GP's point may be rescued by changing this to: Americans may have entered the war because of Pearl Harbor, but the reason why the Allies were the "good guys" is that they "fought the nazis". The reason why nazis needed to be fought is genocide and concentration camps. But, in 1941, most people didn't know (or believe; reports were coming out since 1930s, but fell on deaf ears) in all the atrocities the nazis committed. Back in 1941, for most countries involved this was just a regular war; the extra moral significance was added post-war.