A decade ago, heated controversy marked the 50th anniversary of the dropping of the atomic bomb on Hiroshima in August 1945. Some historians said the bomb had been necessary to end the war and save hundreds of thousands of American lives, while revisionists insisted that Japan had been on the verge of capitulating and would have offered a surrender if only the Truman administration had facilitated it.

Since that contentious anniversary, a “middle ground” school of thought about the bombing has emerged. Writing in Diplomatic History (April 2005), J. Samuel Walker, the historian of the U.S. Nuclear Regulatory Commission, surveys the recent literature. It seems that Japanese leaders were not, in fact, ready to surrender when the bomb was dropped on August 6, but had it (and the second A-bomb, dropped on Nagasaki three days later) not been used, they might have become reconciled to surrender before the U.S. invasion that was planned for 12 weeks later.

Revisionists such as Gar Alperovitz, whose 1965 book, Atomic Diplomacy, initiated the decades-long debate, maintained not only that the planned invasion would not have been as costly as it was later portrayed by President Harry S. Truman and others, but also that there were alternative routes to victory. Japan was already on the verge of surrender, the revisionists contended, citing a July 12, 1945, message from Japanese foreign minister Shigenori Togo to a Japanese ambassador, in which Togo said that “so long as England and the United States insist upon unconditional surrender the Japanese Empire has no alternative but to fight on with all its strength.”

Truman knew of that cable, as an entry in his diary six days later shows. So the administration should have moderated its demand for unconditional surrender by making it clear that the Japanese emperor could remain as titular head of state. It should also have let the shock of a Soviet invasion of Japanese-controlled Man­churia bring Japan to its senses. (“Fini Japs when that comes about,” Truman wrote in his diary after Stalin told him in July that the Soviets would, in Truman’s paraphrase, “be in the Jap war on August 15th.”)

In response to the revisionists, so-called traditionalist historians such as Robert H. Ferrell, of Indiana University, Robert James Maddox, of Pennsylvania State University, and Robert P. Newman, of the University of Pittsburgh, rejected the claim that Japan was ready to surrender. The Japanese officials who favored peace were not in charge; militants controlled the government. For these historians, dropping the bomb was the only way to defeat diehard Japan without an invasion that would have cost a huge number of American lives—at least 500,000 dead and wounded, Truman said in his memoirs a decade later.

But the traditionalists, says Walker, were too ready to accept the high casualty estimates, and they didn’t pay enough attention to the uncertainties in the documentary record. For example, just three days after Hiroshima, Truman said in a radio address that he’d used the bomb “to save the lives of thousands and thousands of young Americans.” If Truman thought then that the bomb would save hundreds of thousands of American lives, why wouldn’t he have said so? The revisionists, on the other hand, failed to understand that the projected number of casualties was “far less important to Truman than . . . ending the war at the earliest possible moment in order to prevent as many U.S. casualties as possible.”

Walker recounts that, “drawing on American sources and important Japanese material opened after the death of Emperor Hirohito in 1989,” historian Richard B. Frank, author of Downfall: The End of the Imperial Japanese Empire (1999), “showed beyond reasonable doubt that the Japanese government had not decided to surrender before Hiroshima.” Sadao Asada, of Doshisha University in Kyoto, also drawing heavily on Japanese wartime sources, reached the same conclusion. “In the end,” he wrote in 1998, “it was the Hiroshima bomb that compelled them to face the reality of defeat.”

Revisionist historians generally ignored the serious drawbacks U.S. officials saw in the alternative approaches to bringing about a Japanese surrender, Walker says. American policymakers were “far from certain that the Soviet invasion of Manchuria would be enough in itself to force a Japanese surrender.” According to a June 1945 analysis by high-ranking planners, “it would require an invasion or ‘imminent’ invasion, combined with Soviet participation in the war, to force a Japanese surrender.” Whatever Truman may have meant by “Fini Japs when [Soviet entry] comes about,” Walker believes that the remark did not reflect the analysis of the president’s top military advisers.

Drawing on Soviet as well as Japanese sources, Tsuyoshi Hasegawa, a historian at the University of California, Santa Barbara, parts company with many other “middle grounders,” says Walker, “by arguing that the bombing of Hiroshima was less important in convincing the Japanese to surrender than Soviet entry into the war.” But in Hasegawa’s view, neither of itself was a “knockout punch.”

Thomas W. Zeiler, a historian at the University of Colorado, Boulder, and author of Unconditional Defeat: Japan, America, and the End of World War II (2003), argues, contrary to the view of Alperovitz and some other revisionists, that the decision to use the bomb was not dictated by a desire to intimidate the Soviets: “The context of the ongoing Pacific War, and the objective of finally crushing an implacable foe, overrode considerations of U.S.-Soviet diplomacy at this time.”

Writing in various scholarly journals during the 1990s, Barton J. Bernstein, a historian at Stanford University, “rejected the revisionist contention that the war could have ended as soon [as] or even sooner than it did without dropping the bomb,” Walker says. “He argued that none of the alternatives available to U.S. policymakers would have brought the war to a conclusion as rapidly as using the bomb. And he doubted that any of the alternatives, taken alone, would have been sufficient to force a prompt Japanese surrender. Bernstein also suggested, however, that it seemed ‘very likely, though certainly not definite,’ that a combination of alternatives would have ended the war before the invasion of Kyushu began on 1 November 1945.”

Traditionalist historians “too lightly” dismissed that possibility, Walker contends. In the 12 weeks before the invasion, “the combination of Soviet participation in the war, the continued bombing of Japanese cities with massive B-29 raids, the critical shortage of war supplies, the increasing scarcity of food and other staples . . . , and diminishing public morale could well have convinced the emperor to seek peace.”

So though the bomb “was probably not necessary to end the war within a fairly short time before the invasion took place,” Walker concludes that it “was necessary to end the war at the earliest possible moment and in that way to save American lives, perhaps numbering in the several thousands.” And for the American president, saving those thousands of American lives that would be lost if the war continued was “ample reason to use the bomb.”