Mongol Marches South
How the Mongols conquered China?

In our mind, Mongol invasion regularly includes archers on horses, bloodshed, slavery or even massacres. Their art of war also has a stereotype. We regularly imagine Mongols riding their Mongolian horses at high speed, encircled the enemies and use their composite bows to knock them down. Indeed, they applied similar tactics on the steppes and deserts. But in China, especially Southern China, the places that consist mountains, land dissected by rivers and also the cities with high walls, Mongols had to adjust their strategies and tactics in order to overwhelm the Chinese.
The opening of the War
After conquering the Jurchen’s Jin(金) dynasty in assisted of the Mongols, the Sung(宋) dynasty’s generals intended to capture the capital Bianjing(汴京) and the important cities of Guanzhong(關中, nowadays’ Shannxi陝西 and Henan河南) for the purpose of taking advantage of terrains to block the Northern barbarian’s invasion.
Emperor Lizong(宋理宗) authorized Zhao Kuei(趙癸) in charge to take back Three Capitals(三京), including Bianjing, Luoyang(洛陽) and Yingtian Fu(應天府). Mongols accused Sung violated the pact that they agreed and attack the Sung forces. The Sung army was crushed by the Mongol’s forces and retreated due to lack of supplies. The Mongol conquest of Sung began.
When the Mongol conquering Jin, they were still adopting the nomad’s strategy, that is attacked on several directions with high speed, lured the enemy out and annihilated them. Instead of sending troops out, the Sung army kept themselves in the walled cities. Some of them constructed fortresses on hills that contains fresh water and available to cultivate.

During the war with Sung, Mongols suffered loses and even Mongke Khan cannot take out the little but fortified Diaoyu fortress(釣魚城). After Kublai became the Great Khan, with the assisted from his Chinese consultants, he started to hold a war in a Chinese way.
Transformation
First, on the level of strategy, Mongol transformed from the quick attack to the enduring confrontation. To overthrow Chinese’s defenses, Mongol did not storm the fortified positions with hasty. They built long walls outside and encamp there to starving out the defenders. Defenders were hard to make a sortie because they will become weak attackers. For example, when the Mongols seized the Sung capital Linan(臨安) and captured the Emperor Gongdi(宋恭帝) to the North. Yangzhou(揚州) did not fall into Mongols yet in the command of Li Tingzhi(李庭芝) and Jiang Cai(姜才). According to The Reading Notes of Twenty-two Historical Books by Zhao Yi(趙翼), a Ming dynasty scholar Ke Weiqi(柯維騏) called the question that, why Jiang Cai held forty thousand men and didn’t intercept the Mongol army that captured Emperor Gongdi? Zhao Yi stated the Mongol constructed massive long walls outside Yangzhou, there were no places for Jiang Cai to intercept. We can see the Mongols did knew how to make pressure on the stronghold of Chinese and the latter had no way to get out or sent forces.
On the other hand, Mongols absorbed Northern Chinese local clans. The latter not only provided military assistance but also stabilizing Chinese peasants and brought Chinese traditional education to the Mongols. In the middle stage of the war, more and more Chinese joint into the Mongol Yuan(元) dynasty force. The Shi(史), Yao(姚) and Dong(董) families as the military commander; Xu Heng(許衡) as the Neo-Confucianism leader of the Yuan dynasty. The Mongol respected the Chinese scholars, this effectively made the Mongol Yuan dynasty also got legitimate from Confucianism and stabilize their ruling in China.
As a universal empire, Mongol made effective uses of different kinds of people. Kublai got Persians to be his technician to built trebuchet. Using the Chinese to fight Chinese. A Mongol general Aju said, “When we-the Mongol forces-face the hills, rivers, and fortifications, cannot without Han forces”.(The History of Yuan, Book 6)
Apart from the family members mentioned above, some Chinese generals deflected to the Mongol Yuan in the case of excluded moral principles of traditional Chinese. For example, the most brilliant military commander Liu Zheng(劉整) deflected to the Mongol Yuan due to the unpleasant relationship and jealousies by other generals of Sung. Anyway, he helped the Mongol to train their navy. With his genius command both on army and navy, Mongol Yuan forces sieged the twin cities Xiangyang(襄陽) and Fancheng(樊城) on the Han River(漢水). They destroyed several Sung dynasty’s reinforcements by the outside fortresses. Just like the case of Napoleon defeated the Austrian’s reinforcement and captured Mantua. In the battle for Xiangyang-Fancheng, the Mongol Yuan had proved they had the ability to defeat the Sung navy. After capturing those strongholds, Mongol Yuan forces– in fact, most of the soldiers were Chinese–led by Lü Wenhuan(呂文煥), the surrendered commander of Xiangyang, headed downstream and capture the capital of Sung dynasty. Six years later, the Sung dynasty destroyed.
The above mentioned the transformation of Mongol during the conquest of Sung dynasty. The Mongol had switched their strategy and made good use of local Chinese’s assistance so that they can overthrow the Sung dynasty like the domino effect in the latter stage of the war. At last, they turned into a Chinese dynasty which is called the Yuan dynasty.
