One major reason that world war ii inspired postwar changes in race relations was the:

The correct answer should be racist nature of the enemies of the United States

Many of the enemies of the United Stats, starting from Germany in world war 2 and later to some eastern countries, have mostly had numerous racist policies. If the United States claimed moral superiority to them then they shouldn't have had the same policies as those other countries. This is why many people claimed that they were no better than their enemies as long as they had racist policies.