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Two German scientists, Otto Hahn and Fritz Strassmann achieved the first splitting of the uranium atom, in a Berlin laboratory in December 1938. This raised the possibility that Nazi Germany might be ahead of the rest of the world in the creation of an atomic weapon. So concerned were many western scientists by these developments that they persuaded Albert Einstein to write personally to President Franklin Roosevelt in August 1939 warning of the dangers of Hitler acquiring the bomb. Roosevelt set up a committee to investigate these claims and in November of that year it recommended that an American atomic bomb be developed.

During 1940-41, even whilst a neutral nation, US atomic research efforts were accelerated. After the United States became involved in the Second World War, Roosevelt and British Prime Minister, Winston Churchill, agreed in June 1942 that the US would bear the main costs of any atomic development. British scientists already at work on their own project would pool their knowledge with their American counterparts and share in the benefits of the joint research.

This combined project was placed under the supervision of the US army engineers of the Manhattan Engineer District commanded by Brigadier General Leslie Groves. Research and development were conducted in different parts of the United States and Canada but the design and construction of the bomb was the responsibility of a centre at Los Alamos, New Mexico under its director, J. Robert Oppenheimer. At first, the funding of the “Manhattan Project” was hidden in the vast American war budget. By 1944 the White House had to inform Congressional leaders about it in order to avoid the costs being questioned in Congress. By mid-1945 the US had spent $2 billion on developing the bomb while Britain was effectively cut out of the program and the Americans alone would possess this new super weapon in these early years.

The bomb was not ready in time for use against Germany which surrendered in May 1945. The first test was successfully completed at a desert site at Alamagordo, New Mexico on 16 July and was therefore ready for use in the war against Japan. Although it was clear by this time that Japan would be defeated, US military planners believed that this would require promised Russian military assistance in China plus an American invasion of the Japanese home islands in order to achieve unconditional surrender. The US planned an invasion of the island of Kyushu in November 1945 followed by an attack upon the main Tokyo plain in March 1946. Military estimates were that these operations would not be completed until the end of 1946 and would result in half a million American and several million Japanese casualties.

The Japanese government had begun peace feelers with the Allies by July 1945 but the US insisted upon unconditional surrender and would not give any guarantees that the Emperor Hirohito would not be tried as a war criminal. The militarist faction in Tokyo then resisted any further attempts at peace discussions despite the Allied leaders meeting in Potsdam, Germany requiring the Japanese to surrender or face prompt and utter destruction.

In the early hours of Sunday 6 August 1945, a B-29 bomber the Enola Gay left the island of Tinian in the Marianas and flew for five and half-hours to the Japanese city of Hiroshima. The city of 340,000 inhabitants had been selected as a target because of its topography rather than its military value. It was located in a valley and the surrounding hills would help maximise the effects of a planned mid-air explosion. At 8:15 local time, a single uranium bomb dubbed “Little Boy” was dropped from the American bomber and denoted at 1800 feet above the city. Thirteen square kilometres of Hiroshima was razed by the explosion, up to 130,000 people perished immediately while another 70,000 would die from blast related illnesses over the next five years. On 9 August a plutonium bomb (”Fat Man”) was dropped on Nagasaki after cloud had covered the city of Kokura the original intended target. Two days later the Japanese government finally agreed to surrender unconditionally and the country was occupied by American troops.