And that too, not once, but twice. I listened to a fascinating podcast from KQED San Francisco's Forum programme about Hiroshima and Nagasaki, around Hiroshima Day, August 6th.
The experts interviewed brought forth many excellent points, but as much as I tried, I could not understand the rationale for the second bomb, the one in Nagasaki. It really was dropped because the Americans could do it and get away with it. With a Japan prone after Hiroshima, the second bomb could surely have been dropped on an uninhabited area if the goal was only to demonstrate American resolve and not to terrorise civilians.
Therefore, the argument that America is a uniquely responsible power (shades of manifest destiny) that should be trusted with weapons of mass destruction is somewhat dubious.
For the answer to above why twice one does not have to exercise much.
The first one was U235 device It worked and data points in terms of death and destruction wasw gathered.
The second was Pu device and some more data points in terms of real death and destruction was accumulated.
It did not end there PU and other radio active material was injected inmto African American soldiers get even more data points to study the effects of radiation..
Read Richard Rhodes Book Dark Sun and The Making of the Atomic Bomb
Colonel E.E. Kirkpatrick of the U.S. Atomic Energy Comission issues a secret document (Document 07075001, January 8, 1947) stating that the agency will begin administering intravenous doses of radioactive substances to human subjects.