Before it entered World War I the United States has declared itself to be

Before the US got involved in WW2 it declared itself to be a neutral country meaning it was not involved in the war. Despite this the US still sold arms to the allies.

so they can differentiate themselves from others as they saw themselves as the superior race. the star was basically a symbol that meant you are a less of a person than i am because of my race. the star was also degrading for those who had to wear it because they were aware of this and the nazis used the notion to make them feel less like a person.