The Portuguese led the way
It is safe to argue that the initiators of the Transatlantic slave trade were the Portuguese. With the blessing of the Catholic Church and the imperial ambitions of Prince Henry the Navigator, the Portuguese are known to have conveyed slaves from Africa to Portugal as far back as 1444.
Other participating European nations took the cue from the Portuguese. But how were the Portuguese the first?
Well, the Portuguese led the way in the business and adventure of exploring the world beyond Europe when they became the best seafaring Europeans at the inception of the 15th century. Exploring new territories came subsequently with desire to colonize peoples and their resources.