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Leslie Groves and J. Robert OppenheimerEVALUATIONS OF TRINITY
(July 1945)
Events > Dawn of the Atomic Era, 1945

Only minutes after the world's first ever atomic explosion, Leslie Groves and Robert Oppenheimer (above) began composing their report for the Secretary of War and President Truman. There was a sense of urgency surrounding this notification, as Truman had already arrived at Potsdam (outside of Berlin) to confer with other Allied leaders on the conclusion of the war with Japan.  Now that the potential of the bomb had been proven, the calculations behind the Potsdam negotiations were dramatically different.  

Harry Truman and James Byrnes (with William Leahy), Potsdam, July 16, 1945The American contingent to the Big Three conference had arrived on July 15, 1945, the day before Trinity.  The leader of the Soviet Union, Joseph Stalin, was a day late, so Truman (left) had additional time to confer with his Secretary of War, Henry Stimson, and his Secretary of State, James Byrnes.  The key issues to be decided were the interrelated questions of Soviet participation in the war against Japan and the wording of an early surrender offer that might be presented to the Japanese.  This draft surrender document received considerable attention, the sticking point being the term "unconditional."  It was clear that the Japanese would fight on rather than accept terms that would eliminateEmperor Hirohito the Imperial House or demean the warrior tradition, but American policy makers feared that anything less than a more democratic political system and total demilitarization might lead to Japanese aggression in the future.  Much effort went into finding the precise formula that would satisfy American war aims in the Pacific without requiring a costly invasion of the Japanese mainland.  In an attempt to achieve surrender with honor, the emperor (right) had instructed his ministers to open negotiations with Russia.  The United States intercepted and decoded messages between Tokyo and Moscow that made it unmistakably clear that the Japanese were searching for an alternative to unconditional surrender.  

On July 16, Secretary of War Stimson received a telegram from his special assistant on atomic issues in Washington, George L. Harrison.  It read: 

Operated on this morning.  Diagnosis not yet complete but results seem satisfactory and already exceed expectations.  Local press release necessary as interest extends great distance.  Dr. Groves pleased.  He returns tomorrow.  I will keep you posted.

George Harrison and Leslie Groves with James Conant and Vannevar Bush, White House, August 9, 1945Stimson immediately informed Truman and Byrnes that the Trinity test had been successful.  The next day Stimson informed the British Prime Minister, Winston Churchill, of the test.  Churchill expressed great delight and argued forcefully against informing the Soviet Union, though he later relented.  On July 18, while debate continued over the wording of the surrender message, focusing on whether or not to guarantee the place of the emperor, Stimson received a second cable from Harrison: 

Doctor has just returned most enthusiastic and confident that the little boy is as husky as his big brother. The light in his eyes discernible from here to HighhoId and I could have heard his screams from here to my farm.

Trinity, July 16, 1945Translation: Groves thought the plutonium weapon would be as powerful as the uranium device and that the Trinity test could be seen as far away as 250 miles and the noise heard for fifty miles.  Initial measurements taken at the Alamogordo site suggested a yield in excess of 5,000 tons of TNT.  Truman went back to the bargaining table with a new card in his hand.  Further information on the Trinity test arrived on July 21 in the form of a long and uncharacteristically excited report from Groves.  Los Alamos scientists now agreed that the blast had been the equivalent of between 15,000 and 20,000 tons of TNT, higher than generally had been predicted.  Groves reported that glass shattered 125 miles away, that the fireball was brighter than several suns at midday, and that the steel tower had been vaporized.  Though he had previously believed it impregnable, Groves stated that he did not now consider the Pentagon safe from atomic attack.  Stimson informed General George Marshall and then read the entire report to Truman and Byrnes.  Stimson recorded that Truman was "tremendously pepped up" and that the document gave him an "entirely new feeling of confidence."

George Marshall and Henry Stimson, 1942The next day, Stimson, informed that the uranium bomb would be ready in early August, discussed Groves's report at great length with Churchill.  The British prime minister was elated and said that he now understood why Truman had been so forceful with Stalin the previous day, especially in his opposition to Russian designs on Eastern Europe and Germany.  Churchill then told Truman that the bomb could lead to Japanese surrender without an invasion and eliminate the necessity for Russian military help.  He recommended that the President continue to take a hard line with Stalin.  Truman and his advisors shared Churchill’s views.  The success of the Trinity test stiffened Truman's resolve, and he refused to accede to Stalin's new demands for concessions in Turkey and the Mediterranean.  

Joseph Stalin, Harry Truman, and Winston Churchill at the Potsdam Conference, July 1945On July 24, Stimson met again with Truman.  He told the President that Marshall no longer saw any need for Soviet help, and he briefed the President on the latest atomic situation.  The uranium bomb might be ready as early as August 1 and was a certainty by August 10.  The plutonium weapon would be available by August 6.  Stimson continued to favor making some sort of commitment to the Japanese emperor, though the draft already shown to the Chinese was silent on this issue.  Truman now had to decide how he would deliver the news of the atomic bomb to Stalin.  Unbeknownst to Truman, the Soviet leader already knew.

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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), 49-50.  The two cables are quoted in 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), 383, 386.  Leslie Groves's comment that he no longer considered the Pentagon safe from attack is from Leslie R. Groves, Now It Can Be Told (New York: Harper & Row, 1962), 434.  Stimson's observations on President Harry Truman's reactions to the news are from Herbert Feis, The Atomic Bomb and the End of World War II (Princeton, NJ: Princeton University Press, 1966), 85.  The photograph of Groves with Robert Oppenheimer is courtesy the Department of Energy.  The photograph of Truman, James Byrnes, and William Leahy and the photograph of George Harrison, Groves, James Conant, and Vannevar Bush are reproduced from Hewlett and Anderson, The New World, opposite 393 and 417, respectively.  Click here for more information on the photograph of Trinity.  The portrait of Emperor Hirohito is courtesy the United States Army Signal Corps (via the Library of Congress).  The photograph of George Marshall and Henry Stimson is courtesy the Center of Military History, United States Army.  The photograph of Joseph Stalin, Truman, and Winston Churchill at the Potsdam Conference is courtesy the Truman Presidential Library.

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