The attacks on Hiroshima and Nagasaki cost at least 200,000 lives but almost certainly saved many millions more

In March 1940, in an unremarkable Birmingham University office, German-born physicist Rudolf Peierls and his Austrian-born colleague Otto Robert Frisch wrote a memorandum. In it they argued that a small quantity of uranium could be used to produce a chain reaction, releasing a force equivalent to many thousands of tons of TNT. The wartime government of Winston Churchill realised the potential of their discovery and soon the highly secret MAUD Committee was created to help advance their work. The committee concluded that a nuclear weapon was feasible and this led to a project with the codename Tube Alloys.

- By Phil Craig

But the scale of the work required was very challenging for Britain and there was fear that any testing site would be in range of the Luftwaffe.

So the preliminary work was made available to the US and soon became a key part of America's own pre-existing Manhattan Project. Under the Quebec Agreement, the two nations agreed to share the results although in the end the US would become most reluctant to do so. At the 'Big Three' Yalta summit of 1944, Stalin requested and received explicit signs from the Americans that the western allies would agree to his complete control of Eastern Europe. In return he promised Roosevelt that he would join the war against Japan. Churchill thought that his frail American partner had been outmanoeuvred.

In fact - although undeniably frail (he would die aged just in April 1945, less than a month before VE Day) - Roosevelt got exactly what he came for, a reduction in the blood price of defeating Japan.

The more Soviet troops that could be brought to bear in the east, the lower US casualties would be. “This makes the trip worthwhile,” he told his Chief of Staff Admiral William Leahy.