Actually, Hiroshima is in Japan, but yes, it was bombed by America. The two Japanese cities of Hiroshima and Nagasaki were bombed by American atomic bombs, the only two ever used in war (so far). The...
I would say US since the size differential gives them the upper hand on resources. Not to disparage japan at all, they are technologically superior/equal in many areas. If they were as big as the US....