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American officials, however, were already having second thoughts about entering into another partnership with the Soviets. Stalin was, to say the least, a troublesome ally in the occupation of liberated Europe. When news of the successful Alamogordo test reached Potsdam, top American officials began to view the Bomb as a way to avoid the need for Soviet involvement in the Pacific war, rather than viewing Soviet involvement as a way to avoid the need for the Bomb. Secretary of State James Byrnes, Truman's closest confidant on atomic matters, was eager to "get the Japanese affair over before the Russians got in" and felt that knowledge of America's new weapon would make the Soviets "more manageable." Secretary of War Henry Stimson, perhaps the most respected U.S. statesman of the century, was wary of using the Bomb as a diplomatic bludgeon, but even he referred to it as a "master card" in Washington's dealings with the Kremlin.
Ways to avoid dropping the Bomb were never really a matter of discussion. At one White House meeting in June, Stimson's assistant John McCloy suggested that Japan be issued a warning about the weapon and offered surrender terms that allowed the retention of the Emperor. McCloy's goal, however, was not so much to prevent the Bomb from being dropped as to avoid the need for the invasion being planned at the meeting. The secrecy surrounding the device known as S-1 was so pervasive that a hush quickly fell over the room and exploration of the options was inhibited. When Japan was issued a warning from Potsdam a month later, no explicit mention was made of either the Bomb or the Emperor. Radio Tokyo broadcast that the Japanese government would treat the warning with "silent contempt." On the island of Tinian that day, a 300-lb. lead cylinder with a core of enriched uranium was being transferred to the headquarters of Colonel Paul Tibbets' 509th Composite Group.
Perhaps the crucial factor in the decision to proceed with the atomic bombing was that none of America's leaders felt any urgency about finding a way to avoid it. The scientists had not stressed that their creation might unleash radioactive fallout that would make the Bomb a more sinister weapon than even chemical warfare. Truman and his advisers knew that the explosion would be phenomenally large, but considered it no more morally repulsive than the massive fire-bombing raids that had cremated much of Tokyo. Stimson, the man who wrestled most with these imponderables, called the Bomb "the most terrible weapon ever known," but even he considered it "as legitimate as any other of the deadly explosive weapons of modern war."
The decision, then, was from their vantage simple. If Truman had not used the Bomb, how could he have explained it to the families of the boys who would subsequently have died, be it 40,000 of them or a million? How could he have justified continuing the war, transferring weary G.I.s to the Pacific to prepare for an invasion, proceeding with the grotesque fire bombings and allowing the Kremlin to expand its grip in the Far East when he had spent $2 billion on a weapon that could produce a quick end to the conflict?