It is a striking historical irony that some slaveholders were at the forefront of efforts to suppress the African slave trade. While partly reflecting humanitarian motives, efforts to restrict the slave trade also expressed a variety of other economic and political interests.
For example, in 1774, the First Continental Congress prohibited the importation of slaves into the United States and banned American participation as a way of asserting the colonists' economic independence and attaching the moral stigma of slavery to Britain. In 1787, South Carolina temporarily prohibited the slave trade in order to prevent debtors from purchasing slaves rather than repaying creditors. Some Virginians feared that continued imports threatened to reduce their slaves' value and diminish the profitable export of Virginia's surplus slaves to the Deep South and West.
The invention of the cotton gin in 1792 had stimulated demand for slaves to raise short-staple cotton. By 1825, field hands, who brought $500 apiece in 1794, were worth $1500.
James Madison, who was serving as Secretary of State at the time he wrote this letter, regarded the African slave trade as America's original sin, but anticipated horrendous upheavals if slaves were emancipated.
It has unfortunately happened, that at no period since the slave-trade was prohibited, have all our citizens abstained from a traffic, deemed worthy of the anxious solicitude of Congress to restrain, as manifested in the several highly penal laws passed on the subject, and alike discountenanced by the regulations of every state in the Union. Now when peace has turned the attention of several nations of the settlement and extension of their colonies, there is danger of the evil increasing, and I must recommend earnestly to the Consuls, especially to those in America, to exert a steadfast vigilance respecting all such infractions of the laws, which may be attempted and to report them, with due precision, to the Department of State.