Final answer:
US Marines hated Japanese soldiers during World War II because of the extreme cruelty and inhumane treatment towards POWs and civilians, as well as racial prejudice fueled by wartime propaganda which depicted Japanese soldiers as subhuman.
Step-by-step explanation:
There are several reasons why US Marines developed a strong hatred towards Japanese soldiers during World War II. The first reason was related to the brutal acts of violence and cruelty towards prisoners of war and civilians, as experienced in events like the notorious Bataan Death March and the high number of casualties at battles such as those on Iwo Jima and Okinawa.
American soldiers were shocked and repulsed by the atrocities rumored and reported, which included the mistreatment and execution of their compatriots as well as local populations.
The second reason stemmed from racial prejudice exacerbated by wartime propaganda. The portrayal of Japanese soldiers in American media as barbaric and the embedded racial animosity intensified the hatred.
This racial bias was evident in the way the Japanese were depicted as subhuman or akin to animals, elevating the conflict to one not just of military strategy, but of a racial war where, in the eyes of many Americans including the military, the Japanese, unlike the Germans in Europe, were not respected as adversaries but rather seen as a race to be annulled.