On August 6th, 1945, a US B-29 bomber dropped an atomic bomb on the Japanese city of Hiroshima, a single bomb equivalent to 20,000 tons of TNT. Hiroshima was reckoned to be a contributor to Japan's war effort, though civilian and military facilities were intermingled, as they were throughout Japan.
The Manhattan Project that created the bomb was so secret that not even Vice-President Truman knew of its existence. However, within minutes of being sworn in following the death of President Roosevelt in April, Truman was given an urgent briefing about the impending weapon. The decision to use it was his alone, though his advisors supported its use, as did Winston Churchill. Truman had four choices. He could continue with conventional bombing of Japan, bombing that had killed an estimated 330,000 people and wounded a further 473,000 between April 1944 and August 1945. More than 80,000 people had died in a single fire-bombing of Tokyo, a greater number than those killed at Hiroshima. Truman later wrote,
“Despite their heavy losses at Okinawa and the firebombing of Tokyo, the Japanese refused to surrender. The saturation bombing of Japan took much fiercer tolls and wrought far and away more havoc than the atomic bomb."
A second option was an invasion of Japan that would have killed millions, including an estimated 400,000 to 800,000 American dead and many more wounded. Iwo Jima had seen 6,200 US troops killed, and Okinawa had cost the lives of 13,000 US soldiers and sailors. The Japanese had fought fanatically, heedless of casualties, and US casualties at Okinawa had been 35%, with one in three participants killed or wounded. An attack on the mainland would, Truman thought, resemble "Okinawa from one end of Japan to the other." The prospect of the bodies of young Americans lying on its beaches and in its hillside jungles, motivated Truman to bring a decisive end to the war without that option.
There was the possibility of a test in an unpopulated area, maybe on an island, to demonstrate the bomb's power and induce Japan's surrender, but it was thought unlikely to achieve that objective. The US side wondered who on the Japanese side would assess the result, a single scientist, or a committee? And would the nation surrender what it saw as its honour on the word of so small a number? The US only had two bombs, so a test would have used half their nuclear arsenal. Truman's advisory committee told him, “We can propose no technical demonstration likely to bring an end to the war."
Truman decided on a military use on a populated area, intending it to shock Japan into a surrender. It was not a decision he made lightly. He wrote, “My object is to save as many American lives as possible but I also have a human feeling for the women and children of Japan.” Nonetheless, he recorded that, "When you have to deal with a beast you have to treat him as a beast.” When Truman announced the use of an atomic bomb to the world, he warned,
“We are now prepared to obliterate more rapidly and completely every productive enterprise the Japanese have above ground in any city. We shall destroy their docks, their factories and their communications. Let there be no mistake; we shall completely destroy Japan’s power to make war... If they do not now accept our terms, they may expect a rain of ruin from the air, the likes of which has never been seen on this earth.”
It took a second bomb, dropped on Nagasaki on August 9th, to bring about Japan's surrender a week later. A 21-year-old American second lieutenant recalled,
"When news began to circulate that [the invasion of Japan] would not, after all, take place, that we would not be obliged to run up the beaches near Tokyo assault-firing while being mortared and shelled, for all the fake manliness of our facades we cried with relief and joy. We were going to live. We were going to grow up to adulthood after all.”
Truman never regretted his decision. He made it and said, “That’s all there was to it.”