The United States knew about that telegram because it had broken the Japanese code early in the war. American officials also knew that the Japanese resistance to unconditional surrender had only one condition enormously important to them: the retention of the Emperor as symbolic leader. Former Ambassador to Japan Joseph Grew and others who knew something about Japanese society had suggested that allowing Japan to keep its Emperor would save countless lives by bringing an early end to the war.

Yet Truman would not relent, and the Potsdam conference agreed to insist on unconditional surrender. Truman was determined to drop those bombs.

But why? Gar Alperovitz, whose research on that question is unmatched (The Decision to Use the Atomic Bomb, Knopf, 1995), concluded, based on the papers of Truman, his chief adviser James Byrnes, and others, that the bomb was seen as a diplomatic weapon against the Soviet Union. Byrnes advised Truman that the bomb “could let us dictate the terms of ending the war.” The British scientist P.M.S. Blackett, one of Churchill’s advisers, wrote after the war that dropping the atomic bomb was “the first major operation of the cold diplomatic war with Russia.”

On August 6, 1945, without either warning or precedent, an American plane dropped a single nuclear bomb on the Japanese city of Hiroshima. The explosion utterly destroyed more than four square miles of the city center. About 90,000 people were killed immediately; another 40,000 were injured, many of whom died in protracted agony from radiation sickness. Three days later, a second atomic strike on the city of Nagasaki killed some 37,000 people and injured another 43,000. Together the two bombs eventually killed an estimated 200,000 Japanese civilians.

Apart from the moral questions, were the atomic bombings militarily necessary? They were not. Japan already had been defeated militarily by June 1945. Almost nothing was left of the once mighty Imperial Navy, and Japan’s air force had been all but totally destroyed. US bombers rained down devastation on her cities, steadily reducing them to rubble.