On August 6, 1945, a B-29 bomber, the "Enola Gay," dropped the world's first atomic bomb on the Japanese city of Hiroshima. Nobody knows how many people died in the attack; estimates range from a "low" of 65,000 to about 250,000. Whatever the number, it was part of the "terrible arithmetic of war" in which whole cities in Europe and Asia were turned into rubble, including the March 9-10, 1945, firebombing of Tokyo in which perhaps over 100,000 people died.
Was the atomic bombing justified? Yes, it was. Truman made the right call. The US naval and air campaign against the Japanese homeland, unlike the Allied bombing of Germany, seriously deteriorated Japan's industrial capacity. The strategic bombing of Japan had by mid-1945 probably cut Japanese industrial production in half. Japan was clearly going to lose the war but its leadership had no intention of recognizing that. As the American invasions of Iwo Jima and Okinawa showed, the Japanese were brave, tough, skillful, and determined defenders of their home islands. American military planners looked at the casualties from those two campaigns and extrapolated to what it would cost to invade Japan proper. The US military estimated that there would be at least one million American casualties, plus hundreds of thousands of other allied dead and wounded, and perhaps twenty to thirty times that many Japanese casualties in the case of an invasion. Such a campaign in Japan might take one to two years, would result in the total devastation of Japan, and produce a legacy of hatred and bitterness that would last a hundred years or more.
One alternative to an invasion was the slow strangulation of Japan via a naval blockade and a continuous bombing and shelling campaign. That, too, would have resulted in the deaths of millions of Japanese, and a war that would have dragged on for perhaps years.
Dropping the atomic bomb was the only viable option. We should note, of course, that even after the second bomb was dropped on Nagasaki, important elements within the Japanese military tried to prevent the Emperor from making his broadcast accepting the Allied terms. The atomic bombings allowed for a peaceful occupation and rebuilding of Japan, and within the parameters of the "terrible arithmetic" it was the least horrible of the horrible options available.
This is a day to stand in awe of science and the power it has to scare us and to amaze us. It is also a time to reflect on what real leadership, skill, and teamwork look like and can produce.