at a forthcoming change in design, probably to the so-called “mixed” implosion bomb with a core of U235 and plutonium alloyed together. Such a core could draw on the resources of both Oak Ridge and Hanford:
First one of tested type [i.e., Fat Man] should be ready at Pacific base about 6 August. Second one ready about 24 August. Additional ones ready at accelerating rate from possibly three in September to we hope seven or more in December. The increased rate above three per month entails changes in design which Groves believes thoroughly sound.
Stimson reported Harrison's several estimates to Truman on Tuesday morning, July 24. The President was pleased and said he would use them to time the release of the Potsdam Declaration. The Secretary took advantage of the moment to appeal to Truman to consider assuring the Japanese privately that they could keep their Emperor if they persisted in making that concession a condition of surrender. Deliberately noncommittal, the President said he had the point in mind and would take care of it.
Stimson left and Byrnes joined Truman for lunch. They discussed how to tell Stalin as little as possible about the atomic bomb. Truman wanted protective cover when Stalin learned that his wartime allies had developed an epochal new weapon behind his back but wanted to give as little as possible away. Byrnes also devised a more immediate reason for circumspection, he told the historian Herbert Feis in 1958:
As a result of his experience with the Russians during the first week of the Conference he had come to the conclusion that it would be regrettable if the Soviet Union entered the [Pacific] war, and… he was afraid that if Stalin were made fully aware of the power of the new weapon, he might order the Soviet Army to plunge forward at once.
But in fact Stalin already knew about the Trinity test. His agents in the United States had reported it to him. It appears he was not immediately impressed. There is gallows humor in Truman's elaborately offhand approach to the Soviet Premier at the end of that day's plenary session at the Cecilienhof Palace, stripped and shabby, where pale German mosquitoes homing through unscreened windows dined on the sanguinary conquerors. Truman left behind his translator, rounded the baize-covered conference table and sidled up to his Soviet counterpart, both men dissimulating. “I casually mentioned to Stalin that we had a new weapon of unusual destructive force. The Russian Premier showed no special interest. All he said was that he was glad to hear it and hoped we would make ‘good use of it against the Japanese.’” “That,” concludes Robert Oppenheimer dryly, knowing how much at that moment the world lost, “was carrying casualness rather far.”
If Stalin was not yet impressed with the potential of the bomb, Truman in his private diary was waxing apocalyptic, biblical visions mingling in his autodidact's mind with doubt that the atom could be decomposed and denial that the new weapon would be used to slaughter civilians:
We have discovered the most terrible bomb in the history of the world. It may be the fire destruction prophesied in the Euphrates Valley Era, after Noah and his fabulous Ark.
Anyway we “think” we have found a way to cause a disintegration of the atom. An experiment in the New Mexican desert was startling — to put it mildly…
This weapon is to be used against Japan between now and August 10th. I have told the Sec. of War, Mr. Stimson, to use it so that military objectives and soldiers and sailors are the target and not women and children. Even if the Japs are savages, ruthless, merciless and fanatic, we as the leader of the world for the common welfare cannot drop this terrible bomb on the old Capital or the new.
He & I are in accord. The target will be a purely military one and we will issue a warning statement asking the Japs to surrender and save lives. I'm sure they will not do that, but we will have given them the chance. It is certainly a good thing for the world that Hitler's crowd or Stalin's did not discover this atomic bomb. It seems to be the most terrible thing ever discovered, but it can be made the most useful.
The Tuesday Truman mentioned the new weapon to Stalin the Combined Chiefs met with their Soviet counterparts; Red Army chief of staff General Alexei E. Antonov announced that Soviet troops were assembling on the Manchunan border and would be ready to attack in the second half of August. Stalin had said August 15 before. Byrnes was anxious that the Soviets might prove uncharacteristically punctual.
That afternoon in Washington Groves drafted the historic directive releasing the atomic bomb to use. It passed up through Harrison for transmission by radio eyes only to Marshall “in order that your approval and the Secretary of War's approval might be obtained as soon as possible.” (A small map of Japan cut from a large National Geographic Society map and a one-page description of the chosen targets, which now included Nagasaki, followed by courier.) Marshall and Stimson approved the directive at Potsdam and presumably showed it to Truman, though it does not record his formal authorization; it went out the next morning to the new commander of the Strategic Air Force in the Pacific:
To General Carl Spaatz, CG, USASTAF:
1. The 509 Composite Group, 20th Air Force will deliver its first special bomb as soon as weather will permit visual bombing after about 3 August 1945 on one of the targets: Hiroshima, Kokura, Niigata and Nagasaki…
2. Additional bombs will be delivered on the above targets as soon as made ready by the project staff…
3. Dissemination of any and all information concerning the use of the weapon against Japan is reserved to the Secretary of War and the President of the United States…
4. The foregoing directive is issued to you by direction and with the approval of the Secretary of War and of the Chief of Staff, USA.
As Groves drafted the directive the metallurgists at Los Alamos finished casting the rings of U235 that fitted together to form the gun bomb's target assembly, the last components needed to complete Little Boy.
Strategy and delivery intersected on July 26 and synchronized. The
Following are our terms. We will not deviate from them. There are no alternatives. We shall brook no delay.
There must be eliminated for all time the authority and influence of those who have deceived and misled the people of Japan into embarking on world conquest…
Until such a new order is established… points in Japanese territory… shall be occupied.
… Japanese sovereignty shall be limited to the islands of Honshu, Hokkaido, Kyushu, Shikoku and such minor islands as we determine.
The Japanese military forces, after being completely disarmed, shall be permitted to return to their homes with the opportunity to lead peaceful and productive lives.
We do not intend that the Japanese shall be enslaved as a race or destroyed as a nation, but stern justice shall be meted out to all war criminals… Freedom of speech, of religion, and of thought, as well as respect for the fundamental human rights shall be established.
Japan shall be permitted to maintain such industries as will sustain her economy…
The occupying forces of the Allies shall be withdrawn from Japan as soon as these objectives have been accomplished and there has been established in accordance with the freely expressed will of the Japanese people a peacefully inclined and responsible government.
We call upon the government of Japan to proclaim now the unconditional surrender of all Japanese armed forces… The alternative for Japan is prompt and utter destruction.
