Sellers, Leila. (1934). Charleston business on the eve of the American Revolution. University of North Carolina Press, Chapel Hill.
. . . The king’s troops could not garrison all of these fortified places. The colony must do its share. The defense of the province was provided for by militia against foreign foes, and by patrols in the country and night watches in the town against domestic enemies. Every citizen, says Hewatt, joined the military to the civil character. Every man between sixteen and sixty years of age able to bear arms had to enroll in the militia. . . . Merchants and tradesmen found military duty inconvenient because it interrupted business. The planters, however, who were more at leisure on account of the task system of managing their plantations, had plenty of time for military exercise and prided themselves upon their martial spirit.
This sounds like reasonable planning to defend the city against other nations' invasion -- and, of course, Indians, right?
But in practice, this is what that militia really was about -- and why that well-regulated militia clause is in the Constitution:
The domestic tranquility of the province was provided for by small patrols drawn every two months from each company. Their duty was to ride along the roads and among the Negro houses in small districts in every parish once every week or as occasion required. To preserve good order in Charleston, where there was danger of insurrection because of the great number of slaves from Africa constantly imported, a night watch was established and paid for by money raised from tavern licenses. In 1770 the night watch consisted of 3 commissioned officers, 3 sergeants, 3 corporals, 3 drums, and 96 privates, armed with muskets and cutlasses; one-third mounted guard every night to “prevent disturbances among disorderly negroes and more disorderly soldiers.” [source: Public Records of South Carolina, XXXII, 385.]
Additionally, imagine what "Their duty was to ride along the roads and among the Negro houses in small districts in every parish once every week or as occasion required" might be like for the Negroes living in those houses. Those plantation fellows out on the town all day and then out into the country, after drinking and whooping it up for hours with their fellows. Imagine how terrifying for the inhabitants in these small houses on a weekly basis. Recall these white fellows could do anything to anyone they felt in the mood to do, because it was all in the name of keeping order. That's how domestic terrorism works to keep the local populations in line.