When a man named Temüjin was given the title of Genghis Khan in 1206, the Mongols were a recently united people, tucked away in the northeast corner of Asia. By the time Genghis Khan died in 1227, they were sunning themselves on the shores of both the Pacific Ocean and the Caspian Sea. By 1241 they were knocking at Vienna’s door, and they remained the terror of eastern Europe for the rest of the century. The Mongols claimed the largest consolidated land empire in history. Seemingly the only way to keep them out was to put the Himalayas between you and them. And many historians believe their power stemmed from an incredibly simple technological innovation: the stirrup.