In 1779 a Philadelphia belle, Margaret Shippen, married a hero of the ongoing Revolution, General Benedict Arnold. Within months Peggy was sending coded messages to an old suitor from England, conveying Arnold's promise to defect. When their plot was discovered, the general fled. Peggy distracted George Washington with hysterics before following her husband. The British government eventually paid Peggy far more than Benedict Arnold ever received.A generation later, the Philadelphia neighborhood where Margaret Shippen had grown up was home to a businessman named James Forten. Due to his...