When the bus carrying O'Dell Wills from Mississippi to Chicago in 1950 neared its destination, the sharecroppers' son could hardly contain his excitement. "When we hit the city limits," he recalls, "I said, 'Wow! I'm home free. This is heaven.'" Fifteen years later, Dorothy Tillman, a civil rights worker arriving from Alabama, saw the high-rise apartment buildings where most blacks then lived and had a different reaction. "Look at all them there factories in the middle of the city," she said to her companion. "Those are not factories," he replied. "People live there."
Between 1940 and 1970, 5 million blacks moved...