Quote Originally Posted by LegioXXXUlpiaVictrix
Technically dropped by the US, but the US were allied to China. If the Japanese hadn't done Nanjing and similar, it's very likely the US policy to Japan wouldn't have been nearly as strict as it were, probably meaning Japan would never have deemed it necessary to attack Pearl Harbor to secure their oil supply, and the US may not have been drawn into war with Japan.
A lot of assumptions that you are making here...

Other examples:
Germany and the Holocaust. How exactly would the situation have been different for Germany without the murder of e.g., 6 million Jews?
Dou you believe there would have been significantly less German casualties without this genocide? If yes - why?

How about Rwanda?
Again I do not think that the Hutu have been "genocided back" after the killing of the Tutsis