The slave trade reached its peak in the 18th century on the West African coast, where merchants from Europe worked in cooperation with native chieftains and slave raiders who were willing to exchange slaves for Western commodities. Denmark made participation in the Atlantic slave trade illegal in 1792 and the USA did so in 1794. In Britain a group of humanitarian Christians, including Thomas Clarkson and William Wilberforce (members of the so-called Clapham Sect), argued that if the Atlantic slave trade were abolished, with its appalling cruelties, plantation owners would treat their slaves more humanely, as being more valuable. They succeeded in getting Parliament to pass a Bill abolishing the British trade in 1807 and at the Treaties of Ghent (1814) and Vienna (1815) Britain agreed to use the Royal Navy to try to suppress the trade, most European countries now supporting the abolition of slavery. However, for the next 40 years illegal smuggling continued, mainly from Africa. Even after slavery was abolished in the British West Indies (1834) trade in slaves between the southern US slave states and such places as Cuba, Costa Rica, Brazil continued. Within the US southern states the breeding, transport, and sale of slaves became highly profitable, there being some four million slaves on the plantations by the time of the Emancipation Proclamation (1863) (see slaves, us proclamation for the emancipation of). Slavery in the Caribbean did not cease until it was banned by such countries as Cuba (1886) and Brazil (1888). In the Muslim world the Arab slave trade from Africa operated from Morocco and Zanzibar and stretched throughout the Ottoman empire into Persia and India. In 1873 the Sultan of Zanzibar was finally persuaded by Britain and Germany to close his markets, while the Moroccan trade gradually dwindled with French and Spanish occupation. However, as late as 1935 there was still evidence of trade in slaves in Africa.