This is the title of a very interesting article at Truthout.

The real reason the Second Amendment was ratified, and why it says "State" instead of "Country" (the Framers knew the difference - see the 10th Amendment), was to preserve the slave patrol militias in the southern states, which was necessary to get Virginia's vote.  Founders Patrick Henry, George Mason, and James Madison were totally clear on that . . . and we all should be too.
In the beginning, there were the militias. In the South, they were also called the "slave patrols," and they were regulated by the states.