Besides cowing the USSR, I think that a valid war aim was fully cowing Japan. One of the lessons of WWI was that military victory wasn't enough, too many Germa

Besides cowing the USSR, I think that a valid war aim was fully cowing Japan. One of the lessons of WWI was that military victory wasn't enough, too many Germans could tell themselves afterwards that they were stabbed in the back, etc. If the goal was to defeat Japanese fascism, then it was important that no Japanese could believe such things afterwards. And for this, a shocking weapon (one plane, one city) was effective in a way that (say) a starvation blockade would not have been.