Which countries took part in the trade?

The answer

Portugal, Spain, the Netherlands, Britain, France, Denmark and Sweden all took part at different times, along with merchants in the Americas. The SlaveVoyages estimates table, by flag of the carrier, lists about 5.8 million people to Portuguese and Brazilian ships, about 3.3 million to British, about 1.4 million to French, about 1.1 million to Spanish, 0.55 million to Dutch, 0.31 million to United States and 0.11 million to Danish and Baltic vessels, out of about 12.5 million.