Manhattan Project: Potsdam and the Final Decision to Use the Bomb, July 1945

POTSDAM AND THE FINAL DECISION TO USE THE BOMB
(Potsdam, Germany, July 1945)
Events > Dawn of the Atomic Era, 1945

After President Harry S. Truman received word of the success of the Trinity test, his need for the help of the Soviet Union in the war against Japan was greatly diminished. The Soviet leader, Joseph Stalin, had promised to join the war against Japan by August 15th. Truman and his advisors now were not sure they wanted this help. If use of the atomic bomb made victory possible without an invasion, then accepting Soviet help would only invite them into the discussions regarding the postwar fate of Japan. During the second week of Allied deliberations at Potsdam, on the evening of July 24, 1945, Truman approached Stalin without an interpreter and, as casually as he could, told him that the United States had a "new weapon of unusual destructive force." Stalin showed little interest, replying only that he hoped the United States would make "good use of it against the Japanese." The reason for Stalin's composure became clear later: Soviet intelligence had been receiving information about the atomic bomb program since fall 1941.

The final decision to drop the atomic bomb, when it was made the following day, July 25, was decidedly anticlimactic. How and when it should be used had been the subject of high-level debate for months. A directive (right), written by Leslie Groves, approved by President Truman, and issued by Secretary of War Henry Stimson and General of the Army George Marshall, ordered the Army Air Force's 509th Composite Group to attack Hiroshima, Kokura, Niigata, or Nagasaki (in that order of preference) as soon after August 3 as weather permitted. No further authorization was needed for subsequent atomic attacks. Additional bombs were to be delivered as soon as they became available, against whatever Japanese cities remained on the target list. Stalin was not told. Targeting now simply depended on which city was not obscured by clouds on the day of attack.

Colonel Paul Tibbets's 509th was ready. They had already begun dropping their dummy "pumpkin" bombs on Japanese targets, both for practice, and to accustom the Japanese to overflights of small numbers of B-29s. The uranium "Little Boy" bomb, minus its nuclear components, arrived at the island of Tinian aboard the U.S.S Indianapolis on July 26, followed shortly by the final nuclear components of the bomb, delivered by five C-54 cargo planes. On July 26, word arrived at Potsdam that Winston Churchill had been defeated in his bid for reelection. Within hours, Truman, Stalin, and Clement Attlee (the new British prime minister, below) issued their warning to Japan: surrender or suffer "prompt and utter destruction." As had been the case with Stalin, no specific mention of the atomic bomb was made. This "Potsdam Declaration" 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. Anti-war sentiment was growing among Japanese civilian leaders, but no peace could be made without the consent of the military leaders. They still retained hope for a negotiated peace where they would be able to keep at least some of their conquests or at least avoid American occupation of the homeland. On July 29, 1945, the Japanese rejected the Potsdam Declaration.

There is probably no more controversial issue in 20th-century American history than President Harry S. Truman's decision to drop the atomic bomb on Japan. Many historians argue that it was necessary to end the war and that in fact it saved lives, both Japanese and American, by avoiding a land invasion of Japan that might have cost hundreds of thousands of lives. Other historians argue that Japan would have surrendered even without the use of the atomic bomb and that in fact Truman and his advisors used the bomb only in an effort to intimidate the Soviet Union. The United States did know from intercepted messages between Tokyo and Moscow that the Japanese were seeking a conditional surrender. American policy-makers, however, were not inclined to accept a Japanese "surrender" that left its military dictatorship intact and even possibly allowed it to retain some of its wartime conquests. Further, American leaders were anxious to end the war as soon as possible. It is important to remember that July-August 1945 was no bloodless period of negotiation. In fact, there were still no overt negotiations at all. The United States continued to suffer casualties in late July and early August 1945, especially from Japanese submarines and suicidal "kamikaze" attacks using aircraft and midget submarines. (One example of this is the loss of the Indianapolis, which was sunk by a Japanese submarine on July 29, just days after delivering "Little Boy" to Tinian. Of its crew of 1,199, only 316 sailors survived.) The people of Japan, however, were suffering far more by this time. Air raids and naval bombardment of Japan were a daily occurrence, and the first signs of starvation were already beginning to show.

Alternatives to dropping the atomic bomb on a Japanese city were many, but few military or political planners thought they would bring about the desired outcome, at least not quickly. They believed the shock of a rapid series of bombings had the best chance of working. A demonstration of the power of the atomic bomb on an isolated location was an option supported by many of the Manhattan Project's scientists, but providing the Japanese warning of a demonstration would allow them to attempt to try to intercept the incoming bomber or even move American prisoners of war to the designated target. Also, the uranium gun-type bomb (right) had never been tested. What would the reaction be if the United States warned of a horrible new weapon, only to have it prove a dud, with the wreckage of the weapon itself now in Japanese hands? Another option was to wait for the expected coming Soviet declaration of war in the hopes that this might convince Japan to surrender unconditionally, but the Soviet declaration was not expected until mid-August, and Truman hoped to avoid having to "share" the administration of Japan with the Soviet Union. A blockade combined with continued conventional bombing might also eventually lead to surrender without an invasion, but there was no telling how long this would take, if it worked at all.

The only alternative to the atomic bomb that Truman and his advisors felt was certain to lead to a Japanese surrender was an invasion of the Japanese home islands. Plans were already well-advanced for this, with the initial landings set for the fall and winter of 1945-1946. No one knew how many lives would be lost in an invasion, American, Allied, and Japanese, but the recent seizure of the island of Okinawa provided a ghastly clue. The campaign to take the small island had taken over ten weeks, and the fighting had resulted in the deaths of over 12,000 Americans, 100,000 Japanese, and perhaps another 100,000 native Okinawans.

As with many people, Truman was shocked by the enormous losses suffered at Okinawa. American intelligence reports indicated (correctly) that, although Japan could no longer meaningfully project its power overseas, it retained an army of two million soldiers and about 10,000 aircraft -- half of them kamikazes -- for the final defense of the homeland. (During postwar studies the United States learned that the Japanese had correctly anticipated where in Kyushu the initial landings would have taken place.) Although Truman hoped that the atomic bomb might give the United States an edge in postwar diplomacy, the prospect of avoiding another year of bloody warfare in the end may well have figured most importantly in his decision to drop the atomic bomb on Japan.

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Sources and notes for this page.

The text for this page was adapted from, and portions were taken directly from the Office of History and Heritage Resources publication: F. G. Gosling, The Manhattan Project: Making the Atomic Bomb (DOE/MA-0001; Washington: History Division, Department of Energy, January 1999), 50-51. For President Harry Truman's account of his informing Stalin about the bomb, see Harry S. Truman, Memoirs: Volume 1, Year of Decisions (New York: Doubleday & Company, 1955), 416. On the Potsdam Declaration, see the History Office publication: Richard G. Hewlett and Oscar E. Anderson, Jr., The New World, 1939-1946: Volume I, A History of the United States Atomic Energy Commission (Washington: U.S. Atomic Energy Commission, 1972), 395. The casualty figures for the Indianapolis and Okinawa are taken from Samuel Eliot Morison, The Two-Ocean War: A Short History of the United States in the Second World War (New York: Little, Brown and Company, 1963), 556, 566, and Allan R. Millett and Peter Maslowski, For the Common Defense: A Military History of the United States of America (New York: The Free Press, A Division of Macmillan, Inc., 1984), 463-464. The photographs of the Potsdam conference and of President Harry Truman are courtesy the Truman Presidential Library. Click here for more information on the picture of Potsdam and the note Truman wrote on the back of it. Click here for more information on the image of the order to drop the atomic bomb. The photograph of Paul Tibbets with his ground crew in front of the Enola Gay is reproduced from Vincent C. Jones, Manhattan: The Army and the Atomic Bomb, United States Army in World War II (Washington: Center of Military History, United States Army, 1988), 535. The photograph of "Little Boy" is courtesy the U.S. Army Corps of Engineers (via the National Archives (NARA)). The photograph of the Marine at Okinawa is courtesy the United States Marine Corps (via NARA).

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