Just some points from that book "now it can be told" -
First, as an aside on the issue of whether or not to "warn" Japan - General Groves relates that this was not done because of a number of quite obvious reasons:
1. The Japanese (probably by design, but at least by mistake) did not declare war on the U.S. before bombing Pearl.
2. The distinct possibility that the enemy would make a much greater effort to shoot down the bomber. (Given the premise that "warning" would also name the target. Otherwise, the warning would be pretty worthless...).
3. The limited possibility that the bomb might fizzle and then the warning itself would be taken as a reason to extend the war.
4. The fact that the Japanese were in fact so fanatical (in their military leadership) that a warning was not thought to be practical.
The idea of "firebombing" almost all of Japan was, of course, presented as a way to avoid the Abombs. It was strategically decided that this was in itself at least as inhumane as the Abomb, and would result in an inordinate loss of American air crews. It is also true that many of the scientists who worked on Manhattan were deeply troubled about the effect, and lobbyied to avoid its use.
The "notification" of the British was really quite limited, but they did in fact concur with Truman on the use of the weapon.
Anyway, this was a horrible event in anybody's eyes, and I for one trust the U.S. far more than say, Iran or North Korea, to keep these weapons out of use again.
James