Genghis wages his first foreign campaign. For his initial foray beyond Mongolia, Genghis presses south some 600 miles (965 kilometers) into China. As he and his hardened troops cross the Gobi desert, they survive by drinking milk and blood from their horses. They attack Xi Xia, a kingdom of some five million subjects, and handily defeat its disorganized army. Xi Xia becomes a Mongol vassal.   © 1997 National Geographic Society. All rights reserved.