Apart from that, the Europeans to a great extent enslaved free population as well. They seized them during raids performed at the shores of Africa. The merchants were later awarded with the license for trading the enslaved people to the colonies of the Spanish empire. It was economically motivated, as the vast lands in America required much labor for a lot of kinds of work, including works at plantations, in mines and in household. In this way, the slave trade process was tightly connected with the process of colonizing the Western Hemisphere.
Klein, Herbert S. and Jacob Klein. The Atlantic Slave Trade. Cambridge: Cambridge University Press, 1999.
Solow, Barbara (ed.). Slavery and the Rise of the Atlantic System. Cambridge, Cambridge University Press, 1991.