What factors made it possible for Europeans like the the Portuguese to begin exporting large numbers of slaves to the New World?

The history of the European seaborne slave trade with Africa goes back 50 years prior to Columbus' initial voyage to the Americas. It began with the Portuguese, who went to West Africa in search of gold. The first Europeans to come to Africa's West Coast to trade were funded by Prince Henry, the famous Portuguese patron, who hoped to bring riches to Portugal. The purpose of the exploration: to expand European geographic knowledge, to find the source of prized African gold, and to locate a possible sea route to valuable Asian spices.