Daniel Snowman interviewed the former US president in 1963 and reveals what he was told. Frank Jackson thinks the bombing of Nagasaki was even more of a war crime

Clifton Daniel is right about his grandfather’s bombing of Hiroshima (‘He felt he had to do it’, 4 August). “That was no decision!” Harry S Truman told me when I interviewed him in July 1963 in the presidential library in Independence, Missouri. I was a postgraduate student and had come to ask him about Hiroshima, the case study at the centre of my recently completed thesis on presidential decision-making. At a stroke, the very core of my research was undercut by the amiable old man.

Truman, who had only become president a few weeks before Hiroshima upon the sudden death of Franklin D Roosevelt, told me how FDR’s advisers, notably the venerable secretary of war, Henry L Stimson, persuaded him that dropping the atomic bomb on people was the one initiative that might bring the war in the Pacific to a rapid end, without the need for a protracted invasion of the Japanese islands at the cost of a million more lives. When I asked Truman about the dangers of nuclear radiation, or whether the bomb was really used to impress or scare the Soviets, all this was dismissed as the dreamings of people who weren’t present at the time and were speculating about matters they weren’t competent to judge. “All the atomic bomb was,” said Truman, “was a big bomb to end the war. And it did end it too!”