Committee decided that Germany would NOT be targeted; the target selected was the Japanese fleet stationed at the Pacific Island of Truk. Manhattan Project scientists other than Bush and Conant, continued to labor under the idea that the bomb would be to deter Germany or perhaps used on it. Bush had decided not to inform them. When I interviewed several leading ones who were still alive in 1995, including Glenn Seaborg and Hans Bethe, none knew of the May 5, 1943 decision, even then though the fact had been public, in fine print, for decades.

May 5, 1943 was the first specific decision to use the bomb; it was the first targeting decision. Germany was not targeted since they might reverse engineer a bomb if it were a dud. Japan was thought to be less likely to do so. The targeting of the fleet at Truk rather than Tokyo was an added precaution — a dud would sink and be hard to recover. From May 5, 1943 onward the use of the bomb was all about the Pacific theater and in 1944, Japan itself.

On September 18, 1944, FDR and Churchill agreed that “when a ‘bomb’ is finally available, it might perhaps, after mature consideration, be used against the Japanese, who should be warned that this bombardment will be repeated until they surrender.” The aide-memoire makes no mention of Germany. In that same time frame, logistical preparations were made in the Pacific theater to use the bomb on Japan. There were none in the European theater.

By December 1944, the Manhattan Project spy mission, Alsos, had enough information to conclude that Germany’s efforts “to develop a bomb were still in the experimental stages,” as Groves wrote in his memoir, Now It Can Be Told. The war was coming to a close; the Soviets were well into Eastern Europe. Paris had been liberated months before. Joseph Rotblatt, a scientist at Los Alamos, decided his job was done; there was no German bomb threat. He quit. He was the only one.

The Project itself was accelerated so the bomb could be used before the war in the Pacific ended. Bush was joined in his determination that weapons developed during the war should be used in the war by others, notably General Groves. A principal motivation was to justify the use of immense resources for the atom bomb project; that meant showing that the bomb had played a big role in ending the war and saving the lives of US armed forces personnel by preventing an invasion.

The last decisions were in May 1945; they were to pick target cities in Japan. While some scientists opposed the use of the bomb on cities, their pleas did not reach President Truman. Jimmy Byrnes, his Secretary of State, explicitly rejected a similar plea from Leo Szilard, who first conceptualized the chain reaction years before it was experimentally achieved. At the top, there was no serious consideration about whether the bombs should be used. It was just a question of when.

The answer: as soon as the bombs were ready and weather permitted — those dates were August 6 and August 9, 1945. The invasion of Kyushu was not due till November 1, 1945. The determination to use the bomb in World War II was realized by its early use. The role of the bombings in ending the war has increasingly come into question with strong evidence pointing to the decisive role of the Soviet entry into the War on August 8, 1945. The Soviets had been neutral with respect Japan before then. Japanese rulers, observing Eastern Europe, did not want to be occupied by the Soviets. So the Japanese submitted to the United States’ demands.

It worth noting that, on April 23, 1945 as part of briefing materials for the newly installed President Truman, Groves wrote. “The target is and was always expected to be Japan.” (italics added). The decision in the direction of Japan and away from Germany was made two years before, on May 5, 1943. Groves knew because he was a part of it. Finally, there is much evidence that the use of the bombs was, in significant measure, a message to the Soviets about who would run the post-War world. Bush too had achieved his objective — the bomb was used in World War II and the U.S. had announced itself as the preeminent global power.