'Less Costly Struggle and Bloodshed': The Atlantic Defends Hiroshima in 1946
Two excerpts from the archives explain how the U.S. government justified its decision to drop the bomb.

Sixty-eight years ago today, the United States dropped an atomic bomb on the Japanese city of Hiroshima, killing approximately 110,000 Japanese citizens and thrusting the world into a nuclear age.
The American public didn't know how to respond to this unprecedented military move. Should they just be glad that the war was over, and conclude that the ends justified the means? Or should they be skeptical, and question whether this final assault was really absolutely necessary?
In December 1946, just over a year after Hiroshima, The Atlantic published Dr. Karl Compton's article, "If the Atomic Bomb Had Not been Used." Compton argues that if the United States had not implemented the Manhattan Project, hundreds of thousands more lives - both American and Japanese - would have been lost. President Truman - who called Compton's article, "the first sensible statement I have seen on the subject" - expressed his support for Compton's position in a letter published in The Atlantic in February 1947. Together, Compton's article and Truman's letter explain how the U.S. government justified its decision to cause massive destruction on a scale the world had never seen before.
In the following excerpt, Compton answers the question, "Did the atomic bomb bring about the end of the war?"
That it would do so was the calculated gamble and hope of Mr. Stimson, General Marshall, and their associated. The facts are these. On July 26, 1945, the Potsdam Ultimatum called on Japan to surrender unconditionally. On July 29 Premier Suzuki issued a statement, purportedly at a cabinet press conference, scorning as unworthy of official notice the surrender ultimatum, and emphasizing the increasing rate of Japanese aircraft production. Eight days later, on August 6, the first atomic bomb was dropped on Hiroshima; the second was dropped on August 9 on Nagasaki; on the following day, August 10, Japan declared its intention to surrender, and on August 14 accepted the Potsdam terms.
On the basis of these facts, I cannot believe that, without the atomic bomb, the surrender would have come without a great deal more of costly struggle and bloodshed.
Exactly what role the atomic bomb played will always allow some scope for conjecture. A survey has shown that it did not have much immediate effect on the common people far from the two bombed cities; they knew little or nothing of it. The even more disastrous conventional bombing of Tokyo and other cities had not brought the people into the mood to surrender.
The evidence points to a combination of factors. (1) Some of the more informed and intelligent elements in Japanese official circles realized that they were fighting a losing battle and that complete destruction lay ahead if the war continued. These elements, however, were not powerful enough to sway the situation against the dominating Army organization, backed by the profiteering industrialists, the peasants, and the ignorant masses. (2) The atomic bomb introduced a dramatic new element into the situation, which strengthened the hands of those who sought peace and provided a face-saving argument for those who had hitherto advocated continued war. (3) When the second atomic bomb was dropped, it became clear that this was not an isolated weapon, but that there were others to follow. With dread prospect of a deluge of these terrible bombs and no possibility of preventing them, the argument for surrender was made convincing. This I believe to be the true picture of the effect of the atomic bomb in bringing the war to a sudden end, with Japan's unconditional surrender.
And here's how President Truman responded:
The White House
December 16, 1946
Dear Dr. Compton:--
Your statement in the Atlantic Monthly is a fair analysis of the situation except that the final decision had to be made by the President, and was made after a complete survey of the whole situation had been made. The conclusions reached were substantially those set out in your article.The Japanese were given fair warning, and were offered the terms which they finally accepted, well in advance of the dropping of the bomb. I imagine the bomb caused them to accept the terms.
Sincerely yours,
Harry S. Truman