In 1202, zealous western Christians gathered in Venice determined to liberate Jerusalem from the grip of Islam. But the crusaders never made it to the Holy Land. Steered forward by the shrewd Venetian doge, they descended instead on Constantinople, wreaking devastation so terrible and inflicting scars so deep that as recently as 2001 Pope John Paul II offered an apology to the Greek Orthodox Church.The crusaders spared no one: They raped and massacred...