Click to enlarge Above, "Little Boy", the first atomic bomb dropped on Hiroshima, Japan.

The Allies were still at war with Japan in July 1945. Immediately after the Trinity test, some scientists suggested that another explosion should be arranged in an uninhabited part of Japan, and witnessed by representatives of the Japanese government. They hoped that the bomb’s awful power would persuade the Japanese to surrender. They hoped that no nuclear bomb would ever be used in war.

The American military had other ideas. On 6^th August ‘Little Boy’, a bomb using uranium-235, was dropped on the Japanese city of Hiroshima killing over 100 000 people. Three days later ‘Fat Man’, a bomb with a different design and using plutonium-239, destroyed Nagasaki. The Japanese Emperor surrendered on 10^th August 1945, ending the Second World War. 

The Manhattan project represented a watershed in the relationship between physics, physicists and politics. Years later, one of the atomic scientists recalled,

Some scientists who worked on building the bomb felt an obligation to inform the public of the awesome consequences of a nuclear war. We made public speeches about the bomb and about or hopes for international understanding. We talked to civic groups, we wrote articles, we tried to create committees of well-known people, such as the Emergency Committee of Atomic Scientists, headed by Albert Einstein.

These last four decades have taught us one important lesson: negotiations to reduce the level of nuclear weapons have little chance of success in a prevailing atmosphere of fear, distrust, confrontation and increasingly offensive and defensive weapons development.

[p212 and p 217, Victor Weisskopf (1989) The Privilege of Being a Physicist]
 

Hiroshima and Nagasaki - the aftermath