Middle Passage
Different countries were more powerful in the slave trade at different times.  For example, for two hundred years, from 1440–1640, Portugal controlled most of the slave trade.  However, over time, the balance of power in Europe changed.  By the 18th century, the British Empire was much more powerful.  During the 18th century, when 6 million Africans were brought over the Middle Passage, British slavers carried almost 2.5 million of them.