At approximately 8.15am on 6 August 1945 a US B-29 bomber dropped an atomic bomb on the Japanese city of Hiroshima, instantly killing around 80,000 people. Three days later, a second bomb was dropped on Nagasaki, causing the deaths of 40,000 more. The dropping of the bombs, which occurred by executive order of US President Harry Truman, remains the only nuclear attack in history. In the months following the attack, roughly 100,000 more people died slow, horrendous deaths as a result of radiation poisoning.
Since 1942, more than 100,000 scientists of the Manhattan Project had been working on the bomb's development. At the time, it was the largest collective scientific effort ever undertaken. It involved 37 installations across the US, 13 university laboratories and a host of prestigious participants such as the Nobel prizewinning physicists Arthur Holly Compton and Harold Urey. Directed by the Army's chief engineer, Brigadier General Leslie R. Groves, the Manhattan Project was also the most secret wartime project in history. At first, scientists worked in isolation in different parts of the US, unaware of the magnitude of the project in which they were involved. Later, the project was centralised and moved to an isolated laboratory headed by physicist J. Robert Oppenheimer in Los Alamos, New Mexico. On 16 July 1945, scientists carried out the first trial of the bomb in the New Mexico desert. President Truman received news of the successful test whilst negotiating the post-war settlement in Europe at the Potsdam Conference.
Although voices within the US Military expressed caution regarding the use of the new weapon against Japan, Truman was convinced that the bomb was the correct and only option. Six months of intense strategic fire-bombing of 37 Japanese cities had done little to break the Hirohito regime's resolve, and Japan continued to resolutely ignore the demand for unconditional surrender made at Potsdam. In such circumstances, the use of the atom bomb was seen as the best means of forcing Japan to surrender, and ending the war. The alternative, of an Allied invasion of the Japanese home islands, was expected to cost hundreds of thousands of casualties.
The effects of the attack were devastating. The predicted Japanese surrender, which came on 15 August - just six days after the detonation over Nagasaki - ended World War II. Yet the shocking human effects soon led many to cast doubts upon the use of this weapon. The first western scientists, servicemen and journalists to arrive on the scene produced vivid and heartrending reports describing a charred landscape populated by hideously burnt people, coughing up and urinating blood and waiting to die.
As questions regarding the ethical implications of the attacks grew, the US Air Force and Navy both published reports which claimed (respectively) that the conventional bombing and submarine war against Japan would have soon forced her to surrender. Joseph Grew, America's last ambassador to Japan before the war started, also publicly alleged that the Truman administration knew about (and ignored) Japanese attempts to open surrender negotiations with the US using the USSR as a mediator. At this time, another interpretation - most famously espoused in 1965 by political economist Gar Alperovitz in his book Atomic Diplomacy - emerged: the atomic bombing of Japan had been motivated by a desire to demonstrate the US's military might to the Soviet Union, about whom the Americans were increasingly nervous.
The moral aspect of the attacks upon Hiroshima and Nagasaki continues to divide historians. While some argue that the terrible long term human cost to the Japanese population can never justify the use of such weapons, others maintain that in the context of total war, it would have been immoral if atomic weapons had not been used to end the war as quickly as possible.