The Battle of Huan Er Tsui

Battle of Huan Er Tsui is perhaps the most important victory in the history of the Mongol empire. In the secret history it was their greatest accomplishment, and the Moslem historian Rashid ad-Din writing over a hundred years later, declared the Mongols still regarded it as one of their greatest victories. This battle secured Mongol supremacy, enabled later conquests and marked the begining of Mongol expansion. Yet ironically, it it very little known to Occidental sources which seem to concentrate on the Mongol's western campaigns which in reality was much less important as the Secret history itself indicate by the much more information on the East.

The numbers of this battle are debated. According to Yuan Shi and Meng Wu Er Shi, the number was 300,000. In the Xin Yuan Shi and Yuan Chao Ming Cheng Shi Liao, Biography of Mukhali, it was 400,000. Yet this number is much less unlikely. In reality the number of 300,000 probably included logistic support and the effective force of the Jin was probably more around 150,000-200,000. Genghis's force is unkown, but taking account of the invading force and those he despatched for patrol, it is more or less around 100,000. The Jin army is composed of Jurchen and Khitan cavalry and Han infantry. The cavalries were the elite of the army and were in the front and wings. While the infantry was placed closely behind.

The two armies were separated by some 15-20 miles. In Yuan Chao Bi Shi, the earliest record, it merely say the Mongol army of the center clashed and defeated the main forces of the Jin. The Yuan Sheng Wu Qin Cheng Lu has a fulle account. According to it "After the Jin army had reached the Ye Hu Ling, two Khitan officers of the staff came to Shi Zong and said: "Since the capture of Fu Zhou, the Mongols are grazing loose near town, so if we now attack them with our cavalry they will be unprepared and we shall win a decisive victory. Shi Zong however, disagreed and suggested to attack the next day with combined cavalry and infantry.

When Genghis herd the Jin army was approaching, he ordered his men to prepare for action and move toward Huan Er Tsui. The two armies then met at that palce.
Apparently the Jin emperor deployed his Khitan and Jurchid horseman on the front and wings of his ar,y and the infantrybehind. The former were immediately attacked by successive waves of Mongol light troops, and evidently with too little space in which to maneuver, at length faltered under the storms of arrows poured into their ranks. Seizing the opportunity, Mukhali delivered a tremendous assault, his troops charging lance in hand. This was supported by the Guard(Keshik) under Genghis Khan, and the Khitan and Jurchid cavalry was hurled back upon its own infantry. With no chance to reform and no room to maneuver do to the packed formation, they trampled the soldiers and involved the whole army into confusion, the battle lasted half day and by noon the disordered Jin was driven from the field. The Mongols pursued the Jin through Huan er Tsui to the valley of Yang below, the Jin suffered tremendous losses, but at Hui He Bao, part of them under the emperor rallied and joining Wan ye Hu Sha, turned to fight a second batle. Hu Sha was proceeding towards Ye hu Ling when he herd the Mongol were coming through the ranges. Surprised, he retired to Xuang Ping. The officers begged to make a stand but he only thought of retreat. But he was slow and at Hui He Po he was first overtaken by the Jin emperor and then by the pursuing Mongols which pursuited relentlessly without rest. Although his troops were more fresh, Hu Sha was alerady in a panic and when he gave battle he was routed and escaped to Xuan De Zhou which he abandoned later and fell to the Mongols.
Meanwhile the emperor who saw the day lost fought his way off the field with 7,000 picked troops, fled to the Sang Kan River. There on the opposite bankof Hu lai He, Ye Lu Tukha of the rebel khitans came up with 3,000 cavalry and he was forced to fight another battle, the battle lasted a whole day until the exhausted Jin finally gave up and fled.

This battle destroyed the core troops of the Jin and paved a way for Mongol conquest.

The main cause for Jin's defeat in the decisive battles of Hu Er Sui, Wu Sha Bao, and Hui He Bao is because the Jin did not utilize the mobility of their cavalry as they had in their early conquests and used a tightly packed formation with the infantry placed close to the cavalry letting little room for maneuver. And when the decisive charge came, the cavalry was forced back to the infantry causing chaos.

I've already mentioned the sources in the post.

The sources of the battle are ""Yuan Chao Bi Shi", the earliest record, it merely say the Mongol army of the center clashed and defeated the main forces of the Jin. The "Yuan Sheng Wu Qin Cheng Lu" has a fuller account."

While Yuan Shi and some other official history also mentions this battle but no details are present.