On 26 February 1638 the governor, John Winthrop speaks of the return to Boston of Captain William Pierce of the Massachusetts ship Desire from the West Indies carrying cotton, and tobacco, and negroes (slaves). Massachusetts were the principal slave trading colonies in New England, and Boston was one of the primary ports of departure for slave ships.
In 1641 Massachusetts Bay Colony was the first of Britain's mainland colonies to make slavery legal.
In 1644 Boston merchants began importing slaves directly from Africa, selling them in the West Indies, and bringing home sugar to make rum, initiating the so-called triangular trade.
In particular, there was a very close economic relationship between the New England colonies and the West Indian slave-based sugar economy. The Caribbean islands found it more profitable to devote all their land to sugar production and import foodstuffs and other staples, while the New England colonies needed a Caribbean export market so that they could purchase manufactured goods from England.
Ships left Boston, Salem, and Newburyport with fish to feed the enslaved Africans laboring on the sugar plantations of the West Indies and lumber to build barrels in which to ship sugar and molasses. Vessels returned from the West Indies loaded with molasses and often carrying a number of enslaved men and women to be sold in the Bay Colony. The molasses was distilled into rum, some of which was sold locally; the rest was shipped to Africa and traded for captured men and women.