Why would Americans be convinced to join because of rumors of concentration camps in Europe? These were unconfirmed reports until the final month when they could be verified on the ground.
Why would they go to war with Japan over brutality in Asia? That was the case in the 1930s also and the nation did not intervene.
Who defeats Japan in World War II? I can see the Soviets defeating the Germans, by how would a partially motivated America do it if they fight to end Japanese oppression in Asia?
First of all, I never said America intervened. But during the Spanish Civil War, there were volunteers who fought in Spain's international brigade. I'm sure there would be no shortage of people who would volunteer to help out England. So my guess is that there would be generations of young people, enriched by the the war economy, horrified by stories of Nazi brutality, and eager to help out whom they consider to be allies of liberty.
As early as 42, there were already stories of concentration camps reaching American shores.
Japan, meanwhile, overextends its forces, and is gradually overwhelmed by the sheer numbers of Chinese and Soviet soldiers. There was also the fact that Japan had little food production. I recently saw a Crash Course video, that argued that Germany and Japan were motivated into invading countries by hunger as much as nationalism. According to John Green, 1 million Japanese soldiers died of starvation alone. Even if America didn't develop a conscience and stop selling Japan food, they could have easily run out of money and resources to pay for food. And since most Americans would have no sympathy for Japan, especially after the Rape of Nanking, there would be no lend-lease to keep them afloat.