The first atomic bomb at Hiroshima

Hiroshima following the dropping of the atomic bomb on 6 August 1945. The prominent building in the foreground was the Industry Promotional Hall, retained in its ruined state as a peace memorial.
Hiroshima following the dropping of the atomic bomb on 6 August 1945. © IWM MH 29427

At 8.15 on the morning of 6th August 1945, the Japanese city of Hiroshima was devastated by the first atomic bomb to be used as a weapon of war. The bomb, nicknamed `Little Boy’, was dropped from the USAAF B29 bomber `Enola Gay’ and exploded some 1,800 feet above the city. Delivering the equivalent of around 12.5 kilotons of TNT, the bomb reduced 5 square miles of the city centre to ashes and caused the deaths of an estimated 120,000 people within the first four days following the blast. Many were instantly vaporised by the explosion, others died afterwards from the effects of burns and radiation.

Three days later, just after 11 on the morning of 9th August, a second atomic bomb nicknamed `Fat Man’ exploded above the city of Nagasaki. Although it was even more powerful than `Little Boy’, the destruction caused by this bomb was less than at Hiroshima due to the nature of the terrain (the original target had been the city of Kokura, but the B29 carrying the bomb had been diverted to Nagasaki because of heavy cloud cover). Nonetheless, over 2 square miles of the city were pulverised and some 73,000 people killed.


Destruction at Nagasaki

A general view of Nagasaki looking towards the hypocentre, a mile behind the Mitsubish Armament and Steel Works, seen across the Urakami River in the centre background. In the foreground is the shell of the Mitsubishi Woodworking Plant, which was unharmed by the blast, but was gutted by fire.

The two atomic explosions had the effects desired by the Allies. On 10th August the Japanese government indicated its readiness to accept defeat, subject to certain conditions. On 14th August it finally accepted the demand for unconditional surrender. The following day was declared `Victory over Japan’ or VJ Day, although it was not until 2nd September that the final Japanese surrender was signed, thereby bringing the Second World War to a formal close.

Why had the Allied powers considered it necessary to inflict such unprecedented destruction on Japanese civilians in order to bring the war to an end? At the Potsdam Conference (17th July – 2nd August 1945) the Allies formulated their terms for ending the war with Japan, which centred on that country’s acceptance of unconditional surrender, as had been the case with Nazi Germany in May. However, the Allies were also aware that whilst the Japanese Emperor Hirohito desired an end to hostilities, and would probably accept the unconditional capitulation demanded, the `hawks’ of the Japanese military and civilian leadership were totally opposed to such a humiliating condition and were ready to fight to the finish – whatever that might look like.

It was this knowledge that informed the contents of the Potsdam Declaration, in particular the statement that failure to accept unconditional surrender would result in “prompt and utter destruction” for Japan. It was no coincidence that on 16th July, the day before the opening of the Potsdam Conference, the world’s first nuclear bomb was detonated in the desert of New Mexico. It demonstrated a destructive power never before seen in a man-made device. In one split second, the face of war changed completely.

Up until that time, the defeat of Japan had been planned and prosecuted with the conventional means of land, sea and air forces. The main debate in early 1945, particularly within the US Joint Chiefs of Staff, was whether that defeat would be best achieved primarily by naval blockade and bombing of infrastructure, or by an invasion of the Japanese home islands. The latter strategy won the day, as being potentially the least costly to the Allies in the long run, and became`Operation Downfall’. It was planned to start in November 1945, and was predicted to run well into the spring of 1946. As intelligence assessments of Japan’s actual defensive dispositions began to build up, however, `Downfall’ came under increasing pressure. Estimates of likely Allied (and especially American) casualties snowballed, some running into the millions, and planners began searching desperately for alternatives. The atomic bomb that was tested at Alamogordo in July seemed to offer a very compelling one.


The Potsdam Conference

Winston Churchill, President Truman and Stalin at the Potsdam conference, 23 July 1945.
© IWM BU 9197
This photograph shows British Prime Minister Winston Churchill, American President Harry Truman and Soviet leader Joseph Stalin at the Potsdam Conference on 23 July 1945.

Winston Churchill, President Truman and Stalin met at the Potsdam conference in July 1945. Their terms for ending the war with Japan hinged on the nation's acceptance of unconditional surrender. 

US President Truman and senior government officials had been aware since June 1945 that atomic weapons were likely to be available in the very near future. In light of Japan’s apparently uncompromising response to the Potsdam Declaration, and the predicted spiralling casualty costs of `Downfall’, there was little hesitation in activating the American plans for the use of “special bombs” on Japan. For the key decision-makers at the time, dropping one or more atomic bombs on Japanese cities seemed very much the lesser evil. Even then, it took two demonstrations of the horrors of nuclear warfare to convince the Japanese hardliners that they should accept the previously unacceptable.

The cost of victory

The mushroom cloud over Nagasaki.
The mushroom cloud over Nagasaki. © IWM MH 2629

Looking back on these events some time later, Lieutenant General Leslie R Groves, former director of the `Manhattan Project’ that had developed the first A-bomb, commented:

“The atomic bombings of Hiroshima and Nagasaki ended World War II. There can be no doubt of that. While they brought death and destruction on a horrifying scale, they averted even greater losses – American, English, and Japanese”.

It was a view that generated controversy then and after as to the justification or otherwise of the use of such weapons on largely defenceless civilian targets, at such terrible cost. But the nuclear genii, once out of the bottle, could not be put back in. The ever-present threat of a nuclear option in the superpower stand-offs of the Cold War defined global politics after 1945. Hiroshima and Nagasaki raised the spectre of Mutual Assured Destruction (MAD) that has haunted the world into our present times.

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