For 300 years, from the beginning of the sixteenth to the beginning of the nineteenth century, the transatlantic slave trade—the forced migration of Africans to work as slaves on the plantations and in the mines of British, French, Spanish, Portuguese and Dutch colonies in North and South America and the Caribbean—was carried on, legally, and on an ever-increasing scale, by the merchants of most Western European countries and their colonial counterparts, aided and abetted by African middlemen. Indeed, until the second half of the eighteenth century, when (at a conservative estimate) 70–75,000 slaves were being transported annually across the Atlantic, scarcely a voice was raised against it. On 25 March 1807, however, after a lengthy and bitter struggle, inside and outside Parliament, it was declared illegal for British subjects (and at this point during the Napoleonic Wars at least half the trade was in British hands) to trade in slaves after 1 May 1808: opposition to the slave trade on moral and intellectual grounds had gathered momentum during the preceding twenty years and changing economic conditions, which to some extent reduced the importance to the British economy of the West Indian colonies, for whom the slave trade was a major lifeline, while at the same time creating new interest groups unconnected with and even hostile to them, had greatly facilitated its abolition.