During Wanyan Yongji's reign, the Mongols, under the leadership of Genghis Khan, had been planning to attack the Jin dynasty. The Mongols first attacked Western Xia, a vassal state under the Jin dynasty. When Western Xia requested aid from the Jin dynasty, Wanyan Yongji ignored them; Western Xia eventually surrendered to the Mongols. In 1211, the Mongols launched a full-scale invasion of the Jin dynasty and defeated the Jin armies in some small battles. In the ninth month, the Mongols besieged the Jin central capital, Zhongdu (中都; present-day Xicheng and Fengtai districts of Beijing), but were forced to retreat because of Zhongdu's strong defences. In 1212, the Mongols attacked the Jin dynasty and besieged the Jin western capital, Datong Prefecture (大同府; present-day Datong, Shanxi Province). In the same year, Yelü Liuge (耶律留哥), a Khitan, led a rebellion against the Jin dynasty in Jilin Province. The rebel movement gained thousands of followers within months. Yelü Liuge submitted to the Mongols and led the rebels to defeat a 600,000-strong Jin army in Dijinao'er (迪吉腦兒; near present-day Changtu County, Liaoning Province).