U.S. Decision to Use Atomic Bombs Against Japan

The U.S. Decision to Use Atomic Bombs Against Japan was not preserved as one simple moment of choice. Harry S. Truman’s authorization took shape amid a secret weapons project, urgent plans to invade Japan, advisers’ estimates of the human cost, and the accelerating timetable at Potsdam. The record traces both the President’s ultimate approval and the uncertainty around when, exactly, he decided that the bomb should be used.

Secret Project and Growing Knowledge

For a time, investigators working for Truman’s Senate committee encountered unexplained expenditures associated with a project identified only as the Manhattan Project. On June 17, 1943, Truman telephoned Secretary of War Stimson. Stimson said that he personally knew the details and that only a handful of people in the world did. Truman accepted his assurance that the work had a specific and important purpose, and asked no further questions.

Information nevertheless continued to reach Truman. By July, he knew the essential nature of the secret development. When Lewis Schwellenbach, formerly a senator and by then a federal district judge, asked about enormous land condemnations for the DuPont Company near Hanford, Truman wrote back to reassure him. The condemned area involved nearly half a million acres. The episode indicates that Truman’s knowledge was no longer confined to the vague assurance given during their 1943 telephone call, even though the project remained secret.

In 1945, Stimson brought the issue directly into Truman’s office. Alone with the President, he presented a typewritten memorandum warning that within four months the United States would probably have completed a weapon capable of destroying a city. The United States and Great Britain had shared in developing it, but only the United States could then produce it. Stimson also cautioned that this advantage would not last and that the Soviet Union might be able to enter production within a few years. The memorandum made the decision’s stakes explicit: the weapon promised immense destructive force, and its development would affect the postwar balance of power.

Invasion Plans and the Cost of Continuing the War

Before the bomb became part of the discussion, Truman faced the question of how to bring Japan to surrender. He was considering an invasion of the Japanese home islands as well as a proposed naval blockade intended to starve Japan into submission. In his diary, he weighed invasion against bombing and blockade, calling the choice his hardest decision to date. He did not mention the secret weapon there.

The omission was also striking at a White House meeting on June 18, 1945, convened to review invasion plans. General Marshall presented a two-stage operation beginning in November with an assault on Kyushu, one of Japan’s southernmost islands, only 350 miles from Okinawa. He described it as an operation as difficult as Normandy. For the first thirty days of the initial phase, he estimated losses comparable to those at Luzon, about 31,000. Assistant Secretary of War John J. McCloy noticed that the discussion proceeded as if the bomb did not exist.

The human cost remained central as the plans developed. Admiral Leahy recorded that Truman wanted estimates of the time, ships, and men needed, as well as likely American killed and wounded. Truman’s stated priority was to minimize the loss of American lives; saving time and money mattered less. At Potsdam, Truman later recalled, Marshall estimated that an invasion of the Tokyo plain and other parts of Japan would cause at least a quarter of a million American casualties. Truman said he reached his conclusion only after long and careful thought and that he disliked the weapon.

The argument was not simply whether Japan had already lost the war. The immediate objective, as the narrative frames it, was to obtain surrender and stop the killing. Japanese forces had continued to resist even when their position appeared hopeless, and American casualties in the Pacific had mounted sharply during Truman’s first months as President. From that perspective, an invasion’s projected toll was not an abstract figure: it was a cost the administration hoped it might avoid.

Potsdam: Advice, Judgment, and an Unclear Turning Point

On July 24, Stimson brought Truman a message from Washington reporting that the project might be ready for use in early August, depending on preparations and atmospheric conditions. Truman welcomed the news. He later described a consensus among Stimson, Leahy, Marshall, James F. Byrnes, and General Arnold that the bomb should be used, and said that the military advice about the lives an invasion might cost reinforced his conclusion.

Yet the account resists reducing the choice to a single, cleanly documented decision. George Elsey later remembered the atmosphere as one in which Truman could no more have stopped the project than a train moving down a track. The narrative likewise observes that there may never have been one clear moment when Truman made up his mind or announced that he had. By the summer of 1945, the project had accumulated momentum, and abandoning it would have meant a dramatic break with the course of events. Truman, for his part, later insisted that the final choice of where and when to use the bomb belonged to him, and that he regarded it as a military weapon.

A likely turning point came on the morning of July 24 at Number 2 Kaiserstrasse, where Truman, Winston Churchill, and their military advisers met around a table. No written record captured a formal decision there, but the account presents the meeting as the point after which use of the bomb, barring an unforeseen development, was settled for the coming weeks. Truman later identified the day he informed Joseph Stalin as the day of his decision.

That afternoon, at the Cecilienhof Palace, Truman walked over to Stalin as a meeting was breaking up and mentioned that the United States had a new weapon of unusual destructive force. He did not name it as an atomic bomb or offer scientific details. Stalin replied that he hoped the weapon would be put to good use against Japan, but did not ask what it was or pursue the subject again during the conference. Churchill, Byrnes, and Charles Bohlen interpreted Stalin’s casual response as evidence that he had not grasped the discovery’s significance. The exchange thus joined the military decision to a diplomatic encounter whose larger implications were not apparent to Stalin in the moment.

Final Authorization and Its Limits

A further message from Washington, received late on July 30 and delivered the next morning, said that a public statement had to be ready by August 1 because the project was moving rapidly. At 7:48 A.M. Berlin time on July 31, Truman wrote his approval on the back of the pink message and handed it to George Elsey for transmission. His instruction authorized the proposal but stipulated that release should occur when ready and not before August 2.

The timing mattered. Elsey later recalled that Truman did not want anything to happen until he had left Stalin and Potsdam. The approval therefore functioned both as a final authorization and as a boundary on when the operation could proceed. Stimson was already back at the Pentagon, and Marshall was due to depart that day as the conference wound down.

The evidence supports neither a portrayal of Truman as unaware of the weapon’s gravity nor a claim that the historical record reveals a single decisive instant. He had received a stark warning about the bomb’s capacity, confronted invasion estimates, heard senior advisers recommend its use, and personally approved its release. He also later said that he did not like the weapon. The decision was made under the pressure of a continuing war and anticipated casualties, while the project’s momentum and the approaching operational timetable narrowed the space in which alternatives could be pursued.