Mary Travers, one third of the 1960s folk group Peter, Paul and Mary, lost her battle with leukemia Wednesday night in Connecticut; she was 72. With songs like “If I Had a Hammer,” “Where Have All the Flowers Gone?” and “Blowin’ in the Wind,” Peter, Paul and Mary were the harmonious voice of the 1960s peace movement. Ms. Travers brought a powerful voice and an unfeigned urgency to music that resonated with mainstream listeners....