In the 1780s, around 40,000 slaves a year were taken from Africa in British ships, on the notorious "Middle Passage," to the Caribbean. In 1787, under an oak tree in Kent, the British Prime Minister, William Pitt, invited his friend William Wilberforce to introduce a parliamentary bill outlawing the slave trade. Neither of them imagined a twenty-year political campaign that would consume the rest of Wilberforce's life. Born in Hull, England,...