I think the more crucial difference is that despite how long and bloody WWII was and how it began with a Japanese surprise attack, the US conspicuously failed to kill large numbers of Japanese people after winning - in contrast with the Mongol's [1] practice of slaughtering the inhabitants of cities that refused to open their doors when asked.

[1] The Mongol's, and many, many other armies at various points in time including many historical European forces.