" 1. Targets had to possess sentimental value in the mind of the Japanese people.
2. Targets had to have some military significance.
3. Targets had to be largely intact, to demonstrate the awesome destructive power of an atomic bomb.
4. The target had to be large in size, suitable for attack by a weapon of an atomic bomb's magnitude.
"

http://en.wikipedia.org/wiki/American_Empire

"That the greatest complication was what might happen at the meeting of the Big Three [the upcoming Potsdam Conference meeting of the U.S., Britain, and the Soviet Union]. He [Truman] told me he had postponed that until the 15th of July on purpose to give us more time. I pointed out that there might still be delay [of the atomic bomb test] and if there was and the Russians should bring up the subject and ask us to take them in as partners, I thought that our attitude was to do just what the Russians had done to us, namely to make the simple statement that as yet we were not quite ready to do it."