Genghis Khan was by far the greatest conqueror the world has ever known, whose empire stretched from the Pacific Ocean to central Europe, including all of China, the Middle East and Russia. So how did an illiterate nomad rise to such colossal power, eclipsing Alexander the Great, Julius Caesar and Napoleon? Credited by some with paving the way for the Renaissance, condemned by others for being the most heinous murderer in history, who was Genghis Khan?