Harry S. Truman’s decision to use the atomic bomb against Japan joined an exceptionally secret weapons program to the immediate question of how to end the war. As plans for an invasion raised the prospect of heavy American losses, Truman weighed bombing and blockade against an assault on Japan itself. The surviving account locates the decision at his Potsdam headquarters and records its tangible outcome in a handwritten order to Henry L. Stimson.

A Secret Program Reaches the Senate Investigator

For some time, staff investigators had encountered unexplained expenditures tied to an enterprise identified only as the Manhattan Project, apparently larger even than the Canol Project. On June 17, 1943, Truman, then a senator, telephoned Stimson, the Secretary of War, to ask about the secret development. Stimson told him that he personally knew all about it and was among the very few people who did. When Stimson assured him that it had a specific and unique purpose, Truman accepted that assurance and ended the inquiry: he did not need to hear more.

Information continued to reach Truman. By July, he knew the essential nature of the development. When Lewis Schwellenbach asked about vast land condemnations for the DuPont Company near Hanford, along the Columbia River, Truman replied in a letter that Schwellenbach should not worry. Nearly half a million acres were involved. The episode shows how the project’s secrecy shaped Truman’s access to knowledge: his understanding came through trusted channels, while the extraordinary scale of the work remained publicly unexplained.

The Choice Against the Invasion Plan

By June 1945, Truman was considering how to bring the war against Japan to an end. He wrote in his diary, “I have to decide Japanese strategy—shall we invade Japan proper or shall we bomb and blockade?” He added, “That is my hardest decision to date.” The immediate alternatives were invasion or a campaign of bombing and blockade; the atomic project was not mentioned in his diary entry.

Nor did the bomb enter the White House meeting on June 18 to review invasion plans, a silence that struck John J. McCloy as strange. George C. Marshall presented a two-phase invasion beginning in November with Kyushu, one of Japan’s southernmost islands, about 350 miles from Okinawa. Marshall expected an operation as difficult as Normandy. For the first 30 days of the initial phase, he estimated losses comparable to those on Luzon—about 31,000. The military estimate gave the invasion option a concrete human cost even as the weapon that might change the decision remained outside the discussion.

The risk of invasion was not the only consideration recorded in the evidence. Stimson brought Truman a multi-page memorandum, written in Stimson’s own hand, whose opening was intended to shock: “Within four months we shall in all probability have completed the most terrible weapon ever known in human history, one bomb of which could destroy a whole city.” The United States and Great Britain had shared in developing the weapon, but only the United States could then produce it. Stimson also warned that this advantage could not last and that Russia might be able to enter production within the next few years.

Authorization at Potsdam

On July 24, Stimson again climbed the stairs to Truman’s office, arriving at 9:20 in the morning with another message. The account identifies the day as almost certainly fateful. Truman’s headquarters during the Potsdam Conference was No. 2 Kaiserstrasse, a house described as the place where the decision on the atomic bomb was made. The location anchors the decision in the conference’s wartime setting, while the surviving objects—the memorandum and the order—show how the question moved from secret briefing to presidential authorization.

Truman’s order to Stimson was handwritten in pencil and directed the use of the atomic bomb on Japan. The decision thus passed through a personal chain of communication: a highly classified program, a warning from the Secretary of War, and a written instruction from the President. The evidence identifies the authorization and its destination, but does not provide the full text of the order or establish every consideration Truman weighed at the moment he issued it.

Announcement and the Decision’s Place in the War

At 7:00 p.m. on August 14, Truman sat at his desk in the Oval Office and announced Japan’s surrender. The juxtaposition of the handwritten authorization and the later broadcast gives the decision its central consequence in the account: the secret weapon contemplated in private became part of the closing sequence of the war, followed by a public announcement of surrender. The evidence does not, by itself, assign the surrender to a single cause.

The decision is presented through the contrast between uncertainty and record. In June, Truman described the invasion-versus-bombing choice as his hardest decision, while the projected invasion carried a specific estimate of casualties. Stimson’s memorandum then set out both the unprecedented destructive power of the weapon and the likelihood that the United States would not retain a monopoly. The pencil order at Potsdam marks the point at which that secret development became an authorized course of action against Japan.