On a midsummer night in 1969 under a full moon, Richard L. Stevens helped capture a Viet Cong guerrilla leader named Hoang Thi Nu, the "Vietnamese Annie Oakley." What he saw her do that night, leaping into a river and running through gunfire, and what she endured afterwards in captivity, changed Stevens's mind about what humans are capable of, and about war. Stevens was the sole American advisor to a South Vietnamese unit of 105 "ex"-Viet...