in the united states since its early history, native americans, african americans, and european americans were classified as belonging to different races. for nearly three centuries, the criteria among whites for membership in these groups were similar, comprising physical appearance, assumption of non-european ancestry, and social circle. the criteria for membership in these races diverged in the late 19th century. during and after reconstruction, after the emancipation of slaves after the civil war, in the effort to restore white supremacy in the south, whites began to classify anyone with "one drop" of "black blood", or known african ancestry, to be black. such a legal definition was not put into law until the early 20th century in most southern states, but many established racial segregation of facilities during the jim crow era, after white democrats regained control of state legislatures in the south.