Sixty years ago, on Aug. 6, 1945, the United States detonated an atomic bomb over Hiroshima, a city of more than 300,000 people.
Just after the blast, the temperature at ground zero exceeded 5,000 degrees Fahrenheit, and those on the ground were roasted alive, vaporized, or grievously injured. Thousands of bodies could be seen floating in the river.
The weapon destroyed 90 percent of Hiroshima's buildings, and later, a radioactive rain, black and deadly, fell upon the city. Some 130,000 people perished that day, including 110,000 civilians.
Three days later, the United States dropped a second atomic bomb on Japan, killing 35,000 to 40,000 people in Nagasaki.
President Harry Truman said the US used the bombs "against those who attacked us without warning at Pearl Harbor."
In the president's estimation, the Japanese got what they deserved. Without the bomb, argued US policymakers, defeating Japan would have required invading the island nation and spilling a vast quantity of American blood.
For years, many have reflected on the motives that drove US policymakers to use atomic weapons against Japan. Though few decisions have been more carefully scrutinized, questions persist.
Wasn't Japan about to surrender? Was the second bomb necessary? Did the United States engage in "atomic diplomacy" to send a warning to Stalin in the emerging struggle against the Soviet Union?
The most recent effort to assess US actions during that fateful summer is Tsuyoshi Hasegawa's "Racing the Enemy: Stalin, Truman, and the Surrender of Japan," a landmark book that brilliantly examines a crucial moment in 20th-century history.
Beyond evaluating the American dimension of the story, Hasegawa, a historian at the University of California, Santa Barbara, considers two related themes: the "tangled relationship" between the Soviet Union and Japan in the war's waning days, and the struggle inside the Japanese government between those who wished to end the war and those who were determined to continue fighting.
These three stories, deftly interwoven by the author, are essential for understanding how and why the war ended as it did, and Hasegawa, who has energetically mined American, Japanese, and Soviet sources, has produced a luminous exploration of a complex question.
Among the book's more provocative conclusions is the contention that the atomic bombs by themselves were not decisive in compelling Japan to surrender.
The Japanese capitulated, Hasegawa argues, only when their negotiations with the Soviet Union broke down and Stalin decided to declare war on Japan. (For several years, Moscow had maintained its neutrality toward Tokyo, but in the summer of 1945, Soviet territorial ambitions in Asia led Stalin to choose war.)
Hasegawa is no less provocative in assessing America's decision to employ atomic weapons. Truman was "eager" to drop the bomb, he writes, and was unwilling to explore alternatives for three reasons.
First, Truman wanted to avoid a land invasion of Japan, which would have killed thousands of Americans.
Second, he was determined to impose unconditional surrender on the Japanese because anything short of that would have made him appear weak. He also worried that a failure to achieve unconditional surrender might fortify those in Japan who wanted to continue fighting.
Finally, Truman hoped to end the Pacific War before the Soviet Union entered the fray against the Japanese, a development that might permit Stalin to obtain territory in Asia or demand a role in America's postwar occupation of Japan. Consequently, Truman was in a hurry to use the bomb.
As Hasegawa writes, "a race was on between the atomic bomb and Soviet entry into the war." For Truman, the bomb was the solution "to all the dilemmas he faced."
Upon learning of the successful attack on Hiroshima, Truman was jubilant. Everything was going to unfold as planned - or so he hoped. An invasion of Japan would be unnecessary, unconditional surrender would be achieved, and Soviet ambitions could be reined in.
In the final pages of this important, enlightening, and unsettling book, the author reflects upon the actions of American, Soviet, and Japanese leaders in the summer of 1945. They were neither heroes nor villains, he observes, "just men."
In a story marked by unspeakable carnage, brutal territorial acquisitiveness, and shameless mendacity, that is the only one of Hasegawa's claims that does not ring true. There was plenty of villainy to go around.
• Jonathan Rosenberg teaches American history at Hunter College of the City University of New York. His book, 'How Far the Promised Land?' on the US civil rights movement, will be published this fall.