But new research findings suggest both judgments are wrong.
A just-published Harvard University Press volume by Professor Tsuyoshi Hasegawa of the University of California, Santa Barbara, is the most comprehensive study yet undertaken of Japanese documentary sources. The highly praised study argues that the atomic bomb played only a secondary role in Japan's decision to surrender.
By far the most important factor, Hasegawa finds, was the entry of the Soviet Union into the war against Japan on Aug. 8, 1945, two days after the Hiroshima bombing.
Japanese military leaders had long been willing to sacrifice civilians and cities to American conventional bombing. What they really feared, Hasegawa points out, was the Red Army, a force which would directly challenge what was left of Japan's dwindling military capacity both on the home islands and in Manchuria. The traditional myth that the atomic bomb ended the war, he writes, "cannot be supported by historical facts."
A similar conclusion has been reached in a recent publication by another eminent Japanese scholar, Professor Herbert Bix, author of a biography of Hirohito, which won the Pulitzer Prize for nonfiction in 2000.
Long before the bombings, top American and British policy-makers were aware that a declaration of war by the Soviet Union, combined with assurances for the Japanese emperor, would likely end the conflict.
As early as April 29, 1945, for instance, U.S. intelligence advised that entry of the Soviet Union into the war would "convince most Japanese at once of the inevitability of complete defeat," and further, that if they were persuaded that unconditional surrender "did not imply annihilation, surrender might follow fairly quickly."
Many scholars have wondered about the timing of the bombings. The invasion of Japan was set to begin in November 1945, three months off. There was plenty of time to test whether the intelligence estimates of the impact of a Soviet declaration of war were correct before striking civilian targets. The bombs could have been used if the Red Army attack did not produce the expected results.
In fact, making sure the Soviet option was available in case the atomic test failed was a major U.S. priority for the first half of 1945.
Once the test succeeded in July, however, the atomic bomb was preferred because, Hasegawa and others argue, U.S. leaders, for political reasons, no longer wanted the Soviets to enter the war.
Strikingly, many American military leaders also believed the atomic bombing was unnecessary. On numerous occasions then-Gen. Dwight D. Eisenhower stated that he had urged the bomb not be used against an already defeated Japan. After the war, he put it bluntly: "It wasn't necessary to hit them with that awful thing."
The diary of President Truman's Chief of Staff, Adm. William D. Leahy, shows that he believed the war could be ended on acceptable terms in June 1945. After the war, Leahy, who also presided over the Combined British and U.S. Chiefs of Staff, wrote that "the use of this barbarous weapon at Hiroshima and Nagasaki was of no material assistance in our war against Japan. The Japanese were already defeated and ready to surrender. ... In being the first to use it, we ... adopted an ethical standard common to the barbarians of the Dark Ages."
The well-known hawk, Gen. Curtis LeMay, publicly declared shortly after the bombings that the war would have been over in two weeks, and that the atomic bomb had nothing to do with bringing about Japan's surrender.
Sixty years later, the moral challenge of Hiroshima and Nagasaki -- and the ongoing threat of future nuclear horrors -- still haunt us.
Gar Alperovitz, Lionel R. Bauman Professor of Political Economy at the University of Maryland, is author of many works on the bombing of Hiroshima, including two widely cited books, "Atomic Diplomacy: Hiroshima and Potsdam" (Simon and Schuster, 1965) and "The Decision to Use the Atomic Bomb" (Knopf, 1995). He can be reached at firstname.lastname@example.org.