In light of the recent BLM/Bundy clash, Maddow had this piece here, that claims that they are part of a movement that grew out of racism during Reconstruction. With out getting into the current events aspect of the piece(as in keeping in line with subreddit rules on scope), is her assessment of this movement's history and its various iterations fair and accurate?
Look at the date on the Posse Comitatus legislation she is citing, 1878. The law had nothing to do with the South, or Reconstruction. It was really passed in response to the 1877 Railroad Riots in which the Federal government had to take up the expense of putting down the riots when state authorities failed. Posse Comitatus was a part of a larger program to improve on the efficiency of State militias, who were then granted Federal funds as long as they could verify certain training and organizational standards. The law is the foundation of the National Guard, who are nearly professional troops who can easily be inducted into the national army in the event of war, but in the meantime can be used by governors for disaster relief and riot suppression at will.
edit: Here's an article I found on the change after finishing hearing the rest of that Maddow clip. I have a link to a published paper on the topic buried somewhere on my harddrive that I could go hunt for if needed.