The did much to educate and provide jobs for newly freed slaves following the end of the civil war.
wade-davis bill
fourteenth amendment
freedmen's bureau
reconstruction act of 1866 which one is it

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.