Which country played the biggest role at the beginning of the Atlantic slave trade?

Portugal

The most active European nation in the trans-Atlantic slave trade was Portugal, which used the forced labor of Africans in their Latin American colonies in present-day Brazil. Almost 3.9 million enslaved Africans were forced to embark on Portuguese ships.

Explanation:

an expansion is one of two basic business cycle phases; the other is contraction. the transition from expansion to contraction is a peak, and the changeover from contraction to expansion is a trough.