I'm not sure it really matters that we agree upon some "point" where it all goes to shit. Like the web, I think it's pretty clear that this sort of thing happens organically over time, and it's hard to pinpoint some particular cornerstone, but easy to highlight contributing factors.
Yes, I should probably clarify that I'm not in favor of oppressing black people. Rather, I'm skeptical of arguments that it's a bad idea for the federal government to take power from the states, because historically the issue of states' rights has been synonymous with the issue of oppressing black people. Forgive me for the sarcasm.
