On October 3, 1775, at his Cambridge headquarters, George Washington gathered his leading officers around a table and laid out a single sheet of paper covered in characters that looked like the husks of an insect’s trail. The talk was quiet and direct. A senior official stood under suspicion, a ciphered message had been opened, and the implications were heavier than the room’s timbered ceiling. After weighing the circumstances, the council resolved to adjourn until morning and bring the physician himself to face the paper. The next day would supply the answers. That night supplied the dread.