I found this interesting - the southern states used militia to suppress slave revolts. The right to a militia independent of the federal government was important to them because of both the need to act quickly to suppress slave revolts as well as the risk that a federal army influenced by free states might not do so.

Focusing too much on this single datapoint in understanding the Second Amendment is surely an oversimplification. The authors of the Fourteenth Amendment stated that they wished to protect the right of recently liberated slaves to keep arms for self defense. It is, nevertheless, an interesting window into the reasons for the Second Amendment and the context of the passage of the Constitution.

Patrick Henry:

The 10th section of the 1st article, to which reference was made by the worthy member, militates against himself. It says, that “no state shall engage in war, unless actually invaded.” If you give this clause a fair construction, what is the true meaning of it? What does this relate to? Not domestic insurrections, but war. If the country be invaded, a state may go to war, but cannot suppress insurrections. If there should happen an insurrection of slaves, the country cannot be said to be invaded. They cannot, therefore, suppress it without the interposition of Congress. The 4th section of the 4th article expressly directs that, in case of domestic violence, Congress shall protect the states on application of the legislature or executive; and the 8th section of the 1st article gives Congress power to call forth the militia to quell insurrections: there cannot, therefore, be a concurrent power. The state legislatures ought to have power to call forth the efforts of the militia, when necessary. Occasions for calling them out may be urgent, pressing, and instantaneous. The states cannot now call them, let an insurrection be ever so perilous, without an application to Congress. So long a delay may be fatal.

Confirmed: Second Amendment Framers Wanted to Protect Slavery | BobCesca.com | Liberal Politics Blog and Podcast