Why do Americans like to compare Pearl Harbor with Hiroshima and Nagasaki?

I'm just wondering...you're Japanese, right? Why the Nanjing Massacre and the other rapes and vivisection and slaughter in Korea, Vietnam, the Philippines, and the Pacific is never brought up is interesting to note...and, well, why are you focusing so much on history, anyway? Don't many Japanese insist that "it's behind us" when the other Asian countries try to bring up World War 2?