The Second World War was the Good War from the Allied perspective, but no nation escaped the vicious conflict with its standards of moral conduct unaltered. The biggest change was probably the breakdown of the prewar consensus that cities and civilians are not legitimate military targets. Germany’s destruction of Rotterdam and Japan’s violation of Nanking were repaid by the Allied firebombings of Hamburg, Dresden and Tokyo. And after these eradications, it must have seemed a small step to President Harry Truman and his advisers to use atomic firestarters over Hiroshima and Nagasaki, killing some 200,000 Japanese civilians.

Many contemporary critics, however, blanched at the new level of horror the atomic bombs visited on noncombatants, including Time magazine founder Henry Luce, the Federal Council of Churches, Albert Einstein-even Dwight D. Eisenhower. Why, then, did the draft script for the 1995 Enola Gay exhibit at the Smithsonian Institution’s National Air and Space Museum, which presented the full historical debate over Truman’s decision, provoke a firestorm of indignation hot enough to cause the original exhibit’s cancellation?

Hiroshima’s Shadow offers a wealth of explanations and rejoinders for what its editors call this “denial of history.” The exhibit’s foremost fault may have been foolish timing. Veterans and other Americans celebrating the 50th anniversary of V-J Day were in no mood to dissect the moral and political complexities of the war’s endgame. But more to the point, much of the public apparently remains satisfied with Truman’s postwar justification that using the bombs was the only alternative to expending the lives of up to 1 million American soldiers in the planned invasion of the Japanese home islands.

In more than 60 archival documents and new essays by prominent historians such as Gar Alperovitz, Barton Bernstein and John Dower, the book lays this notion securely to rest. Military and political leaders knew in the summer of 1945 that Japan was on the brink of surrender, and that the invasion, even if it were necessary, would cost fewer than 50,000 American lives. This somber, impressive volume also offers inescapable evidence, much of it gathered from recently declassified materials, that U.S. decision-makers had a range of other concerns, among them a wish to gain the upper hand in postwar Asia by forcing Japanese capitulation before the Soviet Union’s planned declaration of war and fear of congressional investigations should the war end before the costly Manhattan Project produced demonstrable results. Immediate pressures like these, Hiroshima’s Shadow reminds us, can all too easily obscure moral compromises-until long after the events in question are irrevocable.