The other historically plausible motivation is that Truman wanted to display to Stalin as a deterrent against for soviet expansion into europe that a) they had the bomb b) they were not afraid to use it c) what effect it has on cities. Japan was at war against USA at the time and effects of firebombings with traditional planes and explosives were no less horrible in cost to human lives than the A-bombs so the morale of the whole thing is ambiguous.