I'd phrase it differently, but I think it's basically what happened. Japanese atrocities against civilians in China and Korea were enormous. Japanese treatment

I'd phrase it differently, but I think it's basically what happened. Japanese atrocities against civilians in China and Korea were enormous. Japanese treatment of US PoWs was horrendous. Japanese troops continued to fight literally to the last man, just to try and kill as many Americans as they could. I think when fighting an all-out war against an opponent like this, it's hard to self-limit. Further, the Japanese approach of never surrendering produced some gargantuan casualties. The Battle of Iwo Jima [1] was apparently the turning point in American thinking. After breaking the back of Japanese Imperial Fleet, Americans figured they need to start taking territory, which meant mostly invading islands. Iwo Jima had notable, but not extreme importance. And again, the Japanese would just not surrender. Quoting Wikipedia, of the 21,000 Japanese troops, only 216 were taken prisoner, and these were mostly unconscious or disabled soldiers who could physically not fight. The rest fought tooth, nail and bayonet against each and every American, and died eventually. Add 6k American soldiers who died in that one battle, and suddenly dropping an extra nuke on a city of 100,000 looks, weirdly, like a life-saving manoeuvre. And Iwo Jima wasn't even that important, or particularly close to Japan proper, you could expect casualties to only get significantly worse. This doesn't even count the wounded, which in a military context doesn't usually mean a bloody nose. [1] https://en.wikipedia.org/wiki/Battle_of_Iwo_Jima https://en.wikipedia.org/wiki/Battle_of_Iwo_Jima