" 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."

"Moreover, the enemy has begun to employ a new and most cruel bomb, the power of which to do damage is, indeed, incalculable, taking the toll of many innocent lives. Should we continue to fight, it would not only result in an ultimate collapse and obliteration of the Japanese nation, but also it would lead to the total extinction of human civilization.

Such being the case, how are we to save the millions of our subjects, nor to atone ourselves before the hallowed spirits of our imperial ancestors? This is the reason why we have ordered the acceptance of the provisions of the joint declaration of the powers."