The simple storyline is that the Japanese ignored the Potsdam Declaration (see 17th July in this timeline) and refused to unconditionally surrender so the USA exploded the first atomic bomb used aggressively, in the air above the city of Hiroshima. At the time the single decision to drop this bomb was advertized as an effort to hasten the end of World War II. But subsequent historical research shows there were several reasons feeding into this decision and many people have argued that it was not even necessary to have used this bomb; the Japanese would have surrended a few days later anyway.  This section of the timeline touches briefly on some of these topics, this pivotal event of 1945 deserves your attention to the further readings listed below.

The 11th June entry on this timeline provides you with a simple explanation of how an atom bomb works and a short summary of the history of the Manhattan Project which produced the 2 bombs that essentially destroyed Hiroshima and Nagasaki.  If you haven’t read that entry yet it would be good to do so now.

The release at 08:15 (Hiroshima time) of the bomb, called “Little Boy”, from the B-29 plane called the “Enola Gay” went as planned.  Little Boy, containing about 64 kg (141 lb) of uranium-235, took 44.4 seconds to fall from the aircraft flying at about 31,000 feet (9,400 m) to a detonation height of about 1,900 feet (580 m) above the city. Due to crosswind, the bomb missed the aiming point, the Aioi Bridge, by approximately 800 ft (240 m) and detonated directly over Shima Surgical Clinic. It released the equivalent energy of 16 kilotons of TNT (67 TJ), ± 2 kt. The weapon was considered very inefficient, with only 1.7% of its material fissioning. The radius of total destruction was about 1 mile (1.6 km), with resulting fires across 4.4 square miles (11 km2.)

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People on the ground reported a pika (ピカ)—a brilliant flash of light—followed by a don (ドン)—a loud booming sound. Some 70,000–80,000 people, around 30% of the population of Hiroshima at the time, were killed by the blast and resultant firestorm, and another 70,000 were injured. It is estimated that as many as 20,000 Japanese military personnel were killed. U.S. surveys estimated that 4.7 square miles (12 km2) of the city were destroyed. Japanese officials determined that 69% of Hiroshima's buildings were destroyed and another 6–7% damaged.

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Recall that the Allies had gotten “into the habit” of bombing their enemies’ cities (see timeline entries for 13th February and 9th March). So 330 B-29 bombers had unleashed enough incendiary bombs to kill at least 100,000 people in Tokyo, in contrast one single A-bomb had inflicted about the same number of ultimate deaths (because many 10,000s died later of radiation sickness) and a similar destruction of buildings in Hiroshima.

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President Truman’s explanation to the US public for using this bomb was that it avoided an awful bloodbath of both side’s military personnel  that would have ensued if the Allies had made a land invasion into the mainland of Japan. He was referring to the huge loss of military on both sides and of civilians during the Okinawa invasion (see timeline entry for June 21st).

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But why didn’t the US use the existence of the A-bomb as a threat to ensure a quick surrender? A real demonstration could have been done in Tokyo Bay, within sight of the Emperor and his military leaders. Close enough for an awesome effect but not too close to heavily populated land areas.  But no, other outcomes were desired by the actual exploding of the A-bomb over a populated city: it would punish Japan for Pearl Harbor and war atrocities, help impose American terms in a surrender, justify the enormous expenditures of the Manhattan Project (around two billion US dollars), and as a possible bonus, also frighten the Soviet Union (who did not yet have an A-bomb) and make the Soviets more tractable in the postwar period. The anti-Soviet motives in the use of the A-bomb were important, although not recognized by the US public at the time.

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But the Japanese did not surrender after the Hiroshima bombing (all communication lines out of the city were broken and the Japanese leaders did not know the full extent of the damage right away) and so the US dropped an even stronger A-bomb on Nagasaki 3 days later (see 9th August timeline entry).

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Further readings:

 

https://en.wikipedia.org/wiki/Atomic_bombings_of_Hiroshima_and_Nagasaki

 

http://blog.nuclearsecrecy.com/2015/08/03/were-there-alternatives-to-the-atomic-bombings/