The Jurchen-led Jin dynasty in northern China became a major obstacle to the Mongol Empire's quest for world domination.
However, after realising that this strategy no longer worked, they began to actively prepare for war with the aim of eliminating the Mongol threat in a single campaign.
Earlier in 1204, Genghis Khan had subdued the Ongud, a Mongol tribe helping the Jin dynasty guard its northern border.
The Mongols controlled the area north of the Yin Mountains and started stockpiling resources in preparation for a military campaign against the Jin dynasty.
In March 1211, the Mongols rallied 100,000 troops for a campaign against the Jin dynasty, leaving behind only about 2,000 men to guard their base in Mongolia.
Before embarking, Genghis Khan prayed to the sky deity Tengri along the Kherlen River to bless the Mongols with victory, and made a symbolic vow to avenge his ancestor, Ambaghai, who was executed by crucifixion in 1146 at the order of Emperor Xizong of the Jin dynasty.
The Great Wall, backed up with numerous castles and towers, was defended by nearly a million Jin imperial soldiers which was composed of 800,000 infantry and 150,000 elite cavalry[3] whom Emperor Xingsheng had earlier sent as a defensive measure to counter the Mongol threat.
"central capital"; present-day Beijing); it was also at the plain at the foot of the ridge that the main force of the Jin army, numbering 400,000 troops,[5] was also encamped.
Genghis Khan ordered his third son, Ögedei to lead a separate force to attack the Jin western capital, Xijing (西京; present-day Datong, Shanxi), and block enemy reinforcements.
The Jin imperial court sent Shimo Ming'an (石抹明安), an official of Khitan descent, to meet Genghis Khan and start peace negotiations.
Genghis Khan sent his general Muqali to lead the Balu Unit (八魯營) to launch a surprise cavalry charge on the enemy via a passage at Huan'erzui (獾兒嘴; lit.
Because of the mountainous terrain, the Mongols were unable to deploy their superior cavalry to its full advantage, hence they dismounted and fought on foot.
Wanyan Chengyu managed to rally the scattered Jin forces after the Battle of Huan'erzui and Yehuling and gather at Huihe Fortress (澮河堡).
After the battle, Emperor Xingsheng of the Jin was assassinated in the central capital, Zhongdu (中都; present-day Beijing), by one of his generals, Hushahu (胡沙虎), who then took control of the city.
The Mongols allowed the Jin dynasty to retain control of Zhongdu but forced them to pay a tribute of 500 men, 500 women, and 3,000 horses.
In the summer of 1212, Emperor Xuanzong of the Jin dynasty abandoned Zhongdu and relocated the capital to Bianjing (present-day Kaifeng, Henan) in the south.