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President Obama's visit to Hiroshima on Friday has rekindled public debate about the U.S. atomic bombings of Japan — one largely suppressed since the Smithsonian canceled its Enola Gay exhibit in 1995. Obama, aware that his critics are ready to pounce if he casts the slightest doubt on the rectitude of President Harry S. Truman's decision to use atomic bombs, has opted to remain silent on the issue. This is unfortunate. A national reckoning is overdue.Most Americans have been taught that using atomic bombs on Hiroshima and Nagasaki in August 1945 was justified because the bombings ended the war in the Pacific, thereby averting a costly U.S. invasion of Japan. This erroneous contention finds its way into high school history texts still today. More dangerously, it shapes the thinking of government officials and military planners working in a world that still contains more than 15,000 nuclear weapons.Truman exulted in the obliteration of Hiroshima, calling it "the greatest thing in history." America's military leaders didn't share his exuberance. Seven of America's eight five-star officers in 1945 — Gens. Dwight Eisenhower, Douglas MacArthur and Henry Arnold, and Adms. William Leahy, Chester Nimitz, Ernest King and William Halsey — later called the atomic bombings either militarily unnecessary, morally reprehensible, or both. Nor did the bombs succeed in their collateral purpose: cowing the Soviets.Leahy, who was Truman's personal chief of staff, wrote in his memoir that theMacArthur went further. He told former President Hoover that if the United States had assured the Japanese that they could keep the emperor they would have gladly surrendered in late May.It was not the atomic evisceration of Hiroshima and Nagasaki that ended the Pacific war. Instead, it was the Soviet invasion of Manchuria and other Japanese colonies that began at midnight on Aug. 8, 1945 — between the two bombings.For months, Allied intelligence had been reporting that a Soviet invasion would knock Japan out of the war. On April 11, for example, the Joint Intelligence Staff of the Joint Chiefs of Staff predicted, "If at any time the USSR should enter the war, all Japanese will realize that absolute defeat is inevitable."The Americans, having broken Japanese codes, were aware of Japan's desperation to negotiate peace with the U.S. before the Soviets invaded. Truman himself described an intercepted cable from July 18, 1945, as the "telegram from the Jap emperor asking for peace." Indeed, Truman went to the mid-July summit in Potsdam to make sure that the Soviets were keeping their Yalta conference promise to come into the Pacific war. When Stalin gave him the assurance on July 17, Truman wrote in his diary, "He'll be in the Jap War on August 15. Fini Japs when that comes about." Truman reiterated this in a letter to his wife the next day: "We'll end the war a year sooner now, and think of the kids who won't be killed."In quickly routing Japan's Kwantung army, the Soviets ruined Japan's diplomatic and military end game: keep inflicting military losses on the U.S. and get Stalin's help negotiating better surrender terms.The atomic bombings, terrible and inhumane as they were, played little role in Japanese leaders' calculations to quickly surrender. After all, t. Hiroshima and Nagasaki were just two more cities destroyed; whether the attack required one bomb or thousands didn't much matter. As Gen. Torashirō Kawabe, the deputy chief of staff, later told U.S. interrogators, the depth of devastation wrought in Hiroshima and Nagasaki only became known "in a gradual manner." But "in comparison, the Soviet entry into the war was a great shock."When Prime Minister Kantaro Suzuki was asked on Aug. 10 why Japan needed to surrender so quickly, he explained, "the Soviet Union will take not only Manchuria, Korea, Karafuto, but also Hokkaido. This would destroy the foundation of Japan. We must end the war when we can deal with the United States." Japanese leaders also feared the spread of Soviet-inspired communist uprisings and knew the Soviets would not look kindly upon their paramount concerns — protecting the emperor himself and preserving the emperor system.Truman understood the stakes. He knew the Soviet invasion would end the war. He knew assuring Japan about the emperor might also lead to surrender. But he decided to use the atomic bombs anyway.While at Potsdam, Truman received a report detailing the power of the bomb tested July 16 at Alamogordo, N.M. Afterward he "was a changed man," according to Winston Churchill. He began bossing Stalin around. And he authorized use of the bomb against Japan. If his newfound assertiveness at Potsdam didn't show Stalin who was boss, Truman figured, Hiroshima certainly would.Stalin got the message. Atomic bombs were now a fundamental part of the U.S. arsenal, and not just as a last resort. He ordered Soviet scientists to throw everything they had into developing a Soviet bomb. The race was on. Eventually, the two sides would accumulate the equivalent of 1.5 million Hiroshima bombs. And as Manhattan Project physicist I.I. Rabi astutely observed, "Suddenly the day of judgment was the next day and has been ever since."Oliver Stone is an Academy Award-winning writer and director. History professor Peter Kuznick is director of the Nuclear Studies Institute at American University. They co-authored the Showtime documentary series and book, The Untold History of the United States.