But there was no guarantee that the novel weapons, especially the untested plutonium bomb, would work. A dud bomb or an unimpressive detonation at sea might have only emboldened the Japanese to continue the war.

There were likely only three bombs ready in August. It was not clear when more would be available. So real worries arose that the Japanese might be unimpressed, ignore the warning and ride out the future attacks in hopes there were few additional bombs left.

In the cruel logic of existential war, demonstrating rather than using a new weapon can convey to autocratic belligerents hesitancy seen as weakness to be manipulated rather than as magnanimity to be reciprocated.

By August 1945, six years after the start of World War II in Europe, some 70 million had died, including some 10 million killed by the Japanese military. Millions more starved throughout Asia and China due to the destruction and famine unleashed by Japan ? a brutal military empowered by millions of skilled civilian industrial workers.

To Americans and most of the world 75 years ago, each day in early August 1945 that the Japanese war machine continued its work meant that thousands of Asian civilians and Allied soldiers would die.

In the terrible arithmetic of World War II, the idea that such a nightmare might end in a day or two was seen as saving millions of lives rather than gratuitously incinerating tens of thousands.

It was in that bleak context that Harry Truman dropped the two bombs ? opting for a terrible choice among even worse alternatives.