Genghis Khan, the 13th-century emperor, was infamous for his bloodthirsty, ruthless campaigns, but he was also one of the great commanders of history. Though a master of terror-his campaigns in northern China and Iran were accompanied by a level of slaughter that was not seen again until the 20th century-he was just and generous to his subjects and often magnanimous in victory. His broad, ambitious strategies and elusive tactics were so far ahead of their time that they were acknowledged models for some of the most successful tank commanders of World War II. At the beginning of the 13th century Genghis Khan united the nomad tribes of Mongolia, turned them into a formidable army and led them to rule over the largest empire ever conquered by a single commander. By the time he died, in 1227, his dominions stretched eastward from the Caspian Sea to the shores of the Pacific Ocean.