WWII demonstrated that... the more the Germans bombed London, the more the English hardened against the Germans.
Japan was a different case, as atomic weapons threaten not just defeat but extinction, and there was little doubt that the US would take that route if they had to. Since it's clear that it's not possible for the US to use nukes, that's not a stick that can be used.
Bombing would on the face of it seem to help break morale, but history generally shows that it does the opposite.