Not just the atom bombs but also firebombing of Tokyo ( http://en.wikipedia.org/wiki/Bombing_of_Tokyo
) where the number of casualties on a single raid was "greater than Dresden, Hiroshima, or Nagasaki as single events."
To make this more concrete:
I find it incredible and unbelievable that the endgame of WWII in Japan didn't have a profound impact on both Ueshibas and their students at the time.
Not to minimize the impact on Kisshomaru (who was there the whole time), but Morihei left for Iwama two years before the firebombing really started, and lived out most of the war there in peace and quiet. He even avoided all the wartime and post-war food shortages and rationing.