The dropping of the atomic bomb by the United States was one of the most portentous events in history. Development of the bomb had begun in 1939, after a small group of scientists persuaded the U.S. government that such a weapon was feasible and that Germany was already conducting experiments in atomic energy. The research program that began in October, 1939, ultimately developed into the two-billion-dollar Manhattan Project, which was headed by Leslie R. Groves. The project’s goal was to produce a bomb before the Germans did. Few U.S. political or military officials ever doubted that such a bomb, if produced, would be used. Yet before the first bomb was perfected and tested, Germany surrendered. Only Japan remained at war with the Allies.

Early in 1945, as the first bomb neared completion, some scientists began to have doubts about using it. The wave of horror that might follow its use and the moral burden of unleashing such an awesome weapon might, they thought, offset any immediate advantage the bomb could provide. Several options were possible: The United States might demonstrate the new weapon on a barren island before representatives of the United Nations, who could then warn the Japanese of its destructive power; the bomb might be dropped on a military target in Japan after giving a preliminary warning; or the United States could refuse to drop it at all.

While the scientists pondered such choices, military officials prepared to use the bomb. By the end of 1944, possible targets in Japan had been selected, and a B-29 squadron had begun training for the bomb’s delivery. Two weeks after President Roosevelt died in 1945, Secretary of War Henry L. Stimson, on April 25, met with the new president, Harry S. Truman, informed him about the bomb, and predicted that in four months it would be available for use. Upon Stimson’s recommendation, Truman appointed a special Interim Committee on Atomic Policy to consider use of the bomb. On June l, 1945, the committee recommended to the president that the bomb be used against Japan as soon as possible, be used against a military target, and be dropped without prior warning. By early July, 1945, as Truman left for the Potsdam Conference in Germany to discuss postwar settlements with Great Britain and the Soviet Union, he had decided to use the bomb once it was perfected.

The mushroom cloud rising from the atomic bomb dropped on Nagasaki. (National Archives)

On July 16, in the Trinity Flats near Alamogordo, New Mexico, the first atomic bomb was successfully tested. The United States now had its weapon, although the war in the Pacific had already driven Japan to the brink of surrender. As early as September, 1944, the Japanese had sought to sound out the Allies concerning peace terms. On the eve of the Potsdam Conference, the Japanese ambassador in Moscow asked the Soviet government to mediate with the Allies to end the war. Japan could not accept unconditional surrender, but the Japanese appeared ready to surrender under terms that would allow them to preserve the position of the emperor in the Japanese system. This the Allies would not accept.

The Truman administration faced difficult problems. Total defeat and unconditional surrender of Japan might require a costly and prolonged invasion of the Japanese home islands. The Soviets, as they had promised at Yalta in February, 1945, were scheduled to enter the Pacific war in early August. Although their support had been eagerly sought until the spring of 1945, it now appeared less vital; indeed, Truman now hoped to defeat Japan before the Soviet Union could effectively enter the war and gain any control over the postwar settlement with Japan. Also, use of the atomic bomb in Japan would indicate to the Soviets just how powerful the United States was. On July 26, from Potsdam, the Allies called upon Japan to surrender unconditionally or suffer “the utter devastation of the Japanese homeland,” a veiled reference the significance of which only the Allies fully understood. The Potsdam Declaration did not mention the atomic bomb and did not offer Japan any terms. The Japanese government chose not to reply to the declaration, while it waited for a reply to the peace overtures it had made through the Soviet government. For home consumption, the Japanese government called the Potsdam Declaration “unworthy of public notice.”

In the Mariana Islands, two bombs had been readied for use, and the B-29 crews were standing by. Truman ordered the U.S. Air Corps to drop them. When they were dropped, the age of atomic warfare and the Cold War began.

During the fiftieth anniversary of the dropping of the bomb on Hiroshima, in 1995, there was considerable controversy in the United States concerning why the first bomb over Hiroshima was dropped. The Smithsonian Institution first proposed an exhibit that would have the Enola Gay as its centerpiece with four side displays, two of which would depict the devastation of the Japanese cities of Hiroshima and Nagasaki. With these displays, it intended to depict the Japanese as victims. Veterans’ groups and Congress complained, pointing out that the bomb was dropped not only to end the war but also to retaliate for the Japanese attack on Pearl Harbor, which produced U.S. victims. In spite of the efforts of various scholars, the exhibit ultimately simply contained the Enola Gay. Thus, the significance of dropping the bomb on Hiroshima, the start of the Cold War and the atomic age, is by no means uncontroverted. The question still remains: Was it necessary to drop the bomb on population centers in the first place?