Much of the power of the Declaration of Independence rests on Thomas Jefferson’s opening line: “We hold these truths to be self-evident, that all men are created equal, that they are endowed by their Creator with certain unalienable Rights, that among these are Life, Liberty and the pursuit of Happiness.” For all of its high principle, there is a dark side to this statement. In the century of the country’s founding, British colonies in the New World imported 1.5 million enslaved Africans. That is more than three times the number of European immigrants who sailed westward searching for some version...