Thom Hartmann points out the origin of the militia clause–to defend Southern slave patrols (boldface mine):

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.

In Georgia, for example, a generation before the American Revolution, laws were passed in 1755 and 1757 that required all plantation owners or their male white employees to be members of the Georgia Militia, and for those armed militia members to make monthly inspections of the quarters of all slaves in the state. The law defined which counties had which armed militias and even required armed militia members to keep a keen eye out for slaves who may be planning uprisings.

This ‘service’ was widespread and an integral part of maintaining slavery–slave uprisings were quite common in the 1700s (boldface mine):

…most southern men between ages 18 and 45 – including physicians and ministers – had to serve on slave patrol in the militia at one time or another in their lives.

And slave rebellions were keeping the slave patrols busy.

By the time the Constitution was ratified, hundreds of substantial slave uprisings had occurred across the South. Blacks outnumbered whites in large areas, and the state militias were used to both prevent and to put down slave uprisings. …slavery can only exist in the context of a police state, and the enforcement of that police state was the explicit job of the militias.

If the anti-slavery folks in the North had figured out a way to disband – or even move out of the state – those southern militias, the police state of the South would collapse.

In other words, the Second Amendment has to be read, in part, as a way to maintain militias under state (“State”)–and thus slaver–control, as opposed to letting the “Country” control them (a term used elsewhere in the Constitution to mean the federal government). As Hartmann describes, the Virginian contingent of the drafters of the Constitution were terrified that the Constitution would disband the militias, as Patrick Henry stated:

“If the country be invaded, a state may go to war, but cannot suppress [slave] insurrections [under this new Constitution]. 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 . . . . Congress, and Congress only [under this new Constitution], can call forth the militia.”

The irony is that the federal government was seen as potentially and incorrectly (from the white owners’ perspective) protecting the most oppressed people–slaves–from the oppressive tyranny of the states. It shouldn’t be surprising that an amendment with such an odious purpose is now being used to excuse the blood trade of gun merchants.

Seems this should be pointed out. Loudly and often.