Why did the US choose to use the atomic bomb?

Answer 1

We did not specifically choose either Nagasaki nor Hiroshima for the atomic bombing. We chose a target list of 6 cities, with the primary criteria being they had not already been significantly bombed conventionally. This criteria was used both to help the Japanese quickly see the destructiveness of this new bomb and to help the US Military evaluate bomb effects after the Japanese surrender. On the original list Kyoto was included, but Stimson demanded it be removed due to its cultural significance so Nagasaki was selected to replace Kyoto.

Actual target selections for the attacks were made a couple days before by the field commanders from this list, a primary and a secondary target for each attack. IIRC the primary and secondary target cities for each attack were:

1. August 6, 1945 - primary Hiroshima - secondary Kokura
2. August 9, 1945 - primary Kokura - secondary Nagasaki

The bomb was actually dropped on Nagasaki instead of Kokura on the 9th due to a combination of bad weather and some mechanical problems preventing access to fuel in one tank of the bomber on that mission (Bocks Car). So in a way you could say that the weather choose Nagasaki to atomic bomb as much as the US did.

Had it been necessary to continue the atomic bombing, the Manhattan Project factories were scheduled and ready to produce a total of 23 bombs for delivery on Japanese cities (not just the 2 used) before the end of 1945.

Answer 2

It was that or sending more troops to die in Japan's main land.