Once upon a time, America was split in two by slavery. We fought a Civil War over it. Our martyred president, Abraham Lincoln, died for the cause of reunifying America and freeing enslaved Americans for all time. But Lincoln was not only a martyr: He was also a prophet. In the gathering crisis before our Civil War, he warned his countrymen that “a house divided cannot stand”—that America “could not endure, permanently half slave and half free.” Schoolchildren are still familiar with those