It is the tragedy of warfare that terror, war on civilians as well as their property, has always around. It didn't begin with WWII and the bombing of industrial populations to hinder the war effort, armies have been sowing salt onto fields and poisoning wells, as well as looting and raping and pillaging their way across a landscape in order to demoralize a people and diminish their capacity to resist. When it doesn't happen it is only because some military mind has calculated it might be more effective not to do so.

It can be argued that the Pentagon was a military target, while the WTC was not, but it could just as easily be argued that the latter represented a high value industrial target because so many financial communications links intersected there. It probably caused more damage to America's ability as an economic and industrial power than say, an attack on an Air Force or Navy base.

The possible use of nukes on some Japanese military installation could be endlessly debated, in my opinion, there simply weren't any we had identified that were worth using a weapon of questionable performance and in short supply, and if the base was remote, the Japanese people would have never learned about it. For maximum shock, a civilian target was chosen.

Perhaps a more humane choice, in terms of lives saved, would have been to drop one on the Imperial palace, but I believe it is clear we were saving the Emperor's life to use as a bargaining chip.