6 August 1945, a clear morning. A single B-29 drops one bomb on Hiroshima, a city deliberately kept off the fire-raid lists so the effect could be measured. A light, then a city flattened and burning. Some 70,000 to 80,000 people die that day; by the end of the year about 140,000.
Why then
Japan is beaten and will not surrender on Allied terms. An invasion of the home islands is planned for November. Five months of fire-raids have already killed hundreds of thousands. The bomb is ready, and the weather over the primary target is clear.
What they wanted
Truman hopes for a shock that ends the war without an invasion, and some of his advisers for a lesson Moscow will notice. Japan's leaders hold out for terms that keep the emperor and spare them occupation.
What changed
The Soviet Union declares war on 8 August and its armies pour into Manchuria; on 9 August a second bomb falls on Nagasaki. On 15 August the emperor's recorded voice tells his people to endure the unendurable. A new kind of fear begins.
Still argued
Tsuyoshi Hasegawa argues the Soviet declaration of war shocked Tokyo more than the bombs; others put the bombs first, or a blockade that would have starved Japan anyway. Most of the dead were civilians who never set policy.