Uncertainty over the imminent completion and consequences of the atomic bomb

Uncertainty over the imminent completion and consequences of the atomic bomb was not a single unanswered technical question. It encompassed whether the weapon would work, how destructive it would be, whether it should be used against Japan, and what its existence might mean for civilization and relations with the Soviet Union. Harry S. Truman faced these questions while inheriting a project whose secrecy had kept even its Senate overseer largely uninformed, and whose scientific and political consequences no one could confidently measure.

Knowledge Under Secrecy

The project entered Truman’s awareness through fragments rather than a full account. As chairman of the Truman Committee, he encountered puzzling signs of a vast secret enterprise, including unexplained expenditures associated with the Manhattan Project. On June 17, 1943, he telephoned Henry L. Stimson, who told him that only a handful of people knew about the development and that it served a specific, unique purpose. Truman accepted Stimson’s assurance and did not press him to disclose more. By July, Truman had learned the undertaking’s essential nature, but not its full scope. When Lewis Schwellenbach asked about extensive land condemnations for DuPont Company along the Columbia River near Hanford, involving nearly half a million acres, Truman wrote that there was no reason for concern. The episode shows how the project’s secrecy could leave a senior senator with only partial knowledge even as its physical footprint grew enormous.

That earlier ignorance made the briefing Truman received as president especially consequential. On April 25, 1945, Stimson came to the White House alone, and Leslie Groves, the project’s military head, was brought in discreetly through a side entrance. Stimson gave Truman a freshly prepared memorandum whose opening warned that within four months the United States would probably have completed a weapon capable of destroying an entire city. The warning made the issue immediate, but it did not resolve the central uncertainties: completion remained prospective, the bomb had not been tested, and its ultimate implications were still unknown.

Groves then provided a report of roughly twenty-five pages describing the project’s status, processes, and problems. Truman read it and asked questions, though the surviving accounts do not specify what he asked. According to Groves, Truman showed no concern about the amount already spent and made clear that he accepted the project’s necessity. Stimson also recalled that Truman understood why the project had been withheld from him while he chaired the Senate committee. The briefing therefore closed one gap in Truman’s knowledge without giving him mastery of what came next.

A Weapon Whose Power Was Still Unknown

The scale of the undertaking sharpened the imbalance between what had been accomplished and what could be known. The Manhattan Project was the largest scientific-industrial enterprise to that point, cost $2 billion in less than three years, and involved about 200,000 people. Security was so effective that most participants had only a vague understanding of the work, while senior military figures, including Douglas MacArthur and Chester W. Nimitz, were also kept outside its central secrets. Responsibility rested with the Army Corps of Engineers under Groves; the project had been launched partly from fear that Nazi Germany was developing a similar weapon. That fear had helped justify extraordinary secrecy and speed, but Germany’s defeat altered the context before the bomb’s first test had taken place.

The United States and Britain had shared the project’s technical and scientific work, but it was in practice an American undertaking, directed and financed from Washington. It was also part of Franklin D. Roosevelt’s legacy: Roosevelt had personally backed it, but left Truman no written policy beyond a brief agreement with Winston Churchill. That agreement suggested that the weapon might be used against Japan after mature consideration, with a warning that bombardment would continue until surrender. It did not settle what counted as mature consideration, what kind of target was acceptable, or how the new weapon should be treated after the war.

The most basic technical uncertainty remained. Groves reported that the first test would not be ready until early July, and no one could yet say whether the bomb would work. Even estimates of its explosive force varied widely: the scientific panel’s projections ranged from the equivalent of 2,000 to 20,000 tons of TNT. The uncertainty was not merely about the difference between a smaller and a larger blast. Stimson feared that the weapon could change the political and economic structure of the country and the future of civilization. He regarded it as potentially a revolutionary transformation in humanity’s relation to the universe, one that might lead either toward civilization’s doom or its perfection. No one could establish which future was more likely.

The Decision Before the Test

The decision about Japan developed alongside, but was not initially merged with, conventional invasion planning. In his diary, Truman described the choice as whether to invade Japan proper or to bomb and blockade it, and called it his hardest decision to date. The entry did not mention the atomic project. At the June 18 White House meeting on invasion plans, the bomb again went unmentioned. General George C. Marshall proposed a two-stage invasion beginning in November with Kyushu, only 350 miles from Okinawa. He warned that the operation would be as difficult as Normandy and estimated about 31,000 losses in the first thirty days, while acknowledging that casualties were hard to predict. To John J. McCloy, the discussion proceeded as though the bomb did not exist. The contrast reveals how decision-makers were still weighing an invasion’s foreseeable costs while the weapon’s availability and effects remained uncertain.

Stimson’s concern also extended beyond the immediate military choice. On June 6, he told Truman that the destruction from B-29 fire raids had troubled him, and that precision bombing in Japan was no longer possible because industries were scattered among workers’ homes. The atomic question therefore entered a setting in which the distinction between industrial targets and civilian lives was already difficult to maintain. The possible military advantage of a new weapon had to be considered against an emerging awareness of the scale of devastation inflicted on Japanese cities.

At the final meeting of the Interim Committee on S-1, held at the Pentagon on May 31 and June 1, the committee and its scientific advisers considered the bomb’s use, relations with the Soviet Union, and the possibility of inviting Soviet scientists to observe the first test. The group rejected that proposal and agreed that production and research should move ahead rapidly to maintain an advantage, while efforts to improve political relations with the Soviet Union continued. Its unanimous recommendations were that the bomb be used against Japan as soon as possible, against war plants surrounded by workers’ homes or other vulnerable buildings to create a powerful psychological effect, and without warning. James F. Byrnes reported the conclusions to Truman, who, according to Byrnes’s later account, reluctantly agreed because he could think of no alternative. The recommendations converted uncertainty into a proposed course of action, but they did not eliminate the unknowns on which that course rested.

Dissent and the Unanswered Moral Question

A letter from O. C. Brewster, an engineer who had worked on uranium isotope separation, set out a sharply different response to the project’s changed circumstances. While Germany remained a potential threat, Brewster argued, racing to develop the weapon had seemed necessary. After Germany’s defeat, he urged that the project stop. He proposed demonstrating a single bomb in Japan, then ending further production of nuclear material. He feared that accepting greater casualties in the war against Japan could be preferable to unleashing unrestrained competition in the production of atomic material. His appeal made the future arms race part of the immediate decision rather than a distant afterthought.

Whether Truman read Brewster’s letter is unknown. It reached the White House and was returned without any recorded sign of presidential reaction. The uncertainty surrounding the letter is itself significant: the record cannot establish whether this argument shaped Truman’s thinking. His own surviving diary entry for June 3 offers little explanation, noting only that he had been going through hectic days. Later, he acknowledged that he understood an atomic explosion would cause damage and casualties beyond imagination. The evidence records awareness of dreadful consequences, but does not supply a complete account of how he weighed them at the time.

Stimson, too, pressed Truman to see beyond the weapon’s capacity to shorten the war. He wanted a special select committee to study the implications of the new force and advise the President; Truman authorized him to proceed. At the later committee session, Stimson warned that the bomb was not merely a new weapon but a revolutionary change with consequences that might reach the fate of civilization. The forecasts, the letter, and the committee’s deliberations thus exposed different dimensions of the same uncertainty: what the bomb could do physically, whether its use could be justified militarily, and what precedent its use would establish.

The State of Uncertainty

The defining feature of the episode is that Truman’s responsibility grew faster than the available certainty. He inherited an immense and highly compartmentalized program, received a stark warning from Stimson, and learned that the weapon might be ready within months. Yet the first test lay ahead; estimates of explosive force differed by an order of magnitude; the likely time before the Soviet Union could develop a bomb ranged from three to five years for the scientists and as long as twenty years in Groves’s personal estimate. Even the project’s origin in fear of Nazi progress could not answer what the United States should do after Germany’s defeat.

The evidence does not present uncertainty as an excuse for inaction or as proof that a single alternative was clearly available. It shows choices being made under pressure while technical forecasts, military calculations, diplomatic assumptions, and moral warnings remained unsettled. The Interim Committee recommended use without warning; Brewster urged a demonstration followed by an end to production; Stimson asked for sustained study of the weapon’s larger implications. Truman had authorized that inquiry, but no inquiry could make the consequences fully knowable before the decision had to be faced. The state of uncertainty was therefore both a condition of presidential decision-making and one of the bomb’s first political effects: it forced the government to act before it could know what kind of world the weapon would help create.