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Immediately following the death of his predecessor in April of 1945, brand new President Harry S. Truman had no prior knowledge of the the ultra secret Manhattan Project. He was given a briefing about it by Secretary of War Henry L. Stimson on the day that President Franklin Delano Roosevelt died, following his first Cabinet meeting as President. While in Europe for the Potsdam Conference, he learned the news that the test of the first atomic bomb on July 16, 1945 had been successful. He hinted to Joseph Stalin that the U.S. was about to use a new kind of weapon against the Japanese. Though this was the first time the Soviets had been officially given information about the atomic bomb, Stalin was already aware of the bomb project, having learned about it through his spies long before Truman did.
In August, after the Japanese government refused the terms of the Potsdam Declaration, Truman authorized the use of atomic weapons against Japan. On Sunday morning, August 6, 1945, at 8:15 am local time, the B-29 bomber Enola Gay dropped a uranium-fueled atomic bomb, Little Boy, on Hiroshima. Two days later, after Truman's broadcast warning of further attacks, without any response from the Japanese government, Truman authorized the dropping of a second atomic bomb. On August 9, Nagasaki was devastated using a plutonium implosion-type atomic bomb, Fat Man, dropped by the B-29 bomber Boxcar. The bombs killed an estimated 140,000 people in Hiroshima and 80,000 in Nagasaki by the end of 1945, with roughly half of those deaths occurring on the days of the bombings. Truman received news of the bombing while aboard the heavy cruiser USS Augusta (CA-31) on his way back to the U.S. after the Potsdam Conference. The Japanese surrender came on August 14.
Here is a YouTube video of Truman's warning to Japan:
Supporters of Truman's decision have argued that, given the tenacious Japanese defense of the outlying islands, the bombings saved hundreds of thousands of lives that would have been lost in an invasion of mainland Japan. In 1954, Eleanor Roosevelt said that Truman had "made the only decision he could," and that the bomb's use was necessary "to avoid tremendous sacrifice of American lives." The first day of fighting on Iwo Jima had cost more American casualties than D-day. On Okinawa, 79,000 U.S. soldiers were killed or wounded. As the U.S. readied plans to invade the main islands, Japan was deploying up to 2 million soldiers and additional millions of "auxiliaries" who were clearly prepared to defend their homeland to the death. Truman had been given estimates that an invasion would result in as many as a million American casualties, plus many more Japanese. He viewed the Bomb as a means to ending the war early and saving lives.
Truman always staunchly defended the atomic bombings. Shortening the war, saving American lives, and revenge are the main reasons he gave for using them. In his first public explanation (August 6, 1945, just after Hiroshima was bombed), he said "The Japanese began the war from the air at Pearl Harbor. They have been repaid many fold." On August 9, after Nagasaki was bombed, Truman made another public statement on why the atomic bombs were used. He said:
"Having found the bomb we have used it. We have used it against those who attacked us without warning at Pearl Harbor, against those who have starved and beaten and executed American prisoners of war, against those who have abandoned all pretense of obeying international laws of warfare. We have used it in order to shorten the agony of war, in order to save the lives of thousands and thousands of young Americans."
Dwight Eisenhower, then a General and the Supreme Allied Commander in Europe, had a different view about using the Atomic Bomb. He later wrote in his autobiography The White House Years: Mandate for Change: 1953-1956: A Personal Account as follows:
"In 1945 ... , Secretary of War Stimson visited my headquarters in Germany, [and] informed me that our government was preparing to drop an atomic bomb on Japan. I was one of those who felt that there were a number of cogent reasons to question the wisdom of such an act.... During his recitation of the relevant facts, I had been conscious of a feeling of depression and so I voiced to him my grave misgivings, first on the basis of my belief that Japan was already defeated and that dropping the bomb was completely unnecessary, and second because I thought that our country should avoid shocking world opinion by the use of a weapon whose employment was, I thought, no longer mandatory as a measure to save American lives. It was my belief that Japan was, at that very moment, seeking some way to surrender with a minimum loss of 'face.' The Secretary was deeply perturbed by my attitude, almost angrily refuting the reasons I gave for my quick conclusions."
Even with the benefit of hindsight, the question of the ethics of using the atomic bomb continues to be a subject of controversy. Was Truman right to authorize the use of the atomic bomb on the Japanese, or did he let a very terrible genie out of the bottle, paving the way for future nuclear catastrophe?