In the autumn of 1399, Tamerlane, the Mongol warlord, had just returned to Samarkand flush with the spoils of his recent ransack of Delhi. While the elephants he had brought from India were hauling stone for a magnificent new mosque, he charged south-west to defeat the Ottoman sultan. In signature style, his soldiers stacked up the heads of 90,000 victims in 120 towers around modern Baghdad....