The Manhattan Project: Making the Atomic Bomb

The Potsdam Proclamation

A directive, written by Groves and issued by Stimson and Marshall on July 25, ordered the Army Air Force's 509th Composite Group to attack Hiroshima, Kokura, Niigata, or Nagasaki "after about" August 3, or as soon as weather permitted.⁵⁵ The 509th was ready. Tests with dummies had been conducted successfully, and Operation Bronx, which brought the gun and uranium-235 projectile to Tinian aboard the U.S.S. Indianapolis and the other components on three C-54s, was complete. On July 26 the United States learned of Churchill's electoral defeat and Chiang Kai-Shek's concurrence in the warning to Japan. Within hours the warning was issued in the name of the President of the United States, the president of China, and the prime minister of Great Britain (now Clement Attlee). The Russians were not informed in advance. This procedure was technically correct since the Russians were not at war with Japan, but it was another indication of the new American attitude that the Soviet Union's aid in the present conflict no longer was needed. The message called for the Japanese to surrender unconditionally or face "prompt and utter destruction."⁵⁶ The Potsdam Proclamation left the emperor's status unclear by making no reference to the royal house in the section that promised the Japanese that they could design their new government as long as it was peaceful and more democratic. While anti-war sentiment was growing in Japanese decision-making circles, it could not carry the day as long as unconditional surrender left the emperor's position in jeopardy. The Japanese rejected the offer on July 29.

Intercepted messages between Tokyo and Moscow revealed that the Japanese wanted to surrender but felt they could not accept the terms offered in the Potsdam Proclamation. American policy makers, however, anxious to end the war without committing American servicemen to an invasion of the Japanese homeland, were not inclined to undertake revisions of the unconditional surrender formula and cause further delay. A Russian declaration of war might convince Japan to surrender, but it carried a potentially prohibitive price tag as Stalin would expect to share in the postwar administration of Japan, a situation that would threaten American plans in the Far East. A blockade of Japan combined with conventional bombing was rejected as too time-consuming and an invasion of the islands as too costly. And few believed that a demonstration of the atomic bomb would convince the Japanese to give up. Primarily upon these grounds, American policy makers concluded that the atomic bomb must be used. Information that Hiroshima might be the only prime target city without American prisoners in the vicinity placed it first on the list. As the final touches were put on the message Truman would issue after the attack, word came that the first bomb could be dropped as early as August 1. With the end now in sight, poor weather led to several days' delay.

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