It was 1246, and a Franciscan monk named John de Plano Carpini, the papal envoy to the Mongol court in Karakorum, sat listening very intently to some Russian priests at the coronation of Güyük Khan. Carpini's mind absorbed every detail as the Russian priests spoke of the Mongols' past conquests, reciting the names and locations of the Mongol generals. And when they were done speaking, Carpini had accomplished an amazing thing; He had gathered more intelligence than all of Christendom had ever known about these mysterious, terrifying horsemen from the east. From the Russian priests, he learned of one general...