I had always, maybe somewhat reflexively, argued that the atom bombs convinced a recalcitrant Japan to surrender, saving us from having to engage in an unfathomably brutal invasion. But I just came across this passage, from page 10 of this book:
<blockquote>Eisenhower had little, if any, influence on President Harry S. Truman’s decision. Not until the final Big Three conference at Potsdam in mid-July did he even learn that scientists and engineers working under the direction of the army had been trying to develop a bomb since the beginning of the war. Secretary of War Henry L. Stimson informed him that the first bomb had just been successfully tested and that this revolutionary weapon would be used to force the surrender of Japan. Eisenhower strenuously objected. Japan was already defeated and making overtures for peace, he protested, and the use of such a devastating weapon might tarnish the image of the United States at the moment of its greatest international triumph. Truman’s mind was made up, however, before hearing Eisenhower’s arguments. Truman believed that the bomb would shorten the war and save the lives of American soldiers. Furthermore, accepting the advice of Stimson, he believed that the bomb could be a “master card” in international relations, a weapon of such awesome force that it would give the United States a decisive advantage in peace negotiations with the Soviets.</blockquote>
I have a lot to learn about how our decision was made.